A supported liquid membrane (SLM) with resorcinarene for facilitated transport of methyl glycopyranosides: Parameters and mechanism relating to the transport

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Journal of Membrane Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i

A supported liquid membrane (SLM) with resorcinarene for facilitated transport of methyl glycopyranosides: Parameters and mechanism

relating to the transport

Khalifa Touaj ^a,b , Nabila Tbeur ^a,b , Mustapha Hor ^a , Jean-Franc¸ois Verchère ^b , Miloudi Hlaïbi ^a,b,∗

a

Laboratoire d’Interface Matériaux et Chimie de l’Environnement (LIME), Université HASSAN II, Faculté des Sciences d’Aïn Chock, BP 5366, Maârif, Casablanca, Morocco

b

Laboratoire “Polymères, Biopolymères, Membranes”, UMR 6522 du CNRS, Université de Rouen, 76821 Mont-Saint-Aignan Cedex, France

a r t i c l e i n f o

Article history:

Received 12 July 2008

Received in revised form 19 February 2009 Accepted 15 March 2009

Available online 27 March 2009

Keywords:

Facilitated transport

Supported liquid membrane (SLM) Resorcinarene carrier

Diffusion coefficients Stability constants

a b s t r a c t

A supported liquid membrane (SLM) containing a resorcinarene carrier, previously used for the transport of aldoses and alditols, has been used for the selective transport of methyl-␣- d -glucopyranoside, methyl-

␤-d-glucopyranoside, methyl-␤-d-galactopyranoside, methyl-␣-d-mannopyranoside, and methyl-␤-d- xylopyranoside from concentrated (0.20–0.025 M) aqueous solutions. The membrane is made of a microporous polytetrafluoroethylene film (PTFE), impregnated with a 0.01 M solution of the carrier in CCl

4

. The permeabilities of the SLM for all studied methyl aldopyranosides were calculated. On the basis of the flux dependence on the initial concentrations of carrier and methyl aldopyranoside, the rate-determining step in the transport mechanism is shown to be the migration of the (1/1) carrier–carbohydrate complex in the immobilized organic phase. The flux of sugar is related to the initial concentration of methyl aldopyranoside in the feed phase by a saturation law, which allowed the deter- mination of the apparent diffusion coefficients and the stability constants of the resorcinarene complexes of methyl aldopyranosides formed in the liquid membrane. The stability constants of the complexes fall into two classes: aldopyranosides with trans HO groups form complexes of low stabilities (K ≈ 0.36 ± 0.01) whereas aldopyranosides with cis HO groups form complexes of high stabilities (K ≈ 0.84 ± 0.01).

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The separative methods of carbohydrates based on molecular recognition appear as promising tools. The principle relies on the use of a selective reagent (the host) that can form host–guest com- plexes with carbohydrates (the guests). When the stability of the complex varies with the structure of the sugar, the components of the mixture are complexed in different proportions, and various techniques such as extraction may be used for the isolation of the resulting complexes.

In the case of carbohydrates, a possible process is extraction into organic solvents, which is made possible by forming complexes with lipophilic carriers. Such complexes, contrary to uncomplexed sugars, can be extracted from aqueous solutions. However, for use on the industrial scale, extraction methods should be preferably adapted to liquid membrane processes, which currently offer the best strategy for environmental-friendly separations [1]. Most liquid

∗

Corresponding author at: Université HASSAN II, Faculté des Sciences d’Aïn Chock, BP 5366, Maârif, Casablanca, Morocco. Tel.: +212 2 23 06 80;

fax: +212 2 23 06 74.

E-mail address:

miloudi58@hotmail.com

(M. Hlaïbi).

membranes consist in an organic phase that separates two aque- ous (feed and receiving) phases [1]. In this case, a double extraction process allows the overall transport of water-soluble species across the membrane. Selective transport occurs when the liquid mem- brane contains a carrier agent, which forms complexes of different stabilities with the various sugars present in the feed phase. The main advantage of liquid membranes over classical extraction is that a very small volume of organic solvent is necessary, and since this solvent is trapped in the membrane, the quantity of harmful volatile chemicals released in the atmosphere is reduced. Moreover, operations take place at ambient temperature, with considerable reduction of the energy cost.

Supported liquid membranes (SLM) are the preferred class of liq- uid membranes for practical applications, although other types of membranes are sometimes used, such as bulk liquid membranes or emulsion liquid membranes [1]. In SLM, the organic phase is immo- bilized in a thin, microporous polymer film (the support) that gives mechanical toughness to the device. SLM have been recognized as useful tools for the recuperation of valuable compounds or the elimination of pollutants, mainly metal ions from aqueous solution [1]. The first commercial use of SLM is for chromium removal from wastewater, under the trademark SliM of Commodore Separation Technologies, Inc. [2].

0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.memsci.2009.03.014

Fig. 1.

Structure of resorcinarene

1

(R = CH

3

(CH

2

)

10

).

Our long-term project is aiming at the separation of carbohy- drates by means of SLM processes. For this purpose, the membrane should contain a lipophilic carrier that can specifically recognize sugars by forming host–guest complexes. The chosen host belongs to the family of resorcinarene, that are analogues of calix[4]arenes in which the four phenol units are replaced by resorcinol units [3,4].

Calixarene derivatives have been increasingly used as carriers in liquid membranes, essentially for the extraction of metal ions [5,6].

More generally, the use of various types of macrocyclic carriers in SLM is well documented [7].

Our initial guideline was the discovery by Aoyama et al. [8–10], that resorcinarene 1 (Fig. 1) selectively extracts several sugars from concentrated aqueous solutions into an anhydrous CCl

₄

layer.

The selectivity of the extraction is due to the differences in the stabilities of the host–guest complexes formed between resor- cinarene extractant 1 and the sugars in the organic solvent. Our subsequent studies have shown that extraction also takes place, with a different selectivity pattern, when the organic solvent is not dried before re-extraction of the sugar into water [11]. It may indicate that the species present in the organic layer are indeed resorcinarene–sugar–water ternary complexes. This find- ing opened the way for the first preparation of SLM containing the resorcinarene carrier 1, which selectively effect the transport of sugars between two aqueous solutions [12].

Such SLM offer an alternative to various membranes proposed in the literature for the separation of mixtures of sugars. Ion-exchange membranes have been suggested for the separation of aqueous solutions of sugars as borate complexes [13–15]. Following the pio- neering work of Shinbo et al. [16], liquid membranes containing arylborinic acids as carriers were also designed for transport of car- bohydrates [17–20]. Finally, plasticized cellulose triacetate (PCTA) membranes containing a quaternary ammonium salt as the active carrier have been patented for the separation of mixtures of car- bohydrates (sucrose, fructose and glucose) by facilitated transport [21,22].

A drawback of the systems involving borate or boronate com- plexes is that the aqueous phases must be buffered, because the carbohydrate complexes are formed only in strongly alkaline media.

Moreover, contrary to the newer PCTA membranes [21,22], most SLM described in the literature for the transport of sugars suffer from insufficient stability and/or small fluxes.

We have demonstrated that sugars [12] and alditols [23] are transported across a SLM containing a resorcinarene carrier 1. The permeability P is different for each carbohydrate, and the varia- tions can be assigned to the structural differences of the transported species. Specifically, the variations of P can be related to variations

of the stability constants K of the carrier–carbohydrate complexes and to variations of the apparent diffusion coefficients D. The sta- bility constants measure the intensity of the interaction between the carrier and the carbohydrate. This intensity depends on the ori- entation of the diol group of the carbohydrate that binds with the carrier. It was reported [23] that alditols with threo configuration at C-2,3 form weaker complexes than alditols with erythro config- uration. Moreover, the complexes of alditols are stronger than the complex formed with an aldose, arabinose [12]. It was suggested that the reason for this difference could be the stiffness of the pyra- nose ring of arabinose. On the contrary, the acyclic alditols can easily adapt their conformation in order to match the recognition site of the carrier.

In order to check this hypothesis, we examined the transport of various methyl aldopyranosides. Methyl pyranosides are sugar derivatives (acetals) in which the anomeric hydroxyl group has been exchanged for a methoxy group. Consequently, the structures of these pyranosides are blocked, making the anomerisation impossible. Thus, differences were expected with respect to aldoses that undergo the anomerisation equilibrium. One can also expect that the OMe group would be more lipophilic than the HO group of the aldoses. Finally, this work has further interest in relation with the possible separation of synthetic mixtures of aldoses and pyranosides.

2. Experimental

The pyranosides and other chemicals were commercial products (Aldrich or Fluka) of the purest available grade, used as received.

Resorcinarene 1 was synthesized according to a published proce- dure [9,11,12,24]. Its

¹

H and

¹³

C NMR spectra, measured with Bruker ARX-400 spectrometer, were identical with those of a commercial sample (purchased from Fluka).

The SLM support was a microporous PTFE film (Goodfellow) of thickness 63 ␮ m. Characteristic values are porosity 84% and pore size 0.45 ␮ m. The membrane area available for diffusion was 19.6 cm

²

(diameter, 5.0 cm).

The transport cell [12,23] is made of two compartments of equal volumes (100 mL) separated by the SLM prepared by soaking a square portion of the polymer film (8 cm × 8 cm), into a 0.01 M solu- tion of resorcinarene 1 in pure carbon tetrachloride, during 15 h.

The cell is immersed into a thermostated bath (T, 298 K). The solu- tions in both compartments are stirred with magnetic bars, using a Variomag apparatus.

Initially, the feed compartment contained the pyranoside solu- tion (c

₀

= 0.025–0.20 M) and the receiving compartment contained pure water. Two different techniques were used:

- In the first one (mode a), for the study of the lifetime of the mem- brane and the reproducibility of results, we used for each of the experiments a new pyranoside solution with the same fixed con- centration. With this procedure, we have verified that the results are reproducible with the same membrane was used during 10 days without showing any sign of failure.

- In the second one (mode b), to study the effect of the concentra- tion of the studied aldopyranoside on the parameters of transport, the same membrane and the same solution have been used for all runs. At equilibrium after each run, the contents of both compart- ments were withdrawn and mixed together, then 100 mL of the resulting solution were introduced in the feed compartment and 100 mL of water in the receiving compartment and the follow- ing run was started. This economic procedure for substrates, was typically repeated four or five times.

In these techniques, small aliquots ( v = 1 . 0 mL) of the receiving

phase were withdrawn at known intervals. Some experiments were

continued for 24 h, when samples of both aqueous phases were withdrawn and analyzed to ensure that equal concentrations were present, indicating that equilibrium was reached.

The samples were analyzed using a HPLC apparatus equipped with a 30-cm Phenomenex Rezex column in calcium form, main- tained in an oven at 85

^◦

C. The eluent was pure water, degassed and filtrated with a cellulose ester membrane (Millipore, pore size 0.45 ␮ m). The flow rate was 0.6 mL/min. The pump was a Shimadzu LC-9A model. Detection was achieved with a Varian RI-4 refractometer. Typical retention times (in min) for the methyl aldopyranosides were 13.0 (methyl- ␣ - d -glucopyranoside), 13.0 (methyl- ␤ -d-glucopyranoside), 15.0 (methyl- ␤ -d-galactopyrano- side), 15.0 (methyl- ␣ - d -mannopyranoside) and 15.0 (methyl- ␤ - d -xylopyranoside). The methyl aldopyranoside concentrations were determined by analyzing chromatographic data with the Varian Star software. All experiments were duplicated and were reproducible with 3% as relative standard deviation.

3. Results

3.1. Conditioning of the SLM

The transport of a model methyl aldopyranoside, typically methyl- ␤ - d -xylopyranoside (c

₀

= 0.10 M in feed phase), was studied through a SLM in which the membrane phase was a 0.01 M solu- tion of resorcinarene 1 in pure carbon tetrachloride, supported by a microporous PTFE film.

When the SLM was used for transport immediately after its preparation, the flux of methyl- ␤ -d-xylopyranoside remained small for a long initial period, typically 8–10 h. After this time, the rate of transport increased and the concentration of methyl-

␤ -d-xylopyranoside in the receiving phase c

R

increased rapidly over several hours. This behavior has been observed previously with a SLM containing this carrier [12,23] and was attributed to the slow incorporation of water into the membrane to form presumably a carrier–water complex. After this initial period (induction period), the transport of aldopyranosides increases, prob- ably because the substrate rapidly exchanges with water at the interfaces to form the carrier–substrate complex, or possibly a ternary carrier–substrate–water complex that is the active species for the transport process. Such a phenomenon appears to be com- mon with SLM used for the transport of carbohydrates. For example, the presence of water in the SLM was reported to be important for the transport of carbohydrates through plasticized cellulose triac- etate membranes containing ion-pair carriers [21,25].

The existence of this induction period complicates the analysis of the kinetic studies, because it makes difficult to approximate the exact value of the slope of the plots drawn for a first-order reaction [23]. In this study, in order to suppress this period of slow transport, the SLM was conditioned for 16 h in the cell between two phases of pure water. After this time, the induction period was not observed and the transport of methyl- ␤ -d-xylopyranoside began just after its introduction in the feed phase.

3.2. Transport experiments

Beside their numerous advantages, the main drawback of most SLM is instability, as the organic solution is eventually displaced from the support by the aqueous feed and receiving phases. Vari- ous causes for membrane instability have been discussed elsewhere [26]. However, the SLM studied in this work seem to represent an ideal case, since the transported carbohydrates are almost insol- uble in the organic phase, whereas the carrier is insoluble in aqueous medium, avoiding thus its washing-out from the organic layer.

The high stability of our membranes was previously demon- strated [12,23]. In the case of the transport of sugars, the SLM could be used for 10 days. In the present study with methyl aldopyra- nosides, most experiments were carried out during 5 days with the same membrane, without any observation of leaking. Another advantages of our system is that only facilitated transport of the sugars takes place, as passive diffusion of the sugars across the SLM cannot be detected in the absence of carrier. This result is consis- tent with the hydrophobic nature of the PTFE film, and with the fact that uncomplexed aldopyranosides are not dissolved in carbon tetrachloride.

A special attention was given to the synthesis of carrier 1, because this macrocycle is known to possess several diastereoiso- meric forms [27,28]. We ensured the exclusive formation of the thermodynamically more stable crown form by heating the crude condensation mixture at reflux for at least 4 h [27]. The stereo chemical implications of the complexation of chiral guests with the achiral host 1 have been discussed elsewhere [29].

3.3. Measurement of permeabilities

The principle of the calculation of permeabilities and fluxes has been developed elsewhere [12,23]. The transport rate is measured by determining the increase of the aldopyranoside concentration c

R

in the receiving phase vs. time t. This rate is related to the flux J of aldopyranoside by Eq. (1):

d c

R

d t = JS

V (1)

where S is the membrane area and V is the volume of the receiving phase.

When the system reaches a quasi-steady state, the flux J is related to c, the difference between the concentrations of aldopy- ranoside in the feed (c

F

) and the receiving phases (c

R

), and the membrane thickness l by Eq. (2) derived from Fick’s First Law, J = P c

l (2)

where P is the permeability of the membrane.

Since the flux of aldopyranoside is very large, the concentration (c

R

) of the receiving phase is not negligible vs. the concentration (c

_F

) of the feed phase. Thus, c is calculated using Eq. (3) where c

₀

is the initial concentration of aldopyranoside in the feed phase:

c

F

= c

0

− c

R

and c = c

0

− 2 c

R

(3)

Combining Eqs. (1)–(3) yields differential Eq. (4):

P d t = ( lV/S )(d c

R

/c

0

− 2 c

R

) (4)

Integration of both terms of Eq. (4) yields Eq. (5):

P ( t − t

L

) = _lV

S 1 2

ln

_c

0

c

0

− 2 c

R

(5) which shows that, after an induction period (t

_L

) that may last up to several hours, the term − ln(c

₀

− 2c

R

) is a linear function of t, when the membrane was equilibrated with water for 16 h before starting the experiment, the induction period completely disap- peared. This result demonstrates that the induction period is due to uptake of water by the membrane and the plots drawn (Fig. 2) for the transport of the methyl aldopyranosides were indeed straight lines.

The permeability P values for the various methyl aldopyrano- sides were calculated (Tables 1 and 2), using Eq. (6), from the slopes a of the plots:

P = a _lV

2 S

(6)

Table 1

Transport of methyl-␤-d-glucopyranoside, methyl-␤-d-galactopyranoside and methyl-␤-d-xylopyranoside across a SLM.

Methyl aldopyranoside

c0

(feed phase) (mmol cm

⁻³

) 10

⁵a

(s

⁻¹

) 10

⁷P

(cm

²

s

⁻¹

) 10

⁵Ji

(mmol cm

⁻²

s

⁻¹

)

Methyl-␤-d-Glucop.

0.20 8.19 13.17 4.18

0.10 8.53 13.70 2.185

0.05 8.67 13.93 1.105

0.025 8.75 14.06 0.558

Methyl-␤-d-Galactop.

0.20 3.72 5.98 1.90

0.10 4.00 6.43 1.02

0.05 4.17 6.70 0.532

0.025 4.25 6.83 0.271

Methyl-␤-d-Xylop.

0.40(*) 7.89 12.68 8.05

0.20 8.53 13.70 4.35

0.10 8.78 14.11 2.24

0.05 8.94 14.37 1.14

0.025 9.03 14.50 0.576

T

= 25

^◦

C, [C]

0

= 0.010 mol L

⁻¹

, C is the carrier,

±5% is the confidence interval for values.a

is the slope of a linear plot of Eq.

(5). Permeability

P is calculated from Eq.

(6).

J

i

the initial value of the flux, is calculated using Eq.

(7). (*) independent experiment, not used for calculation of

K. Bold values indicate the beginning of the transport parameters of a new compound of

Tables 6 and 7.

Table 2

Transport of methyl-␣-d-glucopyranoside and methyl-␣-d-mannopyranoside across a SLM. Influence of the methyl aldopyranoside concentration.

Methyl aldopyranoside

c0

(feed phase) (mmol cm

⁻³

) 10

⁵a

(s

⁻¹

) 10

⁷P

(cm

²

s

⁻¹

) 10

⁵Ji

(mmol. cm

⁻²

s

⁻¹

)

Methyl-␣-

d

-Glucop.

0.20 7.08 11.38 3.61

0.10 7.31 11.74 1.86

0.05 7.44 11.96 0.949

0.025 7.51 12.07 0.479

Methyl-␣-d-Mannop.

0.20 6.67 10.71 3.40

0.10 7.19 11.56 1.835

0.05 7.47 12.01 0.953

0.025 7.64 12.26 0.487

Same conditions as

Table 1, all series of runs were performed with a single membrane, by stepwise dilution of the feed phase. Bold values indicate the beginning of the

transport parameters of a new compound of

Tables 6 and 7.

At the initial instant the aldopyranoside concentrations in the aqueous phases are c

_F

≈ c

₀

and c

_R

≈ 0. At this instant, the initial value of the flux, J

_i

, can be calculated by Eq. (7) derived from Eq. (2):

J

_i

= Pc

0

l (7)

3.4. Modeling and parameters of the complex-forming reaction The mechanism for the overall transport of a methyl aldopyra- noside S by the carrier C across the liquid membrane is known to involve five consecutive steps [12]:

Fig. 2.

Plots of

−ln(c0−

2c

R

) vs. time t for the transport of methyl-␤-d- glucopyranoside and methyl-␤-d-galactopyranoside. T = 25

^◦

C, initial concentra- tions of pyranosides in feed phase c

0

= 0.025–0.2 M, concentration of carrier [C]

0

= 0.01 mol L

⁻¹

1. Diffusion of the methyl aldopyranoside through the diffusion layer, from the bulk of feed phase to the interface.

2. Reaction between the methyl aldopyranoside and the carrier at the feed phase–SLM interface, to form a complex soluble in the membrane organic phase.

3. Diffusion into the SLM, where the complex migrates towards the SLM-receiving interface and the carrier migrates back towards the feed–SLM interface.

4. Dissociation of the complex at the SLM-receiving phase interface, to release the methyl aldopyranoside into the receiving phase and the carrier into the membrane.

5. Diffusion of the liberated methyl aldopyranoside through the diffusion layer to the bulk of receiving phase.

Steps (1) and (5) are presumably fast, because both aqueous phases are stirred. Steps (2) and (4) correspond to the fast formation and dissociation of the complex at the interfaces. Hence, as in the most studies with SLM, the rate-determining step is expected to be step (3), the diffusion of the complex into the organic solvent that fills the pores of the polymeric support. The complex was assumed to be a (1:1) methyl aldopyranoside–resorcinarene 1 species. The stoichiometry of such complexes could not be found in the litera- ture. However, in the case of ribose, the existence of a (1:1) complex with resorcinarene 1 in CCl

4

was reported by Aoyama et al. [8]. The extraction of sugars and alditols by resorcinarene 1 also supported the formation of a (1:1) complex [11].

In the transport experiments, the initial methyl aldopyranoside

concentration c

₀

in the feed phase is much larger (50–100-fold)

than the initial carrier concentration ([C]

₀

= 0.01 M) in the organic

phase. Therefore, the concentration [CS] of the (1:1) carrier–methyl

aldopyranoside complex in the membrane will be limited by the

initial concentration of the carrier.

Fig. 3.

Plots of 1/J

i

vs. 1/c

0

for the transport of five methyl aldopyranosides across the SLM.

The relationship between the flux J and the methyl aldopy- ranoside concentration [S]

t

in the feed phase, at any time, was established using a kinetic model in which the diffusion step (3) is assumed to be rate-determining [12]. The reaction between the carrier and the methyl aldopyranoside at both interfaces, may be considered as a fast heterogeneous equilibrium:

C

_(org)

+ S

_(aq)

↔ CS

_(org)

where (org) and (aq) refer to the organic phase and water phase respectively. The concentration [CS]

_i

of complex at the interface obeys the mass action law, Eq. (8):

[ CS ]

_i

= K [ C ]

_i

[ S ]

_i

(8)

where K is the formation constant of the complex, [C]

_i

is the con- centration of the carrier in the membrane, near the interface, and [S]

_i

is the concentration of the methyl aldopyranoside in the feed phase, near the interface.

In the rate-determining step (3), the flux J is determined by Eq.

(9), derived from Fick’s First Law by assuming that the concentra- tion of complex is practically nil at the receiving phase–membrane interface:

J = _D

_∗

l

[ CS ]

_i

(9)

D

^*

is the apparent diffusion coefficient and l is the membrane thick- ness.

Because the methyl aldopyranoside S is in excess in the feed phase, [CS]

_i

[S]

_i

and [S]

_i

=c

0

, the initial concentration of methyl aldopyranoside. The small concentration [C]

₀

of carrier immobi- lized in the SLM is constant, but the free carrier is in equilibrium with the complex. The flux under the initial conditions J

_i

is given by Eq. (10):

J

_i

= ( D

^∗

/l )([ C ]

₀

Kc

0

) / 1 + Kc

0

(10) The postulated mechanism requires that J is proportional to the initial concentration of carrier [C]

₀

and obeys a saturation law with respect to c

0

. In order to test the proposed relationship, Eq. (10) was linearized as a Lineweaver–Burk plot, Eq. (11):

1 /J

_i

= ( l/D

^∗

)(1 / [ C ]

₀

K )(1 /c

0

) + ( l/D

^∗

)([1 /C ]

₀

) (11) Transport experiments with the five methyl aldopyranosides were carried out in which c

₀

varied in the range 0.025–0.20 M. Eq.

(11) was checked by plotting the values of 1/J

_i

vs. 1/c

0

, which yielded very good linear relationships in every case (Fig. 3). From the slopes and intercepts of the above plots, the apparent diffusion coefficients

Table 3

Apparent diffusion coefficients and stability constants for the complexes of

1

with methyl aldopyranosides.

Carbohydrates

K

(mol

⁻¹

L) 10

⁴D^*

(cm

²

s

⁻¹

) 10

⁻³

slope 10

⁻³

intercept

Me-␤-d-Xylop. 0.36 4.00 4.302 1.575

Me-␤-d-Glucop. 0.37 3.79 4.440 1.662

Me-␣-d-Glucop. 0.36 3.33 5.172 1.890

Me-␤-d-Galactop. 0.83 0.836 9.036 7.538

Me-␣-d-Mannop. 0.85 1.471 5.027 4.283

l-Arabinose[12]

0.18 5.95 5.880 1.060

dl-Threitol[23]

0.81 2.53 3.080 2.490

Erythritol

[23]

1.58 1.00 3.970 6.270

Xylitol

[23]

0.46 1.92 7.210 3.280

Slope and intercept were obtained from a plot of 1/J

i

vs. 1/c

0

, Eq.

(11).

K± 0.01 and (D

^*±

0.01)

×

10

⁻⁴

. K is the equilibrium constant for the heterogeneous reaction: C (org) + S (aq)

↔

CS (org).

and stability constants of the complexes were calculated through linear regressions (Table 3):

K = intercept

slope and D

^∗

= l

[ C ]

₀

intercept

To our knowledge, this the first report on the determina- tion of stability constants of the complexes formed between methyl aldopyranosides and resorcinarene 1. Values for a sugar–resorcinarene 1 complexes have been reported in the case of arabinose [12] and some alditols [23].

Examination of Fig. 3 shows that the pair of methyl aldopy- ranosides yields plots with nearly the same intercept. The same phenomenon is observed with the three order methyl aldopyrano- sides. This should be related to the close values of the corresponding apparent diffusion coefficients D

^*

, since intercept =l/(D

^*

[C]

₀

).

Hence, the experimental results support a mechanism in which diffusion of the CS complex is the rate-determining step. However, the values of D

^*

seem very high, compared to usual diffusion coef- ficients for carbohydrates. It suggests that the migration of the complex is not a pure diffusion process, this point is discussed hereafter.

4. Comparison of results

We find that methyl aldopyranosides forming complexes with the resorcinarene 1, are divided into two series:

(a) Methyl- ␣ -glucopyranoside, methyl- ␤ -glucopyranoside and methyl- ␤ -xylopyranoside form with the resorcinarene 1 complexes whose stability constants are equal to 0.36 mol

⁻¹

L.

(b) Methyl- ␤ -galactopyranoside and methyl- ␣ -mannopyranoside form with the carrier, complexes that have the same stability constant whose value is equal to 0.84 mol

⁻¹

L.

By comparing the diffusion coefficients D

^*

and the stability con- stants K noted in Table 3, we note that the values vary from one complex to another. Before proceeding to discuss these results, we will complete our study by a comparison the transport of methyl aldopyranosides with that of the corresponding aldoses and alditols, order to determine all parameters that control the transport of methyl aldopyranosides through our supported liquid membrane.

4.1. Comparison with extraction data

In their pioneering studies, Aoyama et al. reported that ribitol

was not extracted by anhydrous solution of resorcinarene 1

in carbon tetrachloride [8,9]. Instead, Verchère and co-workers

demonstrated that by using a solution of resorcinarene 1 in CCl

₄

saturated with water [11], several alditols were extracted as well as

sugars, although the extraction selectivity was not so good as in anhydrous conditions. Comparison of these results indicates that the extraction of carbohydrates by resorcinarene 1, is considerably modified in the presence of water, probably because the carrier can complex water as well as carbohydrates [9]. This effect of water is obviously present in our SLM transport process. Whether it implies the formation of true ternary complexes, or the formation of water pockets that dissolve the sugar in the organic solvent, cannot be ascertained at this stage of the program. However, the selectivity observed in the transport process suggests that ternary complexes with quite different stability constants may be involved.

Aoyama studied by NMR spectroscopy of the proton an affin- ity that can have various alkyl glycosides for resorcinarene 1 in an anhydrous organic environment, and the composition of the complexes “glycosides–resorcinarene 1”, always formed in the organic medium [30–33]. The results show that studied glyco- sides (mannoside, glucoside, and galactoside, xyloside) form with the resorcinarene 1 the complexes whose stability depends on the stereochemistry of glycoside OH groups [33]. This result confirms the results for the transport of studied glycosides. But unlike the environment in which we work (characterized by the presence of water in the organic phase), Aoyama et al. have studied complexes glycosides–resorcinarene 1 in an anhydrous organic environment, therefore, the parameters relating to the formation of these com- plexes have been modified [9]. Indeed, Aoyama found only in the presence of water, there may be training ternary complex involv- ing resorcinarene 1, sugar and water [8–10]. On the contrary, in an anhydrous organic medium, intramolecular links between gly- cosides are trained in addition to resorcinarene–glycoside links.

The stability constants that result are quite large about 10

⁴

M in the case of Octyl glucopyranoside complex, whose composition (resorcinarene/glucoside) equal to (1/4) with concentrations 1 mM for resorcinarene and 20 mM for octyl glucopyranoside [33]. These work on the extraction and transport of sugars, their derivatives and alditols, show clearly that the results differ greatly from an environ- ment in the presence of water to an anhydrous organic environment [11,12].

4.2. Comparison with transport data of the corresponding aldoses Because methyl aldopyranosides have a blocked structure by the OMe group, which replaces the OH group of anomere carbon of aldoses, and since the parameters of transport varies, with the structure of transported species, we are given to compare, in this paragraph, the results of transporting aldopyranosides with those obtained in the study of correspondent aldoses. This is done to determine the effect of the blocked structure by the OMe group on the parameters of transport through a supported liquid membrane.

The matching aldoses to the methyl aldopyranosides previously studied are: xylose, studied in the transport of aldoses by SLM [34], and the mannose, glucose and galactose studied under the same experimental conditions that methyl aldopyranosides. The permeability and flux of mannose, glucose and galactose through supported liquid membrane are determined following the example of previously established calculation [12]. In Table 4, we represent the permeability P and initial flux J

_i

of methyl aldopyranosides and corresponding aldoses to an initial concentration c

₀

equal to 0.1 M.

We note, according to the results of this table, there is a difference between the parameters on the transport of methyl aldopyranosides and those of correspondent aldoses. To determine the responsible factors for this difference between the transport of methyl aldopyranosides and corresponding aldoses, we will compare the stability constants of complexes (substrate–carrier) formed on interfaces of supported liquid membrane and the diffu- sion coefficients of these complexes through the organic phase of the membrane. All stability constants K and diffusion coefficients

Table 4

Permeabilities and initial fluxes for the transport of methyl aldopyranosides and their corresponding aldoses to an initial concentration c

0

= 0.10 M.

Carbohydrates 10

⁵a

(s

⁻¹

) 10

⁷P

(cm

²

s

⁻¹

) 10

⁵Ji

(mmol Cm

⁻²

s

⁻¹

)

Me-␤-d-Xylop. 8.78 14.11 2.240

Me-␤-d-Glucop. 8.53 13.70 2.185

Me-␣-d-Glucop. 7.31 11.74 1.860

Me-␤-d-Galactop. 4.00 6.43 1.020

Me-␣-d-Mannop. 7.19 11.56 1.835

Xylose (*) 6.53 10.49 1.665

Glucose 8.42 13.53 2.150

Galactose 8.11 13.04 2.070

Mannose 9.06 14.55 2.310

Same conditions as

Table 1, (*) Ref.[34].

D

^*

for the complexes of resorcinarene 1–methyl aldopyranosides and resorcinarene 1–correspondent aldoses, is presented in the following table.

The results of Table 5 show that resorcinarene 1–methyl aldopyranosides complexes have different stability constants and apparent coefficients of diffusion from those of resorcinarene 1–correspondent aldoses complexes. We note that, studied methyl aldopyranosides define two different series of stability constants (0.36 and 0.84 mol

⁻¹

L), while all correspondent aldoses with dif- ferent structures form with the resorcinarene 1 complexes whose same stability (0.20 mol

⁻¹

L) and whose apparent coefficients of diffusion are close and vary approximately between (6.10

⁻⁴

and 7.10

⁻⁴

cm

²

s

⁻¹

). So the parameters of transport through the SLM depend on the difference in structure between methyl aldopyrano- sides and corresponding aldoses.

Indeed, the comparison of permeabilities (Table 4) shows that the membrane is more permeable for aldoses than for correspond- ing methyl pyranosides. Only, xylose behaves differently, since it spends more slowly than other pyranosides. This can be explained by the absence of CH

₂

OH group in xylose, which makes it more lipophilic than other aldoses, therefore more soluble in the organic phase. For this reason, its passage may be slowed by its retention within the membrane.

The behavior of aldoses is very different from that of cor- respondent pyranosides. So, according to this study, we can actually compare the order of the change in the permeability of aldoses (manno. ≈ gluco. ≈ galacto. > xylo.) with that of correspon- dent methyl pyranosides (xylop. > glucop. > mannop. > >galactop.).

We note that when the galactopyranoside migrates very slowly through the SLM, galactose, is transported to the same speed as the mannose and glucose. It should be noted that the difference in structure between studied aldoses, causes a low change in perme- ability and apparent diffusion coefficients (Table 5).

A comparison of stabilities indicates that the methyl pyranoside complexes are stronger than the correspondent aldose complexes, and secondly, the all aldoses have the same stability constant

Table 5

Apparent coefficients of diffusion and stability constants of the complexes formed by the pyranosides and corresponding aldoses with the carrier

1.

Carbohydrates

K

(mol

⁻¹

L) 10

⁴D^*

(cm

²

s

⁻¹

) 10

⁻³

slope 10

⁻³

intercept

Me-␤-

d

-Xylop. 0.36 4.00 4.302 1.575

Me-␤-d-Glucop. 0.37 3.79 4.440 1.662

Me-␣-d-Glucop. 0.36 3.33 5.172 1.890

Me-␤-d-Galactop. 0.83 0.84 9.036 7.538

Me-␣-d-Mannop. 0.85 1.47 5.027 4.283

Xylose 0.19 5.70 5.920 1.037

Glucose 0.20 6.97 4.560 0.904

Galactose 0.21 6.31 4.730 0.999

Mannose 0.20 7.31 4.250 0.862

Same conditions as

Table 3.

Table 6

Permeabilities, stability constants and apparent diffusion coefficients of resorcinarene–methyl aldopyranoside and correspondent alditol complexes.

Carbohydrates 10

⁷P

(cm

²

s

⁻¹

)

K

(mol

⁻¹

L) 10

⁴D^*

(cm

²

s

⁻¹

)

Me-␤-d-Xylop. 14.11 0.36 4.00

Me-␤-d-Glucop. 13.70 0.37 4.00

Me-␣-d-Glucop. 11.74 0.36 3.79

Me-␤-d-Galactop. 6.43 0.83 0.84

Me-␣-d-Mannop. 11.56 0.85 1.47

Xylitol 8.38 0.46 1.91

Glucitol 7.11 1.48 0.55

Galactitol 5.22 1.53 0.39

Mannitol 10.22 1.46 0.89

Similar experimental conditions to

Tables 1–3.

(0.20 mol

⁻¹

L). The latter result is consistent with that of Aoyama on molecular recognition of sugars, where he showed that the galactose, xylose, mannose and glucose have the same affinity for resorcinarene 1 [9].

All these variations cannot be attributed solely to the introduc- tion of OCH

₃

group that represents the only difference between aldoses and corresponding pyranosides. The OCH

₃

group can have a double effect on the transport of methyl pyranosides. On the one hand, it promotes the solubility in organic phase and on the other hand, it removes any possibility of establishing a hydrogen bond with the carrier. Moreover, it blocks the structure of sugar in a sta- ble cyclic form. The difference of stability between pyranosides and correspondent aldoses may be interpreted by the probable involve- ment of the OH anomere group in the complexation of aldoses, as in the case of studied ribose by Aoyama et al. [9]. The same stabil- ity (0.20 mol

⁻¹

L) for studied aldoses complexes can be explained by the fact that monosaccharides are able to change configuration before reacting with the carrier. Hence, depending on the configu- ration ␣ or ␤ of anomere hydroxyl group, there is always creation, in each of studied aldoses, a cis site to be involved in the com- plexation. However, we showed that the cis site of pyranose cycle for the aldopyranosides form complexes with stability constant 0.84 mol

⁻¹

L, whereas in the case of aldoses, stability constant is fixed at around 0.20 mol

⁻¹

L. This difference may be explained by the presence of the lipophilic OCH

₃

group making pyranosides more soluble in the organic phase. Hence, the pyranosides can show more affinity those aldoses for resorcinarene 1. This explanation is very plausible, since the substitution of anomere OH group of glucose by a lipophilic octyl [O(CH2)

₇

(CH3)] group, increases very substan- tially the affinity for resorcinarene 1 [33].

4.3. Comparison with transport data of the corresponding alditols A comparison of permeabilities, stability constants and apparent diffusion coefficients of methyl aldopyranosides with correspon- dent alditol complexes, is presented by the results in Table 6.

According to the permeabilities and apparent diffusion coef- ficients grouped in Table 6, we note that the transport of methyl pyranosides is very different from that of correspon- dent alditols. Indeed, the order of changes in permeabilities of alditols (Man > Xyl > Glc > Gal) differs from that of aldopyranosides (Xylp > Glcp > Manp > Galp), except for the galactitol and galac- topyranoside whose permeabilities are low and almost identical.

Besides, pyranosides migrate faster than correspondent alditols.

They have apparent coefficients of diffusion higher than alditols.

It is likely that the cyclic form of these compounds facilitates the transport of methyl pyranosides compared to the corresponding alditols that exist in the open form. The comparison of stabil- ity constants, from Table 6 reveals that alditols are more strongly associated with resorcinarene 1 than corresponding methyl pyra- nosides. In fact, this result was predictable since the cyclic form

Fig. 4.

Evolution of the permeability of the SLM for each studied aldopyranosides depending on the initial concentration c

0

.

of aldopyranosides may impose an unfavorable orientation of OH groups belonging to the pyranose cycle, necessary for the reac- tion of complexation. When we compare the stability constants of methyl pyranosides and correspondents alditols, two important points needed:

1. The stability constant (0.36 mol

⁻¹

L), characteristic of a trans site of complexation for aldopyranosides, is adjacent to the stability constant (0.46 mol

⁻¹

L) of xylitol whose site of complexation is threo.

2. The stability constant (0.84 mol

⁻¹

L), characteristic of the cis site for the complexation of methyl pyranosides, takes a higher value (1.5 mol

⁻¹

L) for the erythro site on the complexation of alditols [23].

These results show clearly that threo and erythro sites in open forms correspond respectively to trans and cis sites in cyclic forms.

We can therefore conclude that alditols adopt the sickle form for react with the resorcinarene 1.

5. Discussion of results

The diagram in Fig. 4 includes the permeabilities of the prepared SLM for all studied methyl glycopyranosides. The comparison of these permeabilities at different concentrations gives the change order as follows:

P

(Me-␤-D-Xylop.)

> P

(Me-␤-D-Glucop.)

> P

(Me-␣-D-Glucop.)

> P

(Me-␣-D-Mannop.)

>> P

(Me-␤-D-galactop.)

We note that the permeability varies according to two factors:

the size and the configuration of carbohydrate. The size factor is highlighted by the permeability of xylopyranoside, which is higher than those of hexopyranosides. The configuration factor is clarified by comparing glycopyranosides of similar size. Fig. 5 represents the different conformations of studied methyl glycopyranosides

These results show that studied glycopyranosides are divided according to the OH group configurations of pyranose cycle in two series:

– Xylopyranoside and glucopyranoside whose OH groups of posi- tions 2, 3 and 4, are trans.

– Mannopyranoside and galactopyranoside who have two adjacent

diol sites with cis and trans configurations for OH groups.

Fig. 5.

The pyranose forms relating to different conformations of studied methyl aldopyranosides.

The comparison of the permeabilities of glucopyranoside ␣ and

␤ anomeres shows that ␤ anomere is carried a little more than

␣ anomere. However, the effect of anomerisation remains negligi- ble. According to the comparison of methyl glucopyranoside with methyl galactopyranoside, we deduce that the presence of cis con- figuration in the pyranose cycle, slowing transport.

The comparison of results for the two compounds, galactopyra- noside and mannopyranoside which are characterized respectively by the cis OH groups in positions 3, 4 and 2, 3, shows that the cis configuration of carbons 3 and 4 greatly slowing the migration of the methyl galactopyranoside through the SLM. This result can be explained by a greater affinity of the methyl galactopyranose for resorcinarene 1, this affinity is certainly due to the cis orientation of CH

₂

OH adjacent group to the cis site formed by OH groups in positions 3 and 4.

The determination of stability constants K and apparent dif- fusion coefficients D

^*

of resorcinarene 1–methyl aldopyranoside complexes, presented by the results of Table 3, can identify the fac- tors that control the reactions of complexation that occur at the interfaces of the membrane and make hypothesis about the nature of migration of these complexes across the SLM.

5.1. Variation of the stability

The experimental results obtained are in agreement with the mechanism of transport whose decisive step is the diffusion of the complex in the organic phase [12]. These results confirm the (1/1) stoichiometry of complexes which is similar to that of ribose com- plex studied by Aoyama in the CCl

4

[9]. The effect of size on the stability of the complexes is determined by comparing the xylopy- ranoside and glucopyranoside, which have the same configuration.

The stability of the complexes is the same for both glycopyrano- sides. As a result, the size has no effect on the stability of the complexes. The influence of anomerisation is determined by com- paring ␣ and ␤ methyl glucopyranoside anomeres. We note that the inversion of OCH

₃

group configuration does not change the stability of the complexes (K = 0.36 mol

⁻¹

L). We deduce that the stability of the complexes does not depend on the anomerisation.

The effect of configuration on the stability of the complexes is determined by comparing the results of aldopyranosides at differ- ent configurations:

• Methyl- ␤ - d -Glucop., methyl- ␣ - d -Glucop. and methyl- ␤ - d - Xylop., whose OH groups are trans, have the same stability (0.36 mol

⁻¹

L).

• Methyl- ␣ -d-Mannop. and methyl- ␤ -d-Galactop., who possess both a cis diol site and trans diol site have the same stability (0.84 mol

⁻¹

L).

For these complexes we see two types of stability constants:

• K = 0.36 mol

⁻¹

L is characteristic of the presence of a trans diol site on the pyranose cycle (2,3 or 3,4-trans OH groups).

• K = 0.84 mol

⁻¹

L is characteristic of the presence of a cis diol site on the pyranose cycle (2,3-cis OH groups for methyl Mannop. and 3,4-cis OH groups for methyl Galactop.).

These results show that the complexation site is a diol OH groups, which is consistent with studies conducted on sugar [8,10]

and alditol complexes [23]. We also note, that the complexes on a

cis diol site are more stable than those formed on a trans diol site

(0.84 mol

⁻¹

L > 0.36 mol

⁻¹

L). This conclusion is confirmed by the

results of Aoyama, which showed that the cis configuration of sugars has more affinity for resorcinarene 1 that trans configuration [9].

5.2. Variation of the apparent diffusion coefficient

According to the results of Table 3, we find that the values of apparent diffusion coefficients obtained (about 10

⁻⁴

cm

²

s

⁻¹

) for each studied methyl aldopyranoside are higher compared to liter- ature values (about 10

⁻⁶

cm

²

s

⁻¹

) [15,19,35–38]. The same remark was made for alditols and sugars [12,23,39,40]. To explain this phe- nomenon, we suggested in the transport of alditols [23], that the diffusion process is perhaps accompanied by a convection move- ment, which would be responsible for accelerating the diffusion of complexes in the membrane. While the hypothesis of “fixed-site jumping” proposed by Smith et al. in the transport of sugar through a plasticized membrane of triacetate cellulose [18–21,35–38,41–43]

is another suggestion, more likely to clarify this point. This the- ory assumes that the substrate moves by jumping from site to site, inside the membrane. In this case, the transport mechanism has the same kinetic profile as the facilitated transport, nevertheless, its characteristic is to be faster than the diffusion [21].

The pattern of the following figure shows the variation of appar- ent diffusion coefficients and stability constants for all studied complexes of resorcinarene 1–methyl aldopyranosides. As perme- ability, the diffusion of complexes is controlled by the size of glycopyranosides and configuration of their OH groups.

The size factor is considered by comparing the values of D

^*

for methyl- ␤ - d -Xylop. with methyl- ␤ - d -Glucop (D

^*

= 4 × 10

⁻⁴

and 3.79 × 10

⁻⁴

cm

²

s

⁻¹

respectively). According to these values, we find that methyl xylopyranoside diffuse faster than methyl hex- opyranosides. Nevertheless, the effect of size on the diffusion of these aldopyranoses through the SLM remains negligible. As for permeability, the effect of anomerisation on the diffusion through the SLM is very low. Indeed, ␣ and ␤ methyl glucopyranoside anomeres migrate almost the same speed (D

^*

= 3.33 × 10

⁻⁴

and 3.79 × 10

⁻⁴

cm

²

s

⁻¹

respectively).

The results show that the orientation and the position of OH groups belonging to the pyranose cycle, affect the diffusion of the complexes through the SLM. Indeed, the glycopyranosides with trans configuration OH groups migrate faster than glycopyranosides with cis configuration OH groups ( D

^∗(methyl Glucop.)

= 3 . 7 × 10

⁻⁴

>

D

^∗(methyl Mannop.)

= 1 × 10

⁻⁴

cm

²

s

⁻¹

).

The presence of a cis diol site decreases widely the diffusion of these complexes. Just as permeability, low diffusion through the SLM (0.836 × 10

⁻⁴

< 1.253 × 10

⁻⁴

cm

²

s

⁻¹

) of methyl galactopyra-

Fig. 6.

Apparent diffusion coefficients and stability constants for all studied com- plexes of resorcinarene

1–methyl aldopyranosides.

noside compared to methyl mannopyranoside is certainly due to the cis orientation of CH

₂

OH adjacent group to the cis site formed by OH groups in positions 3 and 4, this favorable orientation increases the affinity of methyl galactopyranoside for the resorcinarene 1, which is reflected in slow diffusion of this compound, even if sta- bility does not vary between a cis diol sites of OH groups in positions 3,4 or 2,3.

These results indicate that the transport of methyl aldopyrano- sides through the SLM is similar to the transport of alditols [23];

permeability varies with the concentration of substrate according to a saturation law [12]. We also showed that the diffusion coeffi- cients, such as permeabilities, vary in the same direction depending on the size and configuration of substrate. Indeed, we found that the cis site form a more stable complex than the trans site. However, methyl glycopyranosides of cis configuration migrate more slowly than those of trans configuration.

5.3. Relations between stability constant and diffusion coefficient The comparison factors (stability constants and diffusion coeffi- cients) of Fig. 6 show that the less stable complexes migrate rapidly through the membrane. As a result, the stability constants vary inversely in relation to the apparent diffusion coefficients. This result suggests the hypothesis of “fixed-site jumping” proposed by Smith [21] or the transport of substrate is by skipping one carrier to another, is very likely. Indeed, when the stability of the complexes is low, dissociation constant is large; therefore, the passage of sub- strates from one carrier to another is easy. The apparent diffusion coefficients in this case are larger than those observed in the case of a simple diffusion [12,23,39,40]. We calculated the product KD

^*

for each aldopyranoside. We note that the value is almost constant (about 10

⁻⁴

), varies very little and does not depend on the size or configuration of studied methyl glycopyranosides.

To illustrate the relationship between the stability constant K and the coefficient D

^*

, we studied the evolution of diffusion coeffi- cients D

^*

depending dissociation constants 1/Kon the resorcinarene 1–methyl glycopyranoside complexes. We are seeing an almost linear relationship between the kinetic factors D

^*

and the ther- modynamic constants 1/K on the complexes formed in the organic phase of the SLM.

The study of all results shows that the migration of complexes (carrier–substrate) through the organic phase of the membrane depends on the stability of these complexes, and low stability (small K), results in a large diffusion (high D

^*

) through the membrane and a high permeability of the used SLM for different transported methyl glycopyranosides. However, we can propose three types of move- ments associated with the migration of these complexes through the organic phase of the SLM:

(1) Movement of pure diffusion (D

^*

order 10

⁻⁶

) for a very stable complexes in organic phase.

(2) Composed movement (diffusion + jump from site to site) for unstable complexes in organic phase of the SLM (D

^*

order 10

⁻⁵

).

(3) Movement characterized by the displacement of the substrate by jumping from site to site (too high D

^*

, at around 10

⁻⁴

). This hypothesis of “fixed-site jumping” advanced by Smith for the transport of sugars [21], explains the rapid migration through the organic phase of the SLM for complexes with low stability.

6. Conclusion

The system we have used is a PTFE membrane impregnated

with resorcinarene 1 dissolved in the CCl

4

; the SLM has been

used previously for the transport of sugars [12]. We conducted its

development in incorporating water in the membrane (12 h) before

starting the experiences of transport. This mode has improved the accuracy and reproducibility of our results and reduces the time of the total transport for each of the studied compounds at 7 h only;

this reflects the high permeability for this type of membrane. On the other hand, our system has presented a good stability and the same membrane has been used for 15 days with the same perfor- mances. From all results, we showed that the rate-determining step of the transport mechanism for these compounds is the migration of (1/1) carrier–substrate complexes, formed in the organic phase of the membrane between the carrier (resorcinarene 1) and each of the transported methyl aldopyranosides. This result was deducted from kinetic studies on the transport of these compounds. In addition, the recent studies conducted by Verchère and co-workers [44–46]

with similar membranes for the transport of carbohydrates, con- firm these results and show that the use of carriers with specific configurations, is probably a solution to achieve selective transport of some compounds from mixtures. The established model for this transport mechanism has been verified. The parameters, flux; sta- bility constant and apparent diffusion coefficient of each complex, were determined. Next, we showed that the selectivity of transport is probably linked to the evolution of stability constants and appar- ent diffusion coefficients of “carrier–substrate” complexes, which depends on the structure of the transported compounds. Indeed, it is clear that the factors that control the selectivity depend on the nature and the configuration of transported compounds. For studied glycopyranosides, we have shown that the flux depends on the size and the configuration. The xylopyranoside migrates faster than hexopyranosides and aldopyranosides with trans configura- tion migrate faster than with cis configuration of OH groups. The stability of resorcinarene 1–substrate complexes formed in organic phase of the SLM, depends only on the configuration of transported glycopyranosides, as the cis diol site of OH groups on a pyranose cycle, form more stable complexes than the trans diol site of the same cycle. The analysis of results by comparison, on the transport of aldopyranosides and correspondent aldoses and alditols, show that the cycle forms are better carried than corresponding linear forms. The presence of the OCH

₃

group in pyranosides, can obtain more stable “carrier–substrate” complexes, hence a lower perme- ability compared to the corresponding aldoses. The phenomenon of anomerisation for studied methyl glucopyranosides ␣ or ␤ have no effect on the complexation of these compounds by resorcinarene carrier. However, by combining the results for studied methyl gly- copyranosides and correspondent alditols, we can conclude that the chelation site is a diol OH groups and the complexation of alditols implies an erythro site with an identical stability constant almost to the order of 1.5 mol

⁻¹

L, indicating that the alditols start in the sickle form to react with resorcinarene 1, while, aldopyra- nosides react with the same carrier according to a cis diol site or a trans diol site OH groups of pyranose cycle, to form two types of complexes with two different stabilities (0.84 or 0.36 mol

⁻¹

L). The correspondent aldoses form with the resorcinarene 1 complexes of the same type, with an identical stability (about 0.20 mol

⁻¹

L), this low stability compared to the corresponding aldopyranosides, explains the high permeability of the SLM for these compounds.

For this same stability, it is probably that the site of complexation is an identical diol site for all correspondent aldoses. The rear- rangement of the anomere OH group is necessary for each of these compounds to form with the adjacent OH group this identical same diol site.

Finally, the results of transport, clearly demonstrated this inverse relationship between apparent diffusion coefficients and stability constants of complexes in organic phase, which supports the hypothesis of diffusion by jumping inside of the membrane and helps explain these high values of diffusion coefficients and this great permeability of this prepared SLM for the facilitated transport of these compounds derived from sugars.

Nomenclature

a slope of the plot − ln(c

₀

− 2c

_R

) = f(t)

c

₀

initial concentration of aldopyranoside in the feed phase (mol L

⁻¹

)

c

R

concentration of aldopyranoside in the receiving phase (mol L

⁻¹

)

c

F

concentration of aldopyranoside in the feed phase (mol L

⁻¹

)

P the permeability of studied aldopyranoside (cm

²

s

⁻¹

)

J

_i

initial flux of aldopyranoside (mmol cm

⁻²

s

⁻¹

) D

^*

apparent diffusion coefficient of the complex CS

(cm

²

s

⁻¹

)

K stability constant of the complex CS l the membrane thickness (mm or ␮ m) S the membrane area (cm

²

)

[C]

0

concentration of carrier in the membrane (mol L

⁻¹

) [CS] concentration of the complex in the organic phase

(mol L

⁻¹

)

T temperature (K or

^◦

C)

t time (s)

V volume of the receiving compartment (cm

³

)

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