Glycolipid self-assembly: micellar structure

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Glycolipid self-assembly: micellar structure

Marie-Christine Cecutti, Bonaventura Focher, Bruno Perly, Thomas Zemb

To cite this version:

Marie-Christine Cecutti, Bonaventura Focher, Bruno Perly, Thomas Zemb.

Glycolipid

self-assembly: micellar structure. Langmuir, American Chemical Society, 1991, 7 (11), pp.2580-2585.

�10.1021/la00059a031�. �hal-02094679�

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This is an author’s version published in:

http://oatao.univ-toulouse.fr/23551

To cite this version:

Cecutti, Marie-Christine

and Focher, Bonaventura and Perly, Bruno and

Zemb, Thomas Glycolipid self-assembly: micellar structure. (1991)

Langmuir, 7 (11). 2580-2585. ISSN 0743-7463

1991, 7,

Glycolipid

Self-Assembly:

Micellar

Structure

Christine

_Cecutti,*-*

Bonaventura

Focher,*

Bruno

Perly,8

and Thomas

Zemb§

Ecole

Nationale

Supérieure

de

Chimie

de Toulouse, 118 route de

_Narbonne,

F.31077

Toulouse

cedex,

France,

C.N.R., P.zza

L.

da

Vinci,

1.20133

_Milano,

_Italy,

and

CEA-CEN

de _Saclay,

Service de

Chimie Moléculaire,

F.91191

_Gif

sur

Yvette

_cedex,

France

Small-angle scattering is used _{to investigate} a

_{typical glycolipid}

micelle structure

in

_conjunction

with

NMR determination of

sugar cycle conformation.

It

is shown

that

the ellipsoidal shape

of

the micelle

originates

from

two constraints: sugar rings perpendicular to the interface induce a

limited

area

at

the

chain-head interface. Together

with

the

bulky

hydrated heads,

this

imposes an _{ellipsoidal shape.}

Introduction

Single-chain surfactants

are

_{usually classified}

into

two

families: ionic

and

nonionic

molecules.

In

the

phase

diagram,

a

_{diluted, optically}

_isotropic

fluid

micellar

_phase

Li

exists

for

both types

of

surfactants.

The

micellar

structure,

i.e.,

the number

of

water

molecules

bound per

headgroup,

the

radius

of

the

micellar hydrophobic

core,

and

the

area _per

surfactant

_head, is

determined by

geometric

constraints1 and

_{by the}

balancebetween

head-group

repulsion and

hydrophobic effects

between

_apolar

chains.

In

the

case

of

most

ionic single-chain

molecules

without

added

salt, the

sizeand

structure

of

the_aggregates

in

micellar

phaseis

_{roughly independent}

of

concentration

and temperature at

any

point

of the

phase

diagram

far

from the

_phase

limits.

Nonionic

micelles

exhibit

a_sphere

to rod

transition

in binary solutions.

At

_increasing

tem-perature, the lower

consolute

_point

is due

to

attractive

interactions

appearing between

nonionic

headgroups when

the

interfacial

radius

of

curvature

decreases.2

_Typical

values

of

the_physical

_quantities

_{describing these two types}

of

micelles,

including the

microstructural

parameters,are

given

in

Table

I for

two

classical examples: 2% SDS

in

D2O

and

5% C12E5

in

D2O

at

room

_temperature.

Weexamine here

the

case

of

a

_biologically

_important

molecule, /3-dodecyl maltoside.

This

isa

nonionic

molecule

with

an

_extremely

_large

_hydrophilic

_{headgroup. We}

first

want to

assess

the following

_questions: as

the

_large

head-group

gives rise

to

a_large

sterical repulsive term,

will this

be

sufficient

to

induce

the

characteristic structure

of

ionic

micelles,

with

some

_{particularities}

suchas

the

absence

of

a

salt

effect

due

to the

absence

of

counterions?

On

the

other

hand,

will

glycolipid

micelles present thesame _phase

behavior

and

microstructure

as

other nonionic

_systems?

Our aim

in

this work

is

to

_identify

the

dominant

features

of

glycolipid

self-assembly

in

the micellar

state using

/3-dodecyl

maltoside

(/3C12M) as a

typical

molecule.

/3C12M presentsa_{large and}

flexible headgroup

made

_by

two

_{sugar rings.}

_{Therefore, after}

identification of

the

phases

in

a

_binary

concentration

and

_temperature

_phase

f_{EcoleNationaleSupérieuredeChimiedeToulouse,}

l_C.N.R.

«CEA-CENdeSaclay.

(1)Israelachvili,J.N.;Mitchell,D. J.;Ninham, B. W. J.Chem. Soc.,

Faraday Trans.2 1976, 72, 1525.

(2)Mitchell,D. J.;Tiddy,G. J.;Waring, L.; Bostock, T.; McDonald, . P.J.Chem. Soc.,Faraday Trans1 _{1983, 79, 975.}

Table I.

_Typical

Molecular Parameters

for

theIonic

Surfactant

Sodium Dodecyl Sulfate(SDS),theNonionic

Polyethylene(C12E5), and0-Dodecyl Maltoside(¡9C12M)

SDS C12E5 /SC12M

Vmoi,nm3 0.380 0.620 0.691

headgroupareas a, nm2 0.70 0.35 0.50

headgroupareas

',

nm2 0.60 0.35

chain lengthl,nm 1.8 1.8 1.8

packingparameter,Vmoi/ 0.3 1.0 0.75

volumeofheadgroup,nm3 0.060 0.300 0.376

diagram,

we

studied the

_sugar

_ring

conformation by

NMR

andthe

micellar

_{structure by X-ray}

and

neutron

_scattering.

The

same

methods

have been used

for

other

surfac-tants

with

a

_bulky

nonionic

headgroup.3·4

With

a

mixture

of

lipopolysaccharides isolated

from Escherichia

coli,

with

an _average

of

four sugar rings per molecule,36long

cylinders,

similar

to

those

obtained

with

diheptanoylphosphatidyl-choline®

with

a

_{pH-dependent length}

havebeenevidenced.

Nonspherical

micelles have been

obtained

with

the

gan-glioside

GM1.4

Materials

and

Methods

(1)

Scattering

Techniques. X-raysmall-anglescattering

ofisotropicphases has beenperformedontheD24_{double-crystal}

diffractometerin Lure(Orsay),usinga_wavelengthof0.122nm

anda_sampletodetector distanceof59.3cm. Theresultswere

identicalusing deuterated andprotonatedoctaneassolvent.The

obtainedqrangewas qmin= 0.03 nm"1andq,m= 6 nm"1; i.e.,the

resolutionwas 2v/qmMX= l _nm and_{nolong-rangeorderinghigher}

than_2ir/qmill = 200 _{nm was} considered. The absolute scaling

was _{made by comparison}

with

the isotropiccoherent_scattering®

ofa_sampleof1.5-mmthicknessofwater between25-#tm

thick

Mylarsheets. Dueto water_{compressibility,}eachwater molecule

givesthesame uniform_scatteringas6.35_independentelectrons.

Thistype of normalization,versus a_veryweakbutconstantsignal,

ensures

that

_{the setup}isperfectlyaligned and

that

background correctionis_successfullyachieved.7 Neutronsmall-angle

scat-teringwas_performedon thePACE_{setup at}

LLB

_{Orphée, using}

0.5-1-nm wavelengthswithtwodifferentwavelengths,whichgives

a_qrangeof_qm¡„= 0.08 nm"1to = ₃nm"1. Absolute scaling

(3)Hayter,J. B.;Rivera,M.; McGroarty,E. J.J. Biol.Chem. 1987,

262, 5100.

(4)Cantu,L.; Corti, M.; Degiorgio,V.; Piazza, R.;Rennie,A. Prog.

Colloid Polym.Sci. 1988,76, 216.

(5)Lin,T. L.;Chen,S._{H.; Gabriel, N.E.;Roberts, M.} F.J.Phys.

Chem. 1987,91,406.

(6)Zemb,T.; Charpin,P.J.Phys. 1985,46,249.

(7)Levelut,A.M.Science Phys. Thesis, Orsay, 1968;p334.

was _{made using the incoherent scattering}

of

water.8 _Lyotropic

liquid-crystal identification was _{done using} a Guinier camera

equipped with a linear _{detector, giving} a _qm¡„ = 0.2 nm"1 to q—

= _{8 nm"1.} The _lyotropic

liquid

_{crystals were} identified _{by peak}

spacing. After identification of the symmetry, the area per molecule was derived from measurement of the molecular volumes

by densitometry.

The small-angle scattering of micellar solutions was

first

checked by the measurement of the invariant Q*, which is given

in a _system

with

two scattering length densities bi and b%, with

the volumes

and <fo:9

Q* =

2 ( 1

-b2)% 2= dq

This expression is valid whatever the microstructure of the

solution, isolatedspheres, connected cylinders,or random

bi-layers;10theonlyunderlying assumptionisthat itisatwo-medium

structure. Then, theshapeofthemicelleshastobedetermined

by comparing thescatteringI(q)obtainedwiththe scatteringof

modelstructures. Since thepresenceorabsenceof intermicellar

attractionor _repulsionisnot knowna_{priori, determination of}

the radiusof gyrationhasno _{physical meaning.} The

determi-nationofthemicellar_shapecan _{only rely}on a_{complete calculation} on an absolutescaleofthe scatteringcurve for differentideal

modelshapesofthe micelle. Fortunately, in binarysolutions,

micellarradiiand scattering lengthsare_{known; they}are_imposed

by chemicalcomposition and molecular volumes. There isno

free parameter adjustmentexcept aggregation number, itself

relatedtothearea _{per headgroup,}once the generalshapeofthe

aggregate has been chosen,as _{explained below.}

Our aimis_{to compare theshape}withthe surfactant parameter

p,thelater_{being deduced}fromsterical considerations.

Usually, the volume of the polar headgroups Vp is small

comparedto

that

ofthehydrophobicchains Vc- Thepacking

parameterp isthen definedas10

P= Vool/6.1=

(Vp+Vc)/6.1

= _Vc/6.1

In

the presentcase,the packingofthe molecule requirestaking intoaccountthe whole molecular volume to evaluate to

surfac-tant parameter. The length ofthe molecule is now the total

length(TableI). Using the

first

definition,

(=

18_{A and p}= 0.33

whileusing the totallength (l = 24A) yieldsalsop = 0.33.

Model

of Spherical Micelles.

Wesupposehere

that

the

glycolipid molecules packinto sphericaldroplets

with

hydro-phobic chains inside: one _{singlequantity,}the interfacialarea

per moleculea,determinesthe whole scattering spectrum:

I(q)= _P(q)S(q)

The structure factor S(q) is

first

taken for_{independent hard}

spheres, neglecting other types of interaction. S(q) can be

calculated analyticallyat anyq withgood precision when the radius R

of

the hard core ofmicelles, thedensity n (cm-3) of micelles, and thetotalvolumefraction ofmicellesare known.11 Thesethree_quantitiescan be_{evaluated by molecular parameters}

once thearea _{per molecule} isfixed.

(a)Thesurfaceofmicelles imposesarelationbetweenNand R:

4 R2=

where

N

isthe aggregation number and the single adjustable

parameter, which hasto bebetween 2.5and 7 nm forsterical reasons. _Usually, isdefined atthemicelle-solventinterface.

For typical surfactantssuchasSDS, aisca. 70_A2/molecule.

It

doesnotmake any differencein thiscasetodefine atthe

chain-headgroup interfacesincethe volumeofthe headgroupisonly

10% ofthetotal surfactantmolecular volume. For glycolipids,

however, the sugarrings represent themajor part ofthe

mo-lecular volume. Wethereforedifferentiatebetweenthe

micelle-solventinterface andthe_hydrophobiccore-sugar headgroup

interface '.

(8)Jacrot,B.; Zaccai,G._Biopolymers1981,20, 2413.

(9)PorodG.InSmall AngleX-rayScattering·,Clatter, Krakty,Eds.;

Springer: Berlin,1982.

(10)Duplessix,R.; Cabane,B.;Zemb,T.J. Phys. 1985, 46, 2161.

(11)Hansen, J. P.;Hayter,J.B.Mol.Phys.1982,46, 651.

(b) The micellar volume imposes another relation between

N

andR:

*/3*R3= _NVaol

where Vmoiisthe known molecular volume, measured by

den-sitometryusing theAntonPaar high-precision densitometer.

Aggregation number

N

andmicellarradiusRare now fixed:

N

= 4*R2/a R= _3Vmol/

Thearea _{per molecule}atthe core-headgroupinterfaceisnow

fixed.

If

the_{shaperemains spherical, the radius}

of

the sphere

includingthehydrophobicchainsR'isdefined as

4*R'2=

NS

i/3*R'3

= _NVdu¡n

Therecan bea conflictbetween the values of and '. The

aggregate shape, whennonspherical,isan_{energy-effective packing}

solution tosolvethis conflict.

Theexcluded-volume fraction ofthehydrated micelle is

alsodeducedfromsterical considerations. Thevolumefraction ofthe micelleis_{given by}

N

molecular volumesincludingh

-10water molecules bysurfactantinside_{the hard-sphere volume.}

Thevolumefraction ofthe_{dispersedphase is}therefore known

a

_priori

when isfixed:

= _(Vmol+

hx 30)Mi

Sincethe volumeofone_{single water molecule}is 30A3, atwo-step modelofthemicelleissufficient_{to calculate the form factor P(q)}

in thisqrange:10 wesupposea_hydrophobiccore ofradiusR3and

a_{hydratedheadgroup concentric shell}ofradiusR3. Theinternal

sphereofradiusRicontains_onlythe

N

_{hydrophobic chains.}The

concentric shell betweenRiandR3contains

N

headgroups and

hN

watermolecules. Sincethe molecular volumes and scattering

lengthdensitiesare known, the valuesofRiandR3aswellasthe

contrastisknownonce

N

and hare fixed, hisimposedby the

simulation at high volume fraction. As usual, we make the

assumption

that

hisnotconcentrationdependent. Wefound

h= 10_{an acceptable value}forthiswhole study. The scattering

lengthdensities (electronic densities) are therefore calculated

numericallyforboth X-rayand neutron-scattering densitieswith

thesame _parmaters

N

andh:e

P(q)=

(£(*>,

+ 1-

b^/^fiqR,))2

where

f(x)

= 3(sin_x- xcosx)/x3

If

thestructureisspherical,both X-rayand_{neutron-scattering}

spectracan bereproducedon an absolutescale

with

thissingle parameter . When the calculated scattering cannotbe

fitted

to

the observedone _byvarying , at leastone ofthetwo_underlying

assumptions, i.e., (I) the only interaction between droplets is hard-sphererepulsion, and

(II)

micellesare _spherical,hasto be

modified. Fordouble-chain surfactants,we_{have recently shown}

ina similarcase

that

_assumption

II

is wrong.12

In

thecase

of

_{glycolipids, the}absenceof critical points inthe

phasediagram andno effect of temperature of_{scattering data}

shows

that

it

is also merely assumption

II

which has to be

modified: theshape isnotspherical. We therefore turn now to

thecalculationofindependentcylindricalmicelles. An

infinite

flexiblecylinderis easilydetected byaq-1decayofthescattering.

In thiscase,the scatteringisvery intense at lowqand

it

isgiven

by®

P(q)= _(C-

C^r/qN^B

-bKlnJ2

exp(-W/2)

Where

M

isthe aggregationnumberper

unit

micellarlength,C

the concentration,

Cj*

thecritical micellar concentration,and

5the averagescatteringlength density intheaggregate. The

relevant plotlogIq2versus _logqdoesnot showalinearbehavior

inthe presentcase.

(12)Barnes,I.S.;Hyde,S._{T.;Ninham, B. W.; Derian,}P. J.;Drifford,

2582

Langmuir,

Vol. 7, No. 11, 1991

Cecutti

et al.

Wedonotsee_{such signals}inour measurements,so wecalculate

thecase offinite_cylinders.13 Other_{shapes(bilayers,}

flat

_disks,

etc.)have been_considered,butthe scatteringofthemdoesnot correspondeither on an absolutescaleor _{qualitatively} to the

observedshape.14

Model

of

Finite Cylinders.

Weapproximate finite

sphero-cylinders madeof hydrophobiccore coatedwitha_headgroup

shell byan_{elongatedellipsoid(Figure}4). Now, two parameters

are _{required to}calculatethe whole scatteringcurve for X-ray

and neutron scattering:thearea _persurfactantheadgroup and the

ellipiticity

e. The S(q) termissetto1becauseno theoretical

calculationofS(q)exists forthiscaseandonedoesnotseesensible

decreaseofthe scattering at lowq,whichisthe usualindication

ofsterical hindrance. P(q)iscalculatedfora_prolateellipsoid of

_ellipticity

e;theform factorisgiven by

P(9)=

n(£(6i+1

-b;)2“/a*!?,·3/2X

_

(qRfVcos2 + e2sin2

)

cos d<?)

Thepresenceofthe

_ellipticity

parametereallowssolution ofthe

conflictbetweenthe surface

',

which includesall hydrophobic

tails,and , includingthe wholesurfactantmoleculeaswellas

hydrationwater.

Polydispersity.

Using small-angle scattering, there is no

distinctionon a setof_{scattering data between}size andmass

polydispersity oftheaggregates.16

But

thereisadirect_wayof

determinationofthe variance ofthemass distributionusing

mass action law and the _averagemass variation with

concen-tration:

2=

6 /(

In

(*T-*!))

where is the total surfactant molar fraction, X\ the free

monomer molefraction,and

N

_{the aggregation number.}

Measurement ofmean _{aggregation number}

N

for different

molarfractionsallows thereforethedetermination ofthemass

polydispersity ofthe micelles.

(2)

NMR

Techniques.

NMR

experimentswere _performed

inthe micellar and_{liquid-crystalline}statesto_{getinsight into}the local organizationwithspecialattention toward the local

con-formation of_{the polar}head.

NMR in

the

Micellar

State.

All

experimentswere performed at310Kusinga24mM solution in deuteriumoxide andaBroker

WM500spectrometeroperatingat500.13

MHz

forprotons.

In

a

first

_stage,all_{signalsarising}from nonlabile_protonshave

tobeassignedusing two-dimensionalCOSYand_multistepRelay

experiments.16 This is facilitated by the fact

that

anomeric

protonsfromthe twoglucoseunitsare _clearlyidentifiedon the

spectrum owingtotheirspecific chemicalshiftsandtocoupling constants relatedtolocal anomeric characters.

NOE experiments were then carried out to derive _spatial

proximitybetween_protonsandtomodelthe corresponding overall

average conformation oftheglycolipid molecule.

NMR in

the

Liquid-Crystalline

State. Whenliquidcrystals

are considered, deuterium

NMR

offers the most _powerful

approach to local molecular order and conformation. Forthis purpose,adeuterium-labeled glycolipidwas _preparedasfollows.

Laurieacidwas reducedto the corresponding alcohol using

Li-A1D<andthe obtained

, '-deuterated

dodecanolwas _graftedto

activated maltose using theclassical_{procedures. A}95%labeling

ofthe

first

carbonofthealiphaticchainwas thus achieved. For

NMR

experiments, all samples were _prepared in

deuterium-depleted water (CEA)afterfreeze-dryingofthesolid_glycolipid

from thissolvent. Thisensures that isotropiclines observedin

theforthcoming deuteriumspectraarisefromthe labels and not

fromresidualdeuteriumfromthe water.

All

experimentswere

performed at 310

K

using a Broker MSL300 spectrometer

(13)Hjelm,R. P.,Jr. J._{Appl.Crystallogr.}1985, 18, 452.

(14)Porte,G.J.Phys. Chem. 1983,87, 3541.

(15)Hayter,J.B.InProceedings of theXCCorsoInternationalSchool

ofPhysics·,Degiorgio, V., Corti, M.,Eds.;NorthHolland: Amsterdam,

1985.

(16)Berthault,P.; Bossenec,V.;Perly, B. Analusis, 1990,18,184.

Figure

1. 2HNMR_{spectrum of}/3-dodecylmaltoside in the

liquid-crystallinestate.

Figure

2. Small-angle neutron scattering observed for a6%

solution of d-Cl2M inDzOcompared to the best possiblefits for

spherical micelles

(---)

withafixedarea _{per molecule (</}=

0.50 nm2) andshort ellipsoids (e = _1.2;

—).

operating at 46 MHz for deuterium. The quadrupolar-echo

sequence17 was usedto avoidphasedistorsions and signallosses.

Results

(1) Phase

Diagram.

The

phase

diagram

presents

only

two

regions: an

_isotropic

fluid

micellar

_{phase exists}

_{up to}

50%

_(w/w)

in

water, and

at higher concentration,

aviscous

birefringent

lamellar

phase is

obtained.

The nature

of

thesephases isnot

affected by temperature up to

353

K.

The

_periodicity

D*

and

the thickness 2t

of

the

_bilayer

are

measured by

small-angle

X-ray

scattering at 50%:

2t= 2.9_nm

D*

= _5.3nm

The

area

_{per surfactant headgroup calculated}

with

these

valuesis

therefore

= 0.57

_{nm2/molecule.}

In

the

_case

of

lamellar packing,

a =

'.

A typical NMR

spectrum

in

the

liquid-crystalline

state

is

_{displayed in Figure}

1.

The corresponding 11.4-KHz

quadrupolar

splitting

indicates high molecular ordering

at

_{the corresponding}

carbon, and

the overall

shape

of

the

spectrum

is

_{typical of axially}

_{symmetric bidimensional}

structures. The sharper central line

corresponds

to

iso-tropically tumbling

micelles

in

thermodynamical

equi-librium with

the

_liquid

_crystals.

(2)

Structure of Micelles.

We

_study

now an _aqueous

solution

of

j8-dodecyl

maltoside

(6%

w/v)

at 310

K:

Neutron

and

X-ray

scattering

curves are _compared on

(17)Davis,J.H.; Jeffrey, K.R.;Bloom, M.; Valic, . I.;Higgs,T.P.

Glycolipid

Self-Assembly:

Micellar

Structure

Langmuir,

Vol. 7,No. 11, 1991 2583

TableII.

Structural

Parameters Describing the 0-Dodecyl

MaltosideMicelleat 6%

(w/w) in

Waterat 310

K

Figure

3. Small-angleX-rayscatteringofthesample described

in Figure2_comparedto_{the scattering}of_{perfectspheres}

(---)

andshortellipsoids

(—).

Thesame numericalvalues (areas,

molecular volumes, aggregation numbers)were usedtosimulate

X-ray

andneutron-scatteringspectra.

Figure

4. Schematicviewofthemicellaraggregate.

and absolute

scale (cm-1)

to calculated model

curves

for

spheres

and cylinders (Figures

2

and

3).

The

best

result

is

obtained

with

the

short cylinder model (Figure

4),

for

both neutron and

X-ray

techniques,

taking

the

following

parameter

values:

v - 86 A2 = 51A2

which

gives an _aggregation

number

N

= 82.

Micellar

structural

parameters

are

_reported

in

Table

II.

A

_good

agreement

is

not

obtained

_{from pure spherical}

micelles. The short ellipsoid model

fits

better

on

the

absolute

intensity

scale.

The

best agreement is

obtained

with

an

ellipticity

e= _1.2.

The resulting scattering length

density

is

given

in

Figure

4.

(3)

Polar

Head

Conformation.

Anomeric protons

were hence used as

_starting

_points

for

the complete

assignment

of all

other

signals.

A

typical contour

plot

ofaCOSY

experiment

isdisplayed

in Figure

5.

The corresponding coupling constants

were

derived

further from

computer-assisted spectral

simula-tion. The relevant parameters

are

_reported

in

Table

III.

Ri totalshort radiusofthe micelle 2.4nm

Ri short radiusofthe apolarhydrophobiccore 1.8 nm

N aggregationnumber 82

h water_{molecules per}surfactantmolecule 10

e _ellipticityofthemicelle 1.2

a areapersurfactanthead atthe

water-micelle interface 0.87nm2

(ri area per surfactanthead atthe

chain-headgroup interface 0.50 nm2

61 electronic density ofthemicellarcore 285emn'3

6V electronic density ofthe sugar

headgroupregion 433enm"3

bi scattering length density ofthemicellarcore -0.4X1010cm"*

bi scattering length densityofthe sugar

headgroupregion 3.8 X1010cm'2

TableIII. Chemical

Shifts

andCoupling Constants of

All

Protons of the d-Dodecyl Maltoside Polar Head

proton chemshift, ppm couplingconst,Hz

Hi

Ha H3 H4 h6 He-Hg'

H'i

H'a H's H'4 H'6 H'g-H'g-TableIV. 5.30 3.60 3.65 3.40 3.65 3.80 4.35 3.35 3.70 3.65 3.45 3.80 Ji-2— 4.2 </a-3= 8.7 J3-4= 10.0 J*.g= 9.4 «/fr4= _4.0,Jis-g'= -12.4 J'1-2= _8.3

J'

2-3 = 8.5 =/'3-4= _9.0 c/'4-5= 8.5 t/'g-g'= -12.4

Structure of the Micelles

for

Different

Surfactant Concentrations* concn (w/w),% aggregationno. N Ri Ri e 1 120 0.014 21 36 1.2 4 115 0.056 21 36 1.2 6 82 0.084 18 24 1.2

0_{eistheellipticityfor which thebest}fit_{was achieved. R\and}

Riare the hydrophobiccore radius (short _{axis) and}the external

radius, respectively. The excluded-volume fraction includes 10

watermolecules per headgroup.

A

two-dimensional

NOESY experiment

was

_performed

with

150-ms

_mixing

time.

The corresponding contour

plot

is

_displayed

in

_Figure

6

and

showsa

number

of

structure-related

cross-peaks.

More

accurate

_quantitative

datawere

then obtained by NOE experiments using the

buildup

technique. NOE

buildup

rateswere

obtained at variable

transfer times, and

theserates

could

then

be

converted

into

effective

_interproton

distances.

A

complete

ratio-nalization

of all

data

was achieved using a

molecular

modeling

program,18

allowing proposal

of

a

model

for

a

single

glycolipid

molecule in

the micelle.

In

these

calcu-lations,

a

normal

4C1

chair

conformation

was used

for

both

sugar

units

in

agreement

with

coupling

constants.

_Only

the interglycosidic and the glycosidic

bondswere

consid-ered

for local rotation. The relevant molecular structure

is

depicted

in

Figure

7.

Discussion

The extended chain length for the

/3-dodecyl

maltoside

isLc= 1.8 nm;10

The apolar volume

of

this chain

is _Vc=

0.315nm3.

_{Therefore, the maximum}

_aggregation

number

of

a

_{spherical apolar}

core

without

a hole

at

the center

corresponds

to

N

= _82,when

Ri

= _Lcand = 0.50 nm2.

(18)Langlet,G. 44émeRéunionInternationaledemodélisationdee

structures et propriétés moléculairesenchimie physique et biophysique,

2584

Langmuir,

Vol. 7,No. 11, 1991

Cecutti

et al.

Figure

5. Partial500-MHzcontour plots ofCOSY (a)and single Relay (b) experiments.

Figure

6. _{Contour plot of}a_{phase-sensitive}NOESY experiment(150-msmixing time). Onlynegative levelsare _plotted.

The

size

of

the

extended

maltoside

headgroupis Ry= 1.2

nm, and

the maximum radius

R2isR2 = _Lc+ _Rh= _{ca. 3}

nm.

In

the present

_case,

the external radius

of

themicelle

is 24

A

along

the small

axisand28

A

along

the long

axis,

allowing

enoughroom

for

the

_{sugarheadgroups and also}

satisfying the conditions

of

occupancy

of

thecenter

of

the

micelle;

i.e.,(a) Le

>

Ri

and (b)eR%= _{Lc +Ly¡.}

When

the

long axis reaches30

_{A, linear growth}

of

the

micelle

stops.

When there

is no

_{stopping mechanism}

for

miceller

growth,

infinite cylindrical

micellesare formed.14

In

our

case,

the

cylindrical

structure is

_{forbidden by the volume}

of

_{the headgroup,}as

it

should require dehydration

of

the

sugar

polar

head.

The micelle

_{hence grows}

toward

an

ellipsoid

but

cannot reach

the

_cylindrical

state.

Exper-imentally,

there

is no _way

to

fit

the scattering

with

the

expressions

of

the scattering

of cylindrical

micelles.

The ellipsoid length

is

limited

to

_{the length}

of

the

extended

molecule(3

nm). The two constraints inducing

Glycolipid

Self-Assembly:

Micellar

Structure

Langmuir,

Vol. 7, No. 11, 1991 2585

Figure

7. Schematic lateral view of the _{polar headgroup}

conformation.

the ellipsoidal

shape are as

follows:

_(a)

the

area

at the

chain-headgroup interface

= 0.5 nm2;

this

fixes

the

maximum value

of

R\. (b)

The

total

headgroup volume

between

R\ and

Ri

has

to include

the

2N

_hydrated

sugar

cycles;

this

induces an

_ellipsoidal

_{shape. These}

two

geometrical conditions

are

satisfied by the prolate

ellip-soidal

_shape

of

the

_glycolipid

micelle.

From the

observed

_scattering

_alone, one

could

also

invoke

size

_{polydispersity}

of

micelles

instead

of

a

distri-bution

of

monodisperse

ellipsoids,

as

this

will

_{yield similar}

scattering

curves.

An

_{independent evaluation}

of

poly-dispersity

is

_{however provided by the}

variation

of

the

averageaggregation

number

with

concentration.6

Table

IV

gives

the

aggregation

numbers

at three concentrations.

The lack

of

variation

of

thesenumbers

with

concentration

implies

that

polydispersity should

be

lower

than

a few

percent.

As

this

valueistoo small to

explain

the scattering

curves,a_shape

deformation

has

to

be

invoked instead

of

a

contribution

of

_{polydispersity.}

Conclusion

The two sterical constraints responsible

for

the

shape

of

the

micellar

_aggregateare

_{strongly dependent}

on

the

sugar

ring

conformation and

orientation

relative to the

chain-headgroup interface

plane.

Therefore,

interaction

of

the sugarhead

with

any

other

molecule suchasa

_protein

can

_dramatically

_change

the

value

of

'

when

the

_sugar

conformation

or

orientation

_changes.

The

value

of '

can

easily increase

by

alarge

factor (up to

3)

when

the ring

lies

parallel to the interface.

This

_{should strongly}

decrease

the

size

of

the micelle. Along the

same _{idea, when}

the

glycolipid

is

mixed

with another surfactant,

as

in biological

membranes,

the

spontaneous

curvature

toward oil should

decrease

_drastically.

This

is alsoa _possible

mechanism

for inhibiting

the

decrease

of

the radius

of

gyration

if

binding of

a

_protein

can

induce headgroup conformation

changes.

This

mechanismisnow

under investigation

_using

mixed

_{glycolipid-ionic}

surfactant mixed

micelles.

Acknowledgment.

Weare

_{grateful to Dr.}

C.

Williams

(Lure,

Orsay),

to Dr. L. Auvray (Laboratoire Léon