Sulfur-Containing NDIs: The Role of Self-Sorting

In recent years, much interest was paid towards the self-sorting properties of molecules.¹⁵⁴One motivation for this is the way reactions take place in nature, where at low concentration high reaction rates are observed. This is accom-plished mainly by means of self-recognition of reactive partners, thus excluding other possible reactive partners, and increasing the local concentration of the desired reaction.¹⁵⁵Of particular interest, in material sciences for example, is the self-sorting of aromatic molecules like NDIs, PDIs and other widely used aromatic systems like porphyrins and phthalocyanines. It has been reported, for instance, that the initial self-sorting of potential reactive partners due to the twist angle-driven self-sorting of PDIs favors the formation of one specific product.¹⁵⁶

The potential of compounds120,121and122to display self-sorting was soon noticed. Moreover, compound 121 is a mixture of stereoisomers and, therefore, the possibility to observe chiral self-sorting motivated a more careful study of this system. In addition, the sorting properties due to the gradually increased hindrance on the π-surface made of this system a potential “text-book” example of self-sorting and its arising properties.

Such a system, on the one hand, is likely to show activity in fluorogenic vesicles and, on the other hand, it offers a number of possible conformational changes without touching its main electronic properties. These properties

Conditions for the HPTS assay for Figures 44 and 45: 1930µl buffer (100 mM Li⁺X⁻(X⁻

= F⁻, OAc⁻, Cl⁻, NO⁻₃, ClO⁻₄), 10 mM Hepes, pH 7.0), 25µl EYPC-LUVs⊃HPTS (LiCl).

3 Results and Discussion 65

make it the perfect system to unveil the details of its anion transport mecha-nism.

tert-Butylphenyl Sulfur Containing NDIs

The chosen model system consists of the already described sulfur-containing NDIs bearing o-(tert-butylphenyl) substituents on the imide moieties. Their synthesis, which only differs in the aniline used for the imide formation, was carried out by other members of the Matile group¹⁵⁷ and yields compounds 127to129(Schemes 7 to 9) in addition to the mesovariants of 128.

N

N S S

O O

O O

N

N S S

O O

O O

N

N S S

O O

O O

(M) (P) (meso)

127

Scheme 7:tert-Butylphenyl NDIs with thioether substituents studied for anion transport.

For practical reasons, only the (P)/(M) mixtures of compounds 128a–c were used for transport experiments.

Self-Sorting Properties

The sorting properties of these compounds have been investigated by other members of the Matile group in Geneva and reported elsewhere.¹⁵⁷ In order to better analyze their anion-transport properties, a brief summary follows.

In the solid state, single crystal structures showed that while(M)-127 crys-talizes in a face-to-face manner,(meso)-127does not. A similar observation is made for 128a that crystalizes in an edge-to-face fashion, where 128b,c

N

Scheme 8:tert-Butylphenyl NDIs with sufoxides substituents studied for anion transport.

Scheme 9:tert-Butylphenyl NDIs with sulfone substituents studied for anion trans-port.

3 Results and Discussion 67

crystalizes in face-to-face heterodimers. These observations are mainly ex-plained by the sterics involved: (meso)-127is hindered on both sides by the tert-butyl groups, while in the case of128athe ethyl groups account for the competitive hindrance. The crystal structure also revealed that a shorter π-π distance is observed with increasingπ-acidity.

Concentration-dependent NMR measurements also show that compounds 127and128ado not form face-to-face dimers. Competitive NMR measure-ments show that, for instance,127will give perfect self-sorting in the presence of129, while128would show different levels of self-sorting depending on its stereochemistry (128a–c), indicating that the sorting properties of the 128 family are not constant and a case-by-case study is needed. This should have implications for molecular recognition as well as surface-templated polymer-izations technics as studied by other members of the Matile group and col-laborators. The implications on anion transport are discussed in the following paragraph.

Anion Transport in Fluorogenic Vesicles

To achieve anion transport in vesicles, compounds127,128a–cand129were tested in the HPTS assay. Interestingly, compound129was inactive. This was attributed to a significant lower solubility, as compared to 122 for example, likely causing it to precipitate it before reaching the membrane. This is not uncommon, and has been reported before.¹⁵⁸

The expected increase in activity was observed between the sulfide series and the sulfoxide one, and the drop in activity as compared to the trimethylphenyl series is attributed to solubility issues due to the tert-butyl group that dra-matically changes the hydrophobicity of the molecules.

Gratifyingly, compounds128a–cshowed quite a different transport behav-ior. One difference was observed at the activity levels, where the difference is relevant withEC50s of 8.4±0.1, 12.5±0.4 and 33.0±2.0µM respectively.

The main difference came with the Hill coefficient values, ranging from 2.1± 0.4 for compound128c, 5.0±0.6 for compound128bto an unexpected 7.4

±0.5 for compound128a, underscoring the importance of the conformational changes. This can clearly be seen in Figure 46 and Table 4.

0 0.2 0.4 0.6 0.8 1

5 6 7 8 9 10 20 30 40 50

Y

c / µM

Figure 46: Dose response curves for127and128a–c in the HPTS assay; (meso)-127(),(P)/(M)-127(),(P)/(M)-128a(♦),(P)/(M)-128b(x) and (P)/(M)-128c(+).

From this point on, the relevance of anion-π interactions to generate the transport activity is no longer the focus but the mechanism of this anion transport. A closer analysis of compounds 128a–c, with support of the self-sorting data, suggest that the better the self-self-sorting dimerization properties the worse the observed transport. Indeed,128afor instance, was not observed in its dimeric form while compounds128b,cdid form heterodimers.

The high Hill coefficient values should not necessarily be attributed to dif-ferent active structures but may rather be related to the stability of this struc-ture in the membrane. Indeed, despite the structural differences, all these compounds promote anion transport mainly due to anion-πinteractions and therefore proposing four different active transport structures for five different molecules with comparable activities seems most unlikely.

It is not possible, at this stage, to determine the active structure, but a stack of dimers or a barrel-like tetramer in each leaflet of the membrane are appealing hypothesis. It is clear, however, that this structure is, at least, an octamer. Also, the results here presented point at a higher stability of the active structure, when increasing the dimeric stability, but at a lower activity.

This is due to equilibrium considerations; indeed, if the observed differences are due to the stability of the complex, the smaller the Hill coefficient the higher the local concentration of the complex. This is not reflected in the

3 Results and Discussion 69

Table 4:Summary of transport measurements for some NDIs

NDI¹ EC50/ µM n EC50 (CF) /µM

(P)/(M)-127 >100 n.d. >100

(meso)-127 >100 n.d. >100

(P)/(M)-128a 8.4±0.1 7.4±0.5 22.4±0.8 (P)/(M)-128b 12.5±0.4 5.0±0.6 32.0±5.0 (P)/(M)-128c 33.0±2.0 2.1±0.4 93.0±9.0

(P)/(M)-129 n.d. n.d. n.d.

(meso)-129 n.d. n.d. n.d.

1For structures see Schemes 7 to 9.

EC50values where the trend is the opposite. It is reasonable to think that the dimeric form is present as a competing complex formation which accounts for this effect.

Unfortunately, the low activity of compounds (meso)-127 and (P)/(M)-127 would not allow adequate observation of differences due to the steric hindrance of theπ-surface.