Chiral Assembly Preferences and Directing Effects in Supramolecular Two-Component Organogels

Abstract

The impact of chirality on the self-assembly of supramolecular gels is of considerable importance, as molecular-scale programming can be translated into nanostructuring and ultimately affect macroscopic performance. This paper explores the effect of chirality on the assembly of two-component gels comprised of a second-generation dendritic lysine peptide acid, containing three chiral centres, and an amine. This combination forms an acid⁻amine complex that assembles into nanofibres through peptide-peptide hydrogen bonds, leading to organogels. With achiral amines, a racemic mixture of l,l,l and d,d,d dendritic peptide acids surprisingly forms the best gels-more commonly, mixing enantiomers suppresses gelation. Thermodynamic studies demonstrate that depending on the amine, the greater stability of heterochiral gels can either be entropically or enthalpically driven. With amines possessing "R" chirality, the l,l,l peptide acid consistently forms more effective gels than its d,d,d analogue. Furthermore, in mixed gels, l,l,l sometimes imposes its assembly preference onto d,d,d. In summary, this paper demonstrates a rare example in which heterochiral gels are preferred, and also explores directing effects when each component in a two-component gel is chiral.

Keywords: chirality; organogel; peptide; self-assembly; two-component.

Figure 1

Two-component self-assembling system investigated in the first part of this paper.

Figure 2

Typical examples of the effect of enantiomeric mixing on T_gel, as measured by tube inversion. The concentration of amine in toluene is 10 mM, and the total concentration of dendron (D,D,D + L,L,L) is also 10 mM. The amines for which data are represented are C4 (left) and C7 (right). Data for all amines can be found in the Supplementary Material.

Figure 3

(a) CD spectra of mixtures of L,L,L and D,D,D (total concentration = 0.625 mM in), changing in 10% concentration increments in the presence of C8 (0.625 mM) from 100% LLL (red) to 100% LLL (purple); (b) Data extracted from graph (a) at 220 nm.

Figure 4

Plot of ΔH − TΔS against temperature using the data from Table 3. X’s mark the experimental T_gel values. Enantiopure systems have full lines and racemic mixtures have dotted lines. This graph highlights the differences between systems and the apparent existence of a T_gel threshold.

Figure 5

FEG-SEM images of xerogels formed by C8 and, (a) L,L,L; (b) D,D,D; (c) racemic mixture of L,L,L and D,D,D demonstrating differences in the nanoscale fibrillar morphology of the racemic gel.

Figure 6

Chiral amines used to probe their directing effect on mixtures of peptide enantiomers.

Figure 7

(a) Effect of mixing L,L,L and D,D,D (total combined concentration, 10 mM) on the T_gel value in the presence of C8R (10 mM); (b) CD spectra of mixtures of L,L,L and D,D,D (total concentration = 0.625 mM), changing in 10% concentration increments from 100% LLL (red) to 100% DDD (purple) in the presence of C8R (0.625 mM).

Figure 8

Plot of ΔH − TΔS against temperature using the data from Table 5. X’s mark the experimental T_gel values reported in Table 4. This graph highlights the differences between systems and the apparent existence of a T_gel threshold.

Figure 9

FEG-SEM images of xerogels formed by: (a) L,L,L and C8R; (b) D,D,D and C8R; (c) L,L,L + D,D,D and C8R.