Two‐Component Dendritic Gel: Effect of Stereochemistry on the Supramolecular Chiral Assembly

Abstract

The self-assembly of diaminododecane solubilised by four different stereoisomeric dendritic peptides to form gel-phase materials in toluene was investigated. The second generation dendritic peptides were based on D- and L-lysine building blocks, and each contained three chiral centres. By designing dendritic peptides in which the configurations of the chiral centres were modified, and applying them as gelator units, the assembly of stereoisomers could be investigated. In all cases, the self-assembly of gelator units resulted in macroscopic gelation. However, the degree of structuring was modulated by the stereoisomers employed, an effect which changed the morphology and macroscopic behavior of the self-assembled state. Enantiomeric (L,L,L or D,D,D) gelator units formed fibrous molecular assemblies, whilst the racemic gel (50 % L,L,L : 50 % D,D,D) formed a flat structure with a "woven" appearance. Gelator units based on L,D,D or D,L,L dendritic peptides also formed fibrous assemblies, but small-angle X-ray scattering indicated significant morphological differences were caused by the switch in chirality. Furthermore, the macroscopic stability of the gel was diminished when these peptides were compared with their L,L,L or D,D,D analogues. In this paper it is clearly shown that individual stereocentres, on the molecular level, are directly related to the helicity within the fibre. It is argued that the chirality controls the pattern of hydrogen bonding within the assembly, and hence determines the extent of fibre formation and the macroscopic gel strength.

Positionally Isomeric Organic Gelators: Structure–Gelation Study, Racemic versus Enantiomeric Gelators, and Solvation Effects

“…[14] This can be explained by the fact that the immobilization of solvent is predominantly due to capillary forces, which are stronger for smaller network compartments. Consequently, the specific solvation effects exhibited by structurally different isomeric xylenes may stem from the formation of fibers of different morphology, which in turn give networks of variable density having different gelation capacities.…”

“…[15] In the majority of the published results, the pure enantiomer was seen to be a more effective gelator than the racemate, although several exceptions have also been reported. [14] However, symmetrical meso forms of various gelators have as a rule been found to lack any gelation ability. [15d, 16] Since racemic 17-3-H, 17-3-Na, 15-5-H, and 10-10-Na exhibited surprisingly different G eff values in the gelation of isomeric xylenes, we decided to investigate whether these properties were a consequence of their positional isomerism or a result of their stereochemical form.…”

The transformation of chiral signals into macroscopic properties of materials using chirality-responsive polymers

“…The swelling ratio of a hydrogel in dimethyl sulfoxide was dramatically improved by the addition of chiral amines; this improvement was attributed to the formation of one-handed helices through chiral acid-base reactions. Subsequently, Smith et al 70,71 developed a chiral hydrogel through the self-assembly of diaminododecane with dendritic peptides that contained three groups of L-or D-lysine building units. By adjusting the molar ratios of L,L,L and D,D,D dendritic peptides to form hydrogels, the authors enabled the molecular chirality of the gelator to determine the extent of fiber formation, the helicity within the fibers and the macroscopic gel strength.…”

Exploring molecular recognition pathways within a family of gelators with different hydrogen bonding motifs

“…15, 15a, 15b and 15c These molecules self-assemble to form gels as a consequence of intermolecular hydrogen bond interactions between peptide head groups. We have reported the effect of spacer chain, 13a dendritic generation, 16 chirality, 17 solvent 18 and molar composition 19, 19a and 19b on the gelation process.…”