Regulation of the chiral twist and supramolecular chirality in co-assemblies of amphiphilic L-glutamic acid with bipyridines

A series of amphiphilic L-glutamic acid derivatives with various saturated alkyl chains has been designed and their co-assembly with 4,4'-bipyridine in aqueous media has been investigated. While the individual amphiphiles formed hydrogels with water and self-assembled into fine fiber networks, the addition of 4,4'-bipyridine caused significant changes in the co-assembled nanostructures such that twisted chiral ribbons were formed. In these supramolecular systems, either fine structural changes or adjustment of the stoichiometric ratio of the two components had crucial effects on the formation of the chiral twists. Based on detailed investigations by SEM and XRD analyses, FTIR, CD, and UV/Vis spectroscopies, and molecular simulation, it is considered that a delicate synergistic balance between π-π stacking, hydrophobic, and chiral interactions is responsible for the formation of the chiral twists. An interesting sandwich structure, in which an excess of 4,4'-bipyridine is inserted into the space of primary cages constructed from the amphiphile and 4,4'-bipyridine, is proposed. Remarkably, the handedness of these chiral twists is related not only to the chiral center of the glutamic unit, but also the chain length of the alkyl tails. This work provides a deeper understanding of the formation mechanism of chiral twists, and exemplifies a feasible shortcut to the rational design of chiral structures from basic molecular structures to supramolecular systems.