Hierarchical self-assembly of chiral complementary hydrogen-bond networks in water: Reconstitution of supramolecular membranes

Spontaneous formation of complementary hydrogen-bond pairs and their hierar chical self-assembly (reconstitution) into chiral supramolecular membranes are achieved in water by mixing amphiphilic pairs of glutamate-derived mela mine 6 and ammonium-derivatized azobenzene cyanuric acid 4. Electron micros copy is used to observe formation of helical superstructures, which are dis tinct from the aggregate structures observed for each of the single compone nts in water. In addition, a spectral blue-shift and induced circular dichr oism (ICD) with exciton coupling are observed for the pi-pi* absorption of the azobenzene chromophores. These observations are consistent with the rec onstitution of the hydrogen-bond-mediated supramolecular membrane 6-4. Spec tral titration experiments indicate the stoichiometric integration of the c omplementary subunits with an association constant of 1.13 x 10(5) M-1. Thi s value is considerably larger than those reported for the artificial hydro gen-bonding complexes in aqueous media. The remarkable reconstitution effic iency is ascribed to the hydrophobically driven self-organization of the am phiphilic, linear hydrogen-bond networks in water. Molecular structure of t he complementary subunits plays an important role in the complexation proce ss since it is restricted by the photoisomerized cis-azobenzene subunit. On the other hand, thermally regenerated trans-isomer 4 undergoes facile comp lexation with the counterpart 6. The present reconstitution of supramolecul ar membranes provides the first example of complementary hydrogen-bond-dire cted formation of soluble, mesoscopic supramolecular assemblies in water.