Aggregation Behavior of Amphiphiles Functionalized with Dipeptide Segments and Enantioselective Ester Hydrolysis in Their Bilayer Membranes

Four amphiphiles having dipeptide segments, which consist of one histidyl residue and another amino acid residue, and the dihexadecyl moiety for the double-chain segment (N+C5AlaHis2C16, N+C5LeuHis2C16, N+C5PheHis2C16, and N+C5HisAla2C16) were synthesized, and the aggregate morphology of them was characterized by electron microscopy and differential scanning calorimetry. The amphiphiles, which have the histidyl residue bound to the dihexadecylamine component (N+C5AlaHis2C16, N+C5LeuHis2C16, N+C5PheHis2C16) formed both multi- and single-walled aggregates in the aqueous dispersion state, while their sonicated solutions involved only single-walled vesicles. On the other hand, the amphiphile, which has the alanyl residue bound to the dihexadecylamine component (N+C5HisAla2C16) afforded tubular aggregates in the aqueous dispersion. The hydrolysis of enantiomeric esters of various hydrophobic nature, which was carried out in the single-walled vesicles, showed relatively small enantioselectivity. The reasons for such catalytic performance of the vesicles were discussed from the kinetic and mechanistic viewpoints; the kinetic analysis being carried out on the basis of the organic pseudo-phase concept.