Length of alkyl chain spacer modulates aggregation behavior and coordination capability of histidyl bolaamphiphiles

Histidyl bolaamphiphile is an amphiphilic molecule having hydrophilic histidine motifs at both ends of a hydrophobic alkyl chain spacer. Supramolecular assembly of this molecule has been applied for the preparation of catalysts mimicking metalloenzymes through the coordination of various transition metal ions to imidazole ligands. In this study, the effects of the hydrophobic alkyl chain length in the histidyl bolaamphiphile are investigated on the aggregation and metal-coordination capability that is affected by both the surface areas of aggregates and the dynamic motion of imidazole ligands. The sizes of the histidyl bolaamphiphile aggregates increase to reduce specific surface area with the elongation of the hydrophobic alkyl chain spacer, as verified by experimental measurements and thermodynamic models. Further dissipative particle dynamics (DPD) simulations reveal that dynamic motion of the metal-coordinating moieties on the surface is retarded with the increase of the length of the hydrophobic spacer, facilitating coordination with metal species. Because of these contradictory effects, there is an optimal spacer length at which the metal-coordination capability is maximized, resulting in the highest catalytic activity.