Photoresponsive molecular recognition and adhesion of vesicles in a competitive ternary supramolecular system

摘要

A competitive photoresponsive supramolecular system is formed in a dilute aqueous solution of three components: vesicles of amphiphilic α-cyclodextrin host 1 a, divalent p-methylphenyl guest 2 or divalent p-methylbenzamide guest 3, and photoresponsive azobenzene monovalent guest 5. Guests 2 and 3 form weak inclusion complexes with 1 a (K_a≈10 ~2 M~(-1)), whereas azobenzene guest 5 forms a strong inclusion complex (K_a≈10~4 M~(-1)), provided it is in the trans state. The aggregation and adhesion of vesicles of host 1 a is mediated by guest 2 (or 3) due to the formation of multiple intervesicular noncovalent links, as confirmed by using isothermal titration calorimetry (ITC), optical density measurements at 600 nm (OD600), dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The addition of excess monovalent guest trans-5 to vesicles of 1 a aggregated by divalent guest 2 (or 3) causes the dispersion of vesicles of 1 a because trans-5 displaces 2 (as well as 3) from the vesicle surface. Upon UV irradiation of a dilute ternary mixture of vesicles of 1 a, guest 2 (or 3), and competitor trans-5, compound trans-5 isomerizes to cis-5, and renewed aggregation of vesicles of 1 a by guest 2 (or 3) occurs because 2 (as well as 3) displaces cis-5 from the vesicle surface. Subsequent visible irradiation causes the redispersion of vesicles of 1 a because cis-5 reisomerizes into trans-5, which again displaces guest 2 (or 3) from the vesicle surface. In this way, the competitive photoresponsive aggregation and dispersion of vesicles can be repeated for several cycles.