Self-assembly of coumarin-conjugated acidic proteinoids

Abstract

Photo- and pH-responsive self assembly was prepared by dispersing coumarin-conjugated acidic proteinoid in an aqueous phase. Proteinoid composed of Asp, Ser and Leu (PASL) was prepared as an acidic one by a melt-condensation method. Epoxypropoxy coumarin (EPC) was conjugated to PASL through its reaction with Ser. Assuming all Ser residues reacted with EPC, the calculated molar ratio of Asp/Ser-EPC/Leu in EPC-PASL conjugate was about 202/18/1 on 1H NMR spectrum. EPC-PASL conjugates exhibited the dimerization and the de-dimerization of their EPC residues under cyclic irradiation of 365 nm and 254 nm. They were self-assembled into microparticle (about 500 nm), possibly due to the hydrophobic interaction among Leu residues and EPC residues. The EPC-PASL assemblies were linked each other by the photo-dimerization under the UV irradiation of 365 nm. EPCs are the hydrophobic pendants of the PASL conjugates and they could be easy to be hydrophobically assemblied. This may account for why EPC-PASL assemblies were much greater than PASL assemblies. On the contrary, EPC of PASL conjugates was cleavaged by the photo-dedimerization under the UV irradiation of 254 nm. The size of the assemblies increased and decreased in a cyclic manner under the cyclic irradiation, and the size of the photo cross-linked assembly was larger at a higher pH, possibly due to stronger intermolecular repulsion.