Photophysical and elecron-transfer reaction properties of tris(2,2’-bipyridine)ruthenium(II)-based inhibitors that covalently bound to the active site of chymotrypsin

Abstract

We have prepared two tris(2,2’-bipyridine)ruthenium(II) complex-based new chymotrypsin (CHT) inhibitors, [Ru(bpy)2(bsfbpy)]Cl2 and [Ru(bsfbpy)3]Cl2 {bsfbpy = 4-methyl-2,2′-bipyridine-4′-carboxylic acid [2-(4-benzenesulfonylfluoride)-ethyl]-amide}. They were covalently modified with Ser195 in the active site of CHT. Photoinduced electron-transfer (ET) systems comprising a Ru-CHT complex and an electron acceptor such as methylviologen (MV2+) were studied. After photoirradiation, the photo-excited triplet state of 2CHT-3{[Ru(bsfbpy)3]2+}* and CHT-3{[Ru(bpy)2(bsfbpy)]}* were both quenched by MV2+ through an intermolecular photoinduced ET mechanism, forming oxidized 2CHT-[Ru(bsfbpy)3]3+ and CHT-[Ru(bpy)2(bsfbpy)]3+, respectively. The intramolecular ET from the amino acid residue of Tyr94 near the active site of CHT proceeded. Transient absorption spectra display the formation of a Tyr●+ radical in the 2CHT-[Ru(bsfbpy)3]2+-MV2+ and the CHT-[Ru(bpy)2(bsfbpy)]2+-MV2+ systems, having a lifetime of 2.24 ms for 2CHT-[Ru(bsfbpy)3]2+-MV2+. The present model system could be one of the available models for the further photochemical reaction of Tyr•+ formed by intramolecular photoinduced ET reactions within a protein scaffold. To clarify the photophysical properties of these Ru inhibitors and their multi step photoinduced ET reactions in the protein matrix, the detailed ET reaction kinetics are discussed.