Photoinduced electron and hole transfer involving eosin conjugates of tryptophan derivatives

Abstract

The preparation of several covalent conjugates of the dye, eosin Y (EY2-) with the amino acid, L-tryptophan (Trp), the dipeptide, Trp-Trp, and with poly(L)-tryptophan is described. Intramolecular photochemical electron transfer is responsible for the quenching of dye excited singlet states (flourescence) in these systems. The sacrificial acceptor, tetranitromethane (TNM), was used to irreversibly oxidize pendant dye groups; the transfer of charge (“holes”) from the oxidized eosin moiety located at the N-terminus of the peptides was monitored by laser flash photolysis. Both modes of electron transfer involve charge transfer from indole side chains associated with Trp groups for the various peptide links. Study of the kinetics of hole transfer provided rate constants in the range of 103–104 s-1 for several solvent media including aqueous poly(vinylpyrrolidinone) (PVP), a polymer that provides a protein-like microdomain. The observed temperature dependence of the hole transfer rate is consistent with non-adiabatic electron transfer over distances determined by molecular modeling to be about 1.0 nm (through-space) and involving through-bond interactions via a network of σ-π overlaps. Findings are discussed in terms of demonstration of the entrainment of charge along peptide chains and the efficient separation of charge in (bio)polymer structures of some size that display vectorial properties.