Intramolecular transfer of excitation energy in short linear peptides carrying naphthalene and protoporphyrin molecules. A molecular model of electron transfer/energy transfer transition

Abstract

Sequential oligopeptides carrying naphthalene and protoporphyrin IX as fluorophores, covalently bound to α-amino groups of lysine residues, were investigated in methanol and water/methanol 75/25 (v/v) solutions by steady-state fluorescence, transient absorption spectra and time resolved emission measurements. Quenching of excited naphthalene chiefly takes place by transfer of excitation energy, 1N ∗→P, and proceeds on a time scale of 3–8 ns (25 °C), depending on the helical periodicity of the backbone chain. According to earlier IR and CD spectral results, the oligopeptides investigated attain an α-helix conformation, which is responsible for the periodic variation of the interprobe distances. A slower ( ≈45 ns) and minor fluorescence decay was also measured, which is ascribed to exciplex emission. This finding differs from that earlier obtained with the same chromophores bound to α-helical poly( l-lysine) in water, where quenching of naphthalene was mainly due to electron transfer from ground-state porphyrin, P → 1N ∗ Molecular mechanics calculations show a different topology of N and P molecules bound to the α-helix in the two systems, which is thought to be primarily responsible for the observed transition between photoinduced electron transfer and energy transfer.