Intramolecular transfer of excitation energy in sequential oligopeptides carrying naphthalene and protoporphyrin chromophores

Abstract

The photophysical behavior of sequential oligopeptides, carrying naphthalene and protoporphyrin IX covalently bound to ϵ-amino groups of lysine residues, was investigated in methanol solution by steady-state and time resolved fluorescence measurements, as well as by transient absorption spectra. Quenching of excited naphthalene chiefly takes place by transfer of excitation energy, and proceeds on a time scale of 3-7 ns (25°C). A slower (≈ 45 ns) and minor fluorescence decay was also measured, which may be ascribed to exciplex quenching since it does not depend on the interprobe distances. This finding differs from that earlier obtained with the same chromophores bound to α-helical poly(L-lysine) in aqueous solution, where quenching of naphthalene was mainly due to electron transfer from ground-state porphyrin. IR data on the peptides in methanol solution indicate that intramolecularly H-bonded conformations form, and CD spectra suggest the presence of variable amounts of ordered structure (α-helix), depending on the chain length. The photophysical results can be well described by a lonq-range dipole-dipole interaction model that takes into account the statistical distribution of intramolecular separation distances and mutual orientations of the chromophores.