PHOTOPHYSICAL BEHAVIOR OF POLY(L-LYSINE) CARRYING PORPHYRIN AND NAPHTHYL CHROMOPHORES

The photophysical behavior of protoporphyrin IX (P) and 1-naphthylacetic acid (N), covalently bound to epsilon-amino groups of poly(L-lysine) (PL), was investigated by steady-state and time-resolved fluorescence as a function of pH. Within the whole range of pH explored, i.e., 7-11, exciplex emission is minor and nearly pH independent. Fluorescence quantum yields, decay time measurements, and transient absorption spectra suggest that quenching of the excited naphthyl chromophore chiefly occurs by interconversion to the triplet state when the sample is randomly coiled and by intramolecular electron transfer (ET) from ground-state porphyrin when the polypeptide is in alpha-helical conformation. The kinetic law, based on a two-state model for the polymeric matrix, is presented. The specific rate constant of photoinduced ET is 3.1.10(7) s(-1) (25 degrees C), in excellent agreement with that obtained by taking simply into account the lifetimes of naphthalene fluorescence in alpha-helical PNPL and NPL (pH 11). The relaxation time of the helix-coil transition was found to be definitely shorter than 20 ns. (C) 1994 John Wiley & Sons, Inc.