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

The photophysical behavior of protoporphyrin IX (P) and 1-naphthylacet ic acid (N), covalently bound to epsilon-amino groups of poly(L-lysine ) (PL), was investigated by steady-state and time-resolved fluorescenc e 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 spe ctra suggest that quenching of the excited naphthyl chromophore chiefl y occurs by interconversion to the triplet state when the sample is ra ndomly 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 3.1.1 0(7) s(-1) (25 degrees C), in excellent agreement with that obtained b y taking simply into account the lifetimes of naphthalene fluorescence in alpha-helical PNPL and NPL (pH 11). The relaxation time of the hel ix-coil transition was found to be definitely shorter than 20 ns. (C) 1994 John Wiley & Sons, Inc.