Photoinduced electron transfer in porphyrin-naphthyl pairs covalently and randomly bound to α-helical poly(L-lysine)

The photophysics of protoporphyrin-naphthyl (P-N) pairs covalently and randomly bound to the epsilon-amino groups of the side-chains of poly(L-lysine) (PL) were investigated by steady-state and time-resolved fluorescence measurements. The results indicate that quenching of excited naphthalene (lambda(ex) = 280 nm) chiefly occurs by interconversion to the tripler state when the polymeric matrix is in random coil (pH approximate to 7) and by intramolecular electron transfer from ground-state porphyrin, P --> N-i*, when the polypeptide is in alpha-helical conformation (pH approximate to 11), the specific rate constant of the electron transfer being 3,1 . 10(7) s(-1) (25 degrees C). PNPL exhibits very little exciplex fluorescence, whatever the pH, suggesting both a reduced internal Brownian motion of the chromophores, owing to the amide bond in the substituted side-chains, and a relatively large average interprobe separation distance. This agrees with polarized fluorescence data and with the results of a conformational statistics analysis on the fully ordered PNPL, indicating that the average interchromophoric distance for which the electron transfer has the highest probability to occur is around 12 Angstrom. The computational results allowed us to reproduce the experimental fluorescence decay curves and estimate the parameters governing the electron transfer process. Implications of these findings on the relaxation time of the helix-coil transition in PNPL are briefly discussed.