Photoinduced electron transfer in α-helical poly(L-lysine) carrying randomly distributed donor–acceptor pairs. A kinetic and conformational statistics investigation

Abstract

The photophysics of protoporphyrin IX (P) and 1-naphthylacetic acid (N), covalently bound to ε-amino groups of poly(L-lysine)(PL) in a [P] : [N] molar ratio of 0.25, were investigated as a function of pH. Differential circular dichroism spectra and polarized fluorescence data on PNPL and blank samples (NPL and PPL) suggest that the amide bond in the chromophore linkages slows down the internal rotation of the aliphatic side-chains of the polypeptide. The conformational mobility of these linkages is further reduced by varying the pH from 7 to 11, the α-helical conformation of the backbone chain making the whole structure stiffer. Steady-state fluorescence, transient-absorption spectra and time-decay measurements indicate that quenching of excited naphthalene chiefly results from interconversion to the triplet state when the polymeric matrix is in a random coil conformation (pH ca. 7) and from intramolecular electron transfer, P →¹N*, when it is in an α-helical conformation (pH ca. 11). The kinetic law of these processes, based on a two-state model for the polypeptide matrix, is presented. The specific rate constant of the photoinduced electron transfer is 3.1 × 10⁷ s^–1(25 °C), in very good agreement with that obtained from the lifetimes of naphthalene fluorescence in α-helical PNPL and NPL (pH 11), i.e. 2.7 × 10⁷ s^–1. PNPL solutions exhibit very little exciplex fluorescence, whatever the pH, suggesting a relatively large average separation distance between the chromophores, in agreement with the results of a conformational statistics analysis on the fully ordered PNPL. The probability distribution of centre-to-centre distances between the chromophores, as obtained by adopting a rotational isomeric state model of the probe linkages, allowed us to reproduce the experimental fluorescence decay curves and estimate the parameters governing the electron-transfer process.