Electron transfer from indole and tryptophan

Abstract

Electron photoejection from amino-acid residues such as tryptophan (Trp) and various indole (In) derivatives has received much attention in recent years since (i) it constitutes the major primary photoprocess in such systems and (ii) Trp photodegradation induces in turn the denaturation of Trp-containing proteins.

Electron transfer from electronically excited Trp and In has been investigated by two experimental approaches. Using steady-state irradiation the energetics of electron photoejection was examined (1); using picosecond absorption spectroscopy the kinetics of e^–_aq formation and decay was studied (2). The most significant results are given below.

(1) Photoelectron scavenging as a function of photon energy under low-intensity continuous irradiation which precludes biphotonic processes provides photoionization efficiency curves. The latter depend in several respects upon the solvent and, from the ionization threshold energy, the fundamental energy state V₀ of the quasi-free electron in the corresponding solvent can be deduced. The values thus obtained are –1.3, –1.0 and –0.65 eV for water, methanol and ethanol, respectively.

(2) On the other hand, single pulses of ca. 20 ps duration at 266 nm and hydrated electron (e^–_aq) absorption spectroscopy show that, for both solutes, e^–_aq absorption appears within the laser pulse. Hence, e^–_aq probably originates from the unrelaxed first singlet state and not from the fluorescent state, the lifetime of which is 3.2 and 4.8 ns for Trp and In, respectively.

The e^–_aq optical density remains constant from 50 ps up to ca. 2 ns (the time limit of the apparatus). This observation implies that no appreciable charge recombination occurs during this time range.