Light-induced oxidation of tryptophan and histidine. Reactivity of aromatic N-heterocycles toward triplet-excited flavins

Abstract

Mechanisms of flavin-mediated photooxidation of electron-rich amino acids tryptophan and histidine were investigated for aqueous solutions. Indole, representing the tryptophan side chain in proteins, reacted at nearly diffusion controlled rates (k approximately 2.7 x 10(9) L mol(-1) s(-1) at 293 K) with the triplet-excited flavin state, but reactions of imidazole (and histidine) were significantly slower (k < 2.0 x 10(8) L mol(-1) s(-1)) as determined by laser flash photolysis. Oxidation rates of derivates were invariably susceptible to electronic factors affecting incipient radical cation stability, while no primary kinetic hydrogen/deuterium isotope effect was observed for imidazole. Thus reaction by electron transfer was proposed in contrast to a direct hydrogen abstraction. Unlike indole compounds, imidazole derivatives suffered from the presence of a basic imino nitrogen (=N-), which caused the rate constant of histidine free base (k approximately 1.8 x 10(8) L mol(-1) s(-1)) to drop considerably upon protonation. Complexation of the imino nitrogen with transition metals provoked changes in reactivity, as rate constants decreased after addition of Zn(2+) (k of 4-methylimidazole, as histidine model, decreased from 9.0 x 10(8) L mol(-1) s(-1) in the absence to 4.1 x 10(8) L mol(-1) s(-1) in the presence of ZnCl(2)). The pyrrole nitrogen (-NH-) was not directly involved in complexation reactions, but its electron density increased upon interaction with hydrogen bond-accepting anions and resulted in higher rate constants (k of 4-methylimidazole increased from 9.0 x 10(8) L mol(-1) s(-1) to 2.0 x 10(9) L mol(-1) s(-1) after addition of NaOAc). The high rate constants were in agreement with a large thermodynamical driving force, as calculated from oxidation peak potentials determined electrochemically. After oxidation, resulting radical cations were readily deprotonated and trapped by 2-methyl-2-nitrosopropane, as detected by electron paramagnetic resonance spectroscopy. Indole-derived spin adducts were attributed to selective trapping of C(3)-centered radicals, whereas spin adducts with imidazole-derivatives arose from both carbon and nitrogen-centered imidazolyl radicals.