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Light-Induced Oxidation of Tryptophan and Histidine. Reactivity of Aromatic N -Heterocycles toward Triplet-Excited Flavins

2009; American Chemical Society; Volume: 131; Issue: 23 Linguagem: Inglês

10.1021/ja809039u

1943-2984

Kevin Huvaere, Leif H. Skibsted,

Radical Photochemical Reactions

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 ∼ 2.7 × 109 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 × 108 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 ∼ 1.8 × 108 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 Zn2+ (k of 4-methylimidazole, as histidine model, decreased from 9.0 × 108 L mol−1 s−1 in the absence to 4.1 × 108 L mol−1 s−1 in the presence of ZnCl2). 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 × 108 L mol−1 s−1 to 2.0 × 109 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.