Electron transfer from aromatic amino acids to triplet quinones

The photoreduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone (AQ) and several methylated or halogenated derivatives in argon-saturated acetonitrile–water mixtures by indole, N-acetyltryptophan and N-acetyltyrosine was studied by time-resolved UV–vis spectroscopy using 20 ns UV laser pulses. The quinone triplet state is quenched by the aromatic amino acids and the rate constants are (1–5) × 109 M−1 s−1. The semiquinone radical anion Q− is the major observable transient after electron transfer from amino acids to the quinone triplet state. Termination of Q− and amino acid derived radicals takes place in the μs–ms range. The effects of structure and other specific properties of quinones and amino acids are discussed. The radicals are subjects of intercept with oxygen, whereby hydrogen peroxide is eventually formed. The quantum yield of oxygen uptake (Φ−O2)(Φ−O2) as a measure of formation of hydrogen peroxide increases with increasing amino acid concentration, approaching Φ−O2=0.5Φ−O2=0.5 for AQ in air-saturated solution.