Charge transfer reactions from tryptophan and tyrosine to sulfur-centered dimer radical cation in aqueous -sulfuric acid medium: a pulse radiolysis study

Kinetics and mechanism of one-electron oxidation of N-acetyl methionine (NAM), tryptophan (TrpH), tyrosine (TyrOH), and phenylalanine (Phe) have been studied in 33% v/v H2SO4 solution. The solvent radical (SO4 center dot-) oxidized NAM, TrpH, TyrOH, and Phe to produce NAM(2)(center dot+) (480 nm), TrpH(center dot+) (330, 580 nm), TyrO(center dot) (350, 410 nm), and Phe(-H)(center dot) (320 nm), with rate constants (10(9) M-1 s(-1)) 0.6, 2.7, 3.9, 1.6, respectively. Time resolved radical transformation from NAM(2)(center dot+) to TrpH(center dot+) and TyrO(center dot) have been observed to occur with k(10(8) M-1 s(-1)) = 3.60 and 0.35, respectively. However, NAM(2)(center dot+) to Phe(-H)(center dot) and TrpH(center dot+) to TyrO(center dot) radical transformations have not been observed in this medium. The study shows the kinetics and mechanism of oxidation of some amino acids in strong acidic solutions. To the best of our knowledge, radical cations of amino acids and electron transfer reactions between them could be studied in strong acidic solutions for the first time. Copyright (C) 2016 John Wiley & Sons, Ltd.