| Abstract
| - The reaction between the triplet excited state of riboflavin and amino acids, peptides, and bovinewhey proteins was investigated in aqueous solution in the pH range from 4 to 9 at 24 °C usingnanosecond laser flash photolysis. Only tyrosine and tryptophan (and their peptides) were found tocompete with oxygen in quenching the triplet state of riboflavin in aqueous solution, with second-order rate constants close to the diffusion limit, 1.75 × 109 and 1.40 × 109 L mol-1 s-1 for tyrosineand tryptophan, respectively, with β-lactoglobulin and bovine serum albumin having comparable rateconstants of 3.62 × 108 and 2.25 × 108 L mol-1 s-1, respectively. Tyrosine, tryptophan, and theirpeptides react with the photoexcited triplet state of riboflavin by electron transfer from the tyrosineand tryptophan moieties followed by a fast protonation of the resulting riboflavin anion rather than bydirect H-atom abstraction, which could be monitored by time-resolved transient absorption spectroscopy as a decay of triplet riboflavin followed by a rise in riboflavin anion radical absorption. Forcysteine- and thiol-containing peptides, second-order rate constants depend strongly on pH, forcysteine corresponding to pKaRSH = 8.35. H-atom abstraction seems to operate at low pH, whichwith rising pH gradually is replaced by electron transfer from the thiol anion. From the pH dependenceof the second-order rate constant, the respective values for the H-atom abstraction (k = 1.64 × 106L mol-1 s-1) and for the electron transfer (k = 1.20 × 109 L mol-1 s-1) were determined. Keywords: Amino acids; laser flash photolysis; photooxidation; riboflavin; bovine whey proteins
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