The reaction bet
ween the triplet excited state of riboflavin and amino acids, peptides, and bovine
whey 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 × 10
9 and 1.40 × 10
9 L mol
-1 s
-1 for tyrosineand tryptophan, respectively,
with
-lactoglobulin and bovine serum albumin having comparable rateconstants of 3.62 × 10
8 and 2.25 × 10
8 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 follo
wed 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 follo
wed by a rise in riboflavin anion radical absorption. Forcysteine- and thiol-containing peptides, second-order rate constants depend strongly on pH, forcysteine corresponding to p
KaRSH = 8.35. H-atom abstraction seems to operate at lo
w pH,
which
with 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 × 10
6L mol
-1 s
-1) and for the electron transfer (
k = 1.20 × 10
9 L mol
-1 s
-1)
were determined.Key
words: Amino acids; laser flash photolysis; photooxidation; riboflavin; bovine
whey proteins