A laser flash photolysis study of amino acids and dipeptides using 4-nitroquinoline 1-oxide as a photosensitizer: The pH dependence

The pH effects on the photochemical reaction of amino acids and related dip eptides with 4-nitroquinoline 1-oxide (4NQO) as a photosensitizer have been investigated by laser flash photolysis. The obtained kinetic parameters sh ow that the electron transfer from Tryptophan (Trp), Tyrosine (Tyr) as well as dipeptides containing Trp and/or Tyr residue to triplet 4NQO ((T)4NQO) are efficient, but inefficient from methionine (Met) and dipeptides contain ing neither Trp nor Tyr. The result was supported by the calculated values of the fi ee energy change from measured oxidation potentials for the elect ron transfer. It was demonstrated that Trp and Tyr residues are initial rea ction sites with (T)4NQO, while Tyr/O-. radical may be final species for Tr p-Tyr dipeptide. In acidic aqueous solutions, the self-quenching rate const ants of (T)4NQO and the rate constants of electron transfer from amino acid s to (T)4NQO decrease with decreasing pH. In alkaline solutions, amino acid s are easily oxidized by 4NQO under irradiation of laser pulse, and no tran sient absorption signal was observed.