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PHOTOCHEMICAL REACTION BETWEEN PROTEIN AND NUCLEIC ACIDS: PHOTOADDITION OF TRYPTOPHAN TO PYRIMIDINE BASES *1
Author(s) -
Reeve A. E.,
Hopkins T. R.
Publication year - 1980
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1980.tb02544.x
Subject(s) - chemistry , tryptophan , pyrimidine , thymine , uracil , photochemistry , indole test , amino acid , stereochemistry , biochemistry , dna
— The photochemical interactions between tryptophan and nucleic acid bases were studied. When aqueous solutions of tryptophan were irradiated (Λ > 260 nm) at neutral pH in the presence of each of the nucleic acid bases, pyrimidine bases but not purine bases were altered. Air was found to decrease the rate of reaction. Two classes of photoproducts were isolated by thin layer and ion‐exchange chromatography and partially characterized. One was the dihydro‐pyrimidine forms of the base (see Reeve and Hopkins, 1979) and the other consisted of tryptophan‐pyrimidine photoadducts. Four tryptophan‐uracil and two tryptophan‐thymine adducts each with a 1:1 molecular stochiometry were found. Spectroscopic measurements and a positive reaction with Ehrlich's reagent indicate that the indole nucleus remained intact, but that the pyrimidine base was reduced at the 5, 6 double bond. The absence of a positive ninhydrin reaction and the effect of pH on the quantum yield of the photoadduct formation indicated that the ionized a‐amino acid group of tryptophan was involved in photoadduct formation. Indole derivatives lacking an a‐amino group were also found to form photoadducts with pyrimidine bases. The experimental results are consistent with a reaction mechanism involving tryptophan excitation to the first excited singlet state as the initial event. Radical scavenging experiments indicate that tryptophan free radicals, formed by electron dissociation from the excited state, react with the ground state pyrimidine.