Photosensitized Reduction of Pyrimidine Bases by Tryptophan*

Abstract

Thymine and uracil were chemically altered when irradiated with UV light in aqueous solution containing tryptophan as a photosensitizer. The reaction is inhibited by oxygen and is therefore not an example of photodynamic action. Unlike the pyrimidine bases, purine bases were not altered under similar reaction conditions. Two major photoproducts were identified. One of the products was identified as the reduced base, dihydrouracil or dihydrothymine. The quantum yield for formation of dihydrouracil was 0.24 x depending upon the concentrations of uracil and the reaction temperature. Radical scavengers such as cysteine and nitrous oxide decreased the rate of dihydrouracil formation. Other indole compounds also sensitized the photoreduction of uracil, their quantum yields ranging from < 1 x lo-' for tryptamine to 1.3 x for indole-3-acetic acid. A reaction mechanism is presented whereby the pyrimidines are reduced by electron transfer from a metastable charge transfer complex originating from the first excited singlet state of tryptophan.to 13.6 x

Photochemical Reactions Between Tryptophan and Polynucleotides*

“…The order of reactivity of the polynucleotides towards 14C-trp photoaddition was similar to that found using free pyrimidine bases (Reeve and Hopkins, 1979). These workers reported that free purine bases were unreactive.…”

“…In the absence of oxygen, however, the near-UV destruction of trp proceeds at a greatly diminished rate (Borkman, 1977). This report indicates that photoaddition of trp and perhaps secondary compounds such as trp photopolymers (Reeve and Hopkins, 1979) to synthetic and natural polynucleotides is possible in anaerobic conditions. It is clear that the mechanism@) are more complex than that suggested from the model study of Reeve and Hopkins (1979).…”

“…Many of the biological effects of UV irradiation can be explained in terms of specific photochemical changes t o D N A and RNA. Recent model studies of Reeve and Hopkins (1979) have shown that the UV irradiation of solutions containing trp and pyrimidines resulted in the formation of trp-pyrimidine photo adducts. A logical extension of these model studies is t o determine if excited-state trp can also covalently bind with polynucleotides.…”

The Effect of Tryptophan on UV‐induced DNA Photodamage

“…There have been a number of studies on the photochemistry of nucleic acids and their components in the presence of Trp (6,9–12). Reeve and Hopkins (6,9,10) and Saito et al.…”

“…There have been a number of studies on the photochemistry of nucleic acids and their components in the presence of Trp (6,9–12). Reeve and Hopkins (6,9,10) and Saito et al. (11,12) irradiated the pyrimidine bases, and later the polynucleotides (12), in the presence of Trp with UV light in the range of 270–310 nm and putatively isolated two classes of photoproducts, the reduced base dihydrothymine and the Trp–thymine adduct.…”