Photochemical Reactions of Thymine, Uracil, Uridine, Cytosine and Bromouracil in Frozen Solution and in Dried Films*

Abstract

Abstract— The photochemistry of uracil, uridine, cytosine, thymine and broinouracil has been investigated in frozen aqueous solution and in dried films. Essentially the same photoproducts were obtained in the two conditions; however, the yield of photoproducts was considerably greater in frozen solution. Uracil forms a dimer which can exist in two forms. Some kinetic data are presented for the interconversion of these two forms, The mixed dimer of thymine and uracil can also exist in two forms. Uridine forms only one acid stable photoproduct and does not appear to form mixed photoproducts under the conditions used. Two new photoproducts of thymine other than the dimer are described. Cytosine was at first considered to be completely inert hut using more sensitive detecting equipment it has recently been found to form uracil dinier as a result of dinierization and deamination. The most remarkable response was shown by bromouracil. When irradiated by itself it formed no photoproducts but when irradiated in the presence of uracil, uridine, cytosine or NaOH it formed many photoproducts. Most of these products were devoid of bromide, but two still contained bromine. One of these has been identified as the mixed dimer of uracil and bromouracil while the other has been tentatively identified as the dimer of bromouracil. Dimers of thymine or bromouracil were not formed by X‐rays.

The photodecomposition of bromacil

“…Bromacil's stability to photooxidation is unexpected in view of the normally reactive secondaryalkyt group and the reported photolytic dealkylation of the analogous thymine derivative, 1,3,5-trimethyturacil, in aqueous solution (Atcantara and Wang 1965). The facile addition of water to the 5,6~ouble bond of 1,3-dimethyl-5-fluorouracil followed by dehydrohalogenation (Fikus et al 1964;Fikus et al 1965;Lozeron et al 1964), and the formation of 5-bromouracil dimers (Smith 1963(Smith , 1964 in irradiated aqueous media make the limited reactivity of bromacil even more surprising; differences in the energy of irradiation again may be responsible. Since N-dealkylation was the only type of reaction observed in our experiments, an unstable intermediate carbinolimide such as II would be expected and has been isolated as a product of the metabolism of bromacil by plants (Gardiner et al 1969a) and animals (Gardmer et al 1969b).…”

“…The major product, 6-methyluracil (V), represented a 37% yield. A substance eluting at very high temperature (230°C) provided a mass spectrum identical to that of III, suggesting dissociation of dimer (Smith 1963(Smith , 1964. Consequently, III can be expected to appear at only very low levels during photolysis of I, and the related 5-bromouracit which has been reported to be carcinogenic (McGahen and Hoffman 1963a, 1963b, 1966 was never observed.…”

Comparison of UV-Induced Inactivation and RNA Damage in MS2 Phage across the Germicidal UV Spectrum

“…RNA differs from DNA in composition primarily by the presence of uracil nucleotides instead of thymine. UV irradiation forms several RNA photoproducts, primarily from adjacent pyrimidine nucleotides, such as uracil dimers (20,21), as well as RNA-protein cross-links (22). These lesions, which inhibit RNA synthesis primarily through the formation of 6-4 photoproducts (23), also block the reverse transcriptase enzyme (4) from transcribing strands of cDNA, making the reverse transcription-quantitative PCR (RT-qPCR) assay suitable for detecting damage to the viral RNA.…”

Further Studies on the Uv-Induced Photoreversible and Non-Photoreversible Mutation Through the Action Spectra in Yeast Cells

“…Such a possibility had been speculated by us in vivo system , when an unexpectedly high photoreversion rate was found in mutation. However, because the relative yield of CC, and hence conversion to uracil dimer, appears to be considerably less if compared with TT (Smith 1963 ;Setlow et al 1965), the sole contribution of CC to mutation production, consisting of all of the photoreversible component, is hard to reconcile with the observed mutation cross section unless the size of the gene becomes much more expanded in terms of, especially, C,C sequence.…”