The Photochemistry of Uridylic Acid

Studies of the action spectra for a variety of biological effects of ultraviolet irradiation have indicated in many instances that the nucleic acids are at least the immediate receptors of the effective radiations (1, 2). Relatively few studies have been made, however, of the effects of ultraviolet irradiation on the nucleic acids or their components. Studies of the effects of irradiation on the free purine or pyrimidine bases of the nucleic acids (3) are of limited value because, as will be shown, the attachment of the pentose sugar to the heterocylic ring can profoundly modify the photochemical behavior. Studies of the effects of ultraviolet irradiation on the intact nucleic acids (4, 5) have been difficult to interpret because of the limited criteria of photochemical change which can be applied to these large molecules and the consequent necessity of using doses of radiation considerably in excess of those adequate to produce the biological effects. In this and a subsequent paper, the results are presented of the effects of ultraviolet irradiation on mononucleotide components of nucleic acids, in the hope that these results may provide a basis for extrapolation to the effects of ultraviolet irradiation on the polymeric nucleic acids. As many of the biological effects of ultraviolet irradiation have been shown to be, at least in part, reversible by light or heat (6-8), particular attention has been paid to certain effects of ultraviolet irradiation on nucleotides which have been found to be of such a nature that they are readily reversed.