A Reversible Photochemical Alteration of Uracil and Uridine

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Abstract
Irradiation of uracil soln. (6.2 ug./ml. in 001 [image] PO4 buffer at pH 7) for 16 hrs. with u.-v. light of 2537 A. resulted in destruction of 63% of the absorption of the uracil at 2590 A. If the pH of the irradiated uracil soln. was shifted to 1 by addition of HC1, the absorption at 2590 A rose exponentially, following lst-order kinetics, with a rate constant of 14 min. at room temp.; in this way 74% of the initial absorption could be recovered. Absorption spectra, and shifts in same upon shifting pH to 11.4 indicate that the recovered absorption is actually due to uracil. Uridine showed similar responses, but was 16 times as labile to u.-v. as uracil. Thymine, cytosine, guanine, adenylic acid, and guanylic acid were not decomposed under these conditions.