The photochemistry of uracil: a reinvestigation.

Results from a re-examination of the photochemical reactions undergone by uracil (Ura) are presented. Irradiation of Ura in frozen aqueous solution at -78.5°C produces two diastereomeric (6-4) products, namely the cis and trans isomers of 5-hydroxy-6-4'-(pyrimidin-2'-one)-5,6-dihydrouracil. Upon heating in 0.1m HCl, each of these compounds decomposes to form 6-4'-(pyrimidin-2'-one)uracil. In addition, evidence for production of a hydrate of a trimer of Ura is presented, Irradiation of this compound at 254nm forms Ura and a (6-4) adduct as products. The compounds 5-4'-(pyrimidin-2'-one)uracil and 6-hydroxy-5,6-dihydrouracil were also present after Ura was irradiated in frozen aqueous solution. Cyclobutane dimers (CBDs) are formed when Ura is irradiated in the frozen aqueous state, in fluid aqueous and acetonitrile solution and in the presence of photosensitizers (e.g. acetone). Published information, concerning the identity and relative quantitative importance of the four CBD isomers (cis-syn, cis-anti, trans-syn and trans-anti) formed in these photochemical systems, is incomplete and often in substantial disagreement. Using chemical methods in conjunction with HPLC, the identity and relative amounts of the four dimers have been determined in each of these systems. Consequently, a number of inconsistencies found in the literature concerning dimer product identity and quantitative distribution have been resolved.