PHOTOREACTIONS AT 3655Å BETWEEN PYRIMIDINE BASES AND SKIN-PHOTOSENSITIZING FUROCOUMARINS

Abstract After irradiation at 3655 Å of an aqueous frozen solution containing thymine and psoralen, a new photocompound was isolated by column chromatography. It contains a furocoumarin and a pyrimidine-moiety linked together by the formation of a cyclobutane ring (see formulas II and III). By irradiation at 2537 Å in acetic acid solution, the photocompound breaks up again yielding psoralen and thymine. From an aqueous frozen solution containing cytosine and psoralen irradiated at 3655 Å, an analogous photocompound was obtained, which, however, consists of the addition to psoralen of a uracil molecule, instead of a cytosine one (IV, V). It has been stated that the hydrolytic deamination of the cytosine moiety to the uracil one takes place during the working up of the photocompound in aqueous solution after irradiation. Substances with properties similar to those above were obtained from bergapten (5-methoxy-psoralen) and thymine, from psoralen and thymidine or thymidylic acid, irradiated at 3655 Å.
The new substances may be considered as model compounds in explaining the photoreactions which take place between the skin-photosensitizing furocoumarins and DNA upon irradiation at 3655 Å.