Photochemistry of Nucleic Acids and Related Compounds

Smith and Aplin (1966) have irradiated a solution of uracil (2.8 x 10 M) and cysteine HCl (10 M) with ultraviolet light (2537 A). The irradiation produced a heterodimer, 5-S-cysteine-6-hydrouracil, the structure of which was determined by ultraviolet, infrared, NMR, and mass spectroscopy. This compound may serve as a model for the mechanism by which DNA and protein are cross-linked in vivo by 

Cysteine had also been found to add photochemically to poly U, poly C, and DNA, thus adding credence to Smith and Aplin s postulate that the formation of a heterodimer between a pyrimidine and a sulfur (or hydroxy) amino acid may constitute a mechanism for the photochemical cross-linking of DNA and protein in vivo (Smith, 1962, 1964 Smith et al, 1966). Infrared data suggested that the COOH and amino groups of the cysteine residue were free, implying that the linkage to the uracil skeleton was through the sulfur bond. 

Khattak and Wang (1969) have isolated two products from uracil irradiated with ultraviolet light in frozen aqueous solution. Evidence from NMR, UV, IR, and mass spectra suggest that one is a photopolymer, Uj, and the other is probably 6-4 -[pyrimidin-2 -one]-uracil. 

Fritzsche, H Dieu, N. Q and Te, N. T. unpublished results, cited in (Fritzsche, 1967a). 

Kyogoku. Y. Ph.D. Thesis. University of Tokyo. 1963. cited in Tsuboi (1969). 

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