A new type of tandem base lesion has been observed in d(CpGpTpA) X-irradiated in aqueous solution. The lesion is attributed to the formation of a covalent bond between the C8 carbon atom of guanine and the methyl carbon atom of thymine. This tandem base lesion is formed in the absence of oxygen. It is the main product produced by ionizing radiation under these conditions.

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Free radicals interact with DNA bases to produce secondary radicals. The secondary radicals are reactive species and tend to interact with neighboring bases, resulting in DNA lesions with two adjacent modified bases. In this study the DNA oligomers d(CpApTpG) and d(CpGpTpA) were exposed to free radicals generated in anoxic aqueous solution by X irradiation. Four new lesions were identified in which adjacent guanine and pyrimidine bases are covalently bonded. One of the tandem lesions formed in d(CpGpTpA) has the C5 carbon atom of cytosine covalently bonded to the C8 carbon atom of guanine. Interestingly, the same bond is formed between the terminal bases in d(CpApTpG), resulting in a cyclized molecule.

Two oxidation products and one reduction product were identified .in single crystals of tyrosine Hel irradiated at 4.2"K. Both oxidation products are associated with the loss of an electron from the phenol ring of tyrosine. The reduction product is the result of electron addition to the carboxyl group. The identification of these species followed from an analysis of proton hyperfine couplings measured by the ENDOR technique.