The dinucleoside monophosphates d(TpG), d(TpC), and d(TpT) were X-irradiated in oxygenated solution. In each case the modification of the dinucleoside in which the thymine base is degraded to a formamido remnant was observed as a principal product. The hydrolysis of the phosphoester bond of formamido-modified dinucleosides is much slower than that of the corresponding unmodified dinucleosides. This effect is also observable in the hydrolysis of irradiated DNA, where hydrolysis by nuclease P1 (plus acid phosphatase) generates the modified dinucleosides${\rm d}({\rm T}^{{\rm F}}{\rm pN}),{\rm T}^{{\rm F}}$ being the modified thymidine. The total yield of the formamido lesion in all its forms,${\rm d}({\rm T}^{{\rm F}}{\rm pN})$, exceeds the yield of any other base modification.

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