Photochemistry of pyrimidine bases as studied by e.s.r. and spin-trapping

The direct photoexcitation of pyrimidine bases in D2O solutions yields free radicals which could be conveniently identified by spin-trapping with 2-methyl-2-nitrosopropane. Most of the radicals formed were attributed to D-addition to one end of the 5,6 double bond. However, orotic acid and iso-orotic acid yielded N(3) centred free radicals, formed by homolytic cleavage of the N-H bond. No indication could be found for a free radical involvement in the photocleavage of cyclobutane-type pyrimidine dimers.