Free Radicals in Crystals of Sulfur-Containing Purines and Pyrimidines. 2-Thio-5-Aminocytosine and 2-Mercaptobenzimidazole

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Abstract
Single crystals of 2-thio-5-aminocytosine.cntdot.2HCl.cntdot.2H2O (2T5AC) and 2-mercaptobenzimidazole (2MBI) were irradiated at 77.degree. and 295.degree. K and the induced radicals were studied by ESR. At 77.degree. K, a resonance with a large g-value variation (between 1.99 and 2.18) was observed in both compounds. The resonance is assigned to a cation or a deprotonated cation radical. At 295.degree. K another resonance was observed with principal g values typical for the S radicals observed at elevated temperatures in S-containing amino acids and proteins (about 2.060, 2.025 and 2.002). The present data are consistent with a perthiyl RSS radical. Other radical species, formed by H abstraction from the NH2 group at C4 in 2T5AC, by hydrogen atom addition to S in 2T5AC, and by H addition to the benzene part of 2MBI, were also observed. Their ESR properties are discussed in relation to the proposed radical models.

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