engleski

Radiation-Induced Formation of Sigma Radicals in Crystals of Thiothymine

Thio-nucleobases have been used as intrinsic photolabels to probe the structure and organization of RNA molecules in solution and to identify the contacts between nucleic acids and proteins in nucleoprotein complexes. The thio-analogs of the nucleic-acid bases absorb light at long wavelengths and could be selectively photoactivated into the electronic triplet state, that leads to their high affinity to crosslink to other nucleic-acid bases and amino-acid residues in contact. Here we report that thio-nucleobases behave different from the regular nucleic-acid bases also when irradiated by radiation of much higher energy (gamma-rays). In irradiated crystals of regular nucleic-acid bases only pi-type radicals have been observed. In crystalline thio-analogs of the nucleic-acid bases also sigma-type radicals are formed. The EPR features of a sigma-type radical in gamma-irradiated single crystal of thiothymine has been studied in detail. The characteristics of the g and A(14N) tensors are consistent with the idea that the radical is formed by a loss of an electron from thiothymine and subsequent deprotonation at N1.The unpaired electron is shared by the free sp2 hybrid orbital on N1 and the non-bonding p orbital at S.

EPR; free radicals; sigma radicals; thio-nucleobases

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