PHOTO-CIDNP STUDY OF THE SPLITTING OF THE DINUCLEOTIDE CIS,SYN-THYMINE DIMER WITH REDUCED FLAVIN AS A SENSITIZER - EVIDENCE FOR A THYMINE RADICAL-ANION INTERMEDIATE

Photochemically induced dynamic nuclear polarization (photo-CIDNP) stu dies were performed on the splitting of cis,syn 2'-deoxythymidylyl-(3' -->5')-2'-deoxythymidine cyclobutane dimer (cs-dTp[]dT), using reduced flavin as a sensitizer. This system serves as a model for the light-i nduced repair mechanism of thymine dimers in DNA by the enzyme photoly ase. The CIDNP spectrum shows enhanced absorption of the two C6-H prot ons of the corresponding monomer dTpdT, which demonstrates that a thym ine radical anion is involved in the splitting of the dimer. This is s upported by a similar CIDNP spectrum that is obtained with the electro n-donor N(alpha)-acetyl tryptophan as a sensitizer. This result sugges ts that the light-induced splitting of thymine dimers in DNA by photol yase also proceeds via the thymine radical anion. The small difference in intensity of the two CIDNP signals belonging to the C6-H protons s hows that the unpaired electron in the monomer radical has a slight pr eference for the thymine moiety at the 5' terminus.