Electron delocalization in pyrimidine dimers and the implications for enzyme-catalyzed dimer cycloreversion

Photo-CIDNP study of the splitting of the dinucleotidecis, syn-thymine dimer with reduced flavin as a sensitizer: Evidence for a thymine radical anion intermediate

“…On the basis of molecular orbital and thermodynamic calculations it has a P.J.W. Pouwels been rationalized that the cyclobutyl bonds in the dimer radical anion are more labile than those in the neutral dimer [17,18], which leads to dissociation into dTpdT-', the corresponding monomer and monomer radical anion. Back electron transfer ~om monomer radical anion to the tryptophan radical cation produces the diamagnetic product dTpdT, at which moment repair has been achieved.…”

Dna Repair Enzymes

“…Recent observation of energy transfer from the pterin chromophore to F ADH2 makes the former mechanism more likely (40). Regardless of which chromophore actually donates the electron to the dimer, this model predicts the formation of a pyrimidine dimer anion, which is known to be unstable and to collapse to pyrimidine monomers (43). Recent model studies and molecular orbital cal culations suggest that C4 of the dimer is a likely site for electron donation and that the carbanion fomled as a consequence would favor nonsynchronous cleavage (43) of the cyclobutyl bonds at C5 and C6 ( Figure I).…”

Splitting of Cis-syn Cyclobutane Thymine-thymine Dimers by Radiolysis and Its Relevance to Enzymatic Photoreactivation

“…Enzymes contain an antenna molecule (5,10-methenyl-tetrahydrofolate or an 8-hydroxy-5-deazaflavin) which absorb a photon and transfers energy to FADH 2 at the catalytic centre, which in turn transfers an electron to (equation 1 a) or from (equation 1 b) the pyr < > pyr dimer to generate a radical ion pair, resulting in splitting (quantum yield of repair=0 .4-0 .6) followed by charge neutralization . Molecular orbital calculations predict that both the pyr < > pyr cation radical (Pabon and Bauld 1984) and anion radicals (Hartmann et al 1987) are prone to decay to their constituent monomers by ring splitting. Recent studies employing picosecond flash photolysis (Okamura et al .…”