A POSSIBLE CHAIN-REACTION IN PHOTOSENSITIZED SPLITTING OF PYRIMIDINE DIMERS BY A PROTONATED, OXIDIZED FLAVIN

Pyrimidine dimer cycloreversion was photosensitized by protonated 2',3',4',5'-tetraacetylriboflavin (ac4rfH+), whereas the protonated flavin radical (ac4rfH2.+) failed to sensitize detectable dimer splitting. In acetonitrile containing 0.075% perchloric acid, the ac4rf absorption bands at 375 nm and 445 nm are replaced by a new band at 390 nm due to ac4rfH+. Irradiation of ac4rfH+ in the presence of cis,syn- or trans,syn-1,3-dimethyluracil dimer resulted in efficient cycloreversion to 1,3-dimethyluracil, probably induced by electron abstraction from the dimer by ac4rfH+ to produce the dimer radical cation. The quantum yeilds of splitting were large (PHI(spl) = 0.35 and 1.5 for cis,syn and trans,syn isomers, respectively, at [dimer] = 1.2 mM). Splitting quantum yields increased with dimer concentration in a quadratic fashion. These results are consistent with the occurrence of a chain reaction propagated by monomer.+ + dimer --> monomer + dimer.+ in the case of the dimethyluracil dimers. The fluorescence of the protonated flavin (lambda(em) = 507 nm) was found to be quenched by the dimers. In the presence of small amounts of added water, however, both the fluorescence quenching and splitting were abolished. Two cis,syn dimers in which the N(1)- and N(1')-atoms were tethered via a trimethylene bridge exhibited quantum yields of approximately 0.02 at the above dimer concentration. A compound in which ac4rf is covalently bound to a pyrimidine dimer was also found to undergo splitting in acidified acetonitrile (PHI = 0.03). These values are more than 1 order of magnitude greater than those reported for unprotonated flavins.