HEMATOPORPHYRIN-SENSITIZED DEGRADATION OF DEOXYRIBOSE AND DNA IN HIGH-INTENSITY NEAR-UV PICOSECOND PULSED-LASER PHOTOLYSIS

The photosensitized degradation of deoxyribose and DNA, using hematopo rphyrin (HP) and picosecond laser pulses at high intensities (pulse du ration 30 ps, lambda(exc) = 355 nm, light intensity range 10(8)10(10) W/cm(2)) was studied. Aldehyde formation from 2-deoxy-D-ribose and lon g-chain double-stranded DNA, when analyzed as a function of light inte nsity, followed a non-linear dependence, suggesting the involvement of multiphoton light absorption by HP. The degradation mechanism was stu died by analysis of the yield dependence on excitation intensity and t he effect of added radical scavengers. The participation of OH radical s in the degradation process was confirmed by spin trapping techniques . At low light intensities added N2O largely increased product formati on, suggesting that HP photoionization predominates under these condit ions. At higher intensities (I greater than or equal to 3 GW/cm(2)) th e product yield was not affected by N2O which, combined with spin trap ping data, suggested that OH radical formation occurred, but that neit her HP photoionization nor peroxy radical formation was involved. Sing le and double strand breaks in supercoiled plasmid DNA (pBR 322) confi rmed the generation of OH or OH-like radicals during high-intensity ex citation of HP. A mechanism involving a multistep excitation of HP, fo llowed by resonance energy transfer to H2O resulting in dissociation t o yield OH and H atoms, is proposed.