LASER PHOTOCHEMISTRY OF DNA - 2-PHOTON ABSORPTION AND OPTICAL-BREAKDOWN USING HIGH-INTENSITY, 532-NM RADIATION

Formation of cyclobutylpyrimidine dimers and strand breaks in double-stranded DNA was investigated by using 532-nm, 28-ps pulses from a frequency-doubled, mode-locked Nd:YAG laser at intensities below and above the threshold for optical breakdown. Two-photon absorption by DNA was detected in the absence of optical breakdown by measuring the yields of cyclobutylpyrimidine dimers formed in supercoiled pBR322 DNA. The yield of cyclobutylpyrimidine dimers per laser pulse was measured at seven peak intensities between 1.03 and 8.04 GW/cm2. A plot of the In (dimer yield/pulse) versus In (photon flux) was linear with a slope of 1.88 ± 0.26. The two-photon cross section for absorption at 532 nm was calculated to be 0.5 (±0.2) × 10-52 cm4 s photon-1 per nucleotide. Experiments performed by using intensities above the threshold for optical breakdown caused breaks in the DNA strands but no cyclobutylpyrimidine dimers. The free-radical quencher, mannitol, partially inhibited formation of the strand breaks, indicating that mechanical processes initiated by the plasma also contribute to the creation of DNA strand breaks. © 1990, American Chemical Society. All rights reserved.