PHOTOLESIONS IN DNA UPON 193 NM EXCITATION

Aqueous solutions of plasmid (pBR322 and pTZ18R) and calf thymus DNA were excited by 20 ns laser pulses at 193 nm. The quantum yields of single- and double-strand break formation, interstrand cross-links, locally denatured sites, (6-4)photoproducts and biological inactivation (PHI(ssb), PHI(dsb), PHI(icl), PHI(lds), PHI6-4 and PHI(ina), respectively) were measured. The quantum yields are virtually independent of intensity, demonstrating a one-quantum process. The obtained values in aerated neutral solution in the absence of additives are PHI(ssb), almost-equal-to 1.5 x 10(-3), PHI(dsb) almost-equal-to 0.06 x 10(-3) (dose: 10-200 J m-2), PHI(icl) almost-equal-to PHI(lds) almost-equal-to 0.1 X 10(-3) and PHI(6-4) = 0.5 x 10(-3) Both PHI(ssb) and PHI(dsb) decrease strongly with increasing concentrations of TE buffer (0.01-10 mM). Biological inactivation of the pTZ18R plasmid was determined from the transformation efficiency of Escherichia coli bacteria strains AB1157, AB1886 uvr and AB2480 uvr rec; the PHI(ina) values are 1.4 x 10(-3), 2.1 x 10(-3) and 3 x 10(-3), respectively. The monoexponential survival curves in all cases show that a single damage site leads to inactivation (one single hit). The biological consequences of different photoproducts are discussed.