STRAND BREAKAGE IN POLY(C), POLY(A), SINGLE-STRANDED AND DOUBLE-STRANDED DNA INDUCED BY NANOSECOND LASER EXCITATION AT 193 NM

Single- and double-stranded calf thymus DNA and two polynucleotides (0.4 mM) were studied in aqueous solution at pH almost-equal-to 7 using pulsed, 20 ns laser excitation at 193 nm. Monophotonic ionization of the nucleic acids is suggested from the linear dependences of the concentration of ejected electrons and the number of single- and double-strand breaks (ssb, dsb, respectively) on laser intensity (I(L)) in the range (0.2-3) x 10(6) W cm-2. The quantum yields of formation of hydrated electrons (PHI(e-)) and ssb and dsb ((PHI(ssb) and PHI(dsb)) are therefore independent of I(L). In contrast, under 248 nm excitation these quantum yields increase linearly with I(L) under otherwise comparable conditions. Nevertheless, several effects and mechanistic implications are analogous using lambda(exc) = 193 and 248 nm. For polycytidylic acid, poly(C), in Ar-saturated solution for example, the efficiency of ssb per radical cation (eta(RC) = PHI(ssb)/PHI(e-)) is similar to the efficiency of ssb per OH radical (eta(OH)). For polyadenylic acid, poly(A), and single- and double-stranded DNA eta(RC) (lambda(exc) = 193 nm) is significantly smaller than eta(OH). The ratio PHI(ssb)(N2O)/PHI(ssb)(Ar) is almost-equal-to 2 for poly(C), almost-equal-to 4 for poly(A) and almost-equal-to 10 for DNA; the conversion of hydrated electrons into OH radicals in N2O-saturated solution and smaller eta(RC) than eta(OH) values in the case of DNA account for these results. For double-stranded DNA PHI(dsb) does not depend on I(L) but increases linearly with the dose, indicating an accumulative effect of two ssb to generate one dsb. The critical distance for this event is 60-85 phosphoric acid diester bonds.