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

Abstract

Single- and double-stranded calf thymus DNA and two polynucleotides (0.4 mM) were studied in aqueous solution at pH approximately 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 (IL) 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 IL. In contrast, under 248 nm excitation these quantum yields increase linearly with IL 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 approximately 2 for poly(C), approximately 4 for poly(A) approximately 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 IL 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.

The reactions of cytidine and 2′-deoxycytidine with SO4˙– revisited. Pulse radiolysis and product studies

“…In poly(C), the kinetics of SO 4 ؒϪ -induced strand breakage shows a ∼15% slow contribution that is very similar to that of the ؒ OH-induced reaction. 31 In the present context, the most important observation is that strand breakage induced by photoionisation 32 which has to proceed via a free radical cation is not suppressed by dioxygen in contrast to the SO 4 ؒϪ -induced reaction. 31 This is very strong evidence that a free cytosine radical cation is not involved to a major extent in the SO 4 ؒϪinduced reaction and that the reaction proceeds mainly by a nucleophilic attack by the C(2Ј)OH as depicted in reaction (9).…”

“…These results will also be relevant for the understanding of the marked mechanistic differences of strand break formation in poly(C) induced by SO 4 ؒϪ radicals 31 and by photoionisation. 32 The now available information will allow us to support but also to modify in some essential aspects the present view 23,27 on the two so differently behaving systems, 1a and 1b.…”

Irradiation of DNA with 193 nm Light Yields Formamidopyrimidine‐DNA Glycosylase (Fpg) Protein‐Sensitive Lesions

“…In our study, a very slight difference in the yield of prompt strand breakage was found for samples irradiated under aerated conditions, as compared with argon-saturated conditions ( Table 1). Gomer et al (2) found that the quantum yield of ssb induced by photolysis with 193 nm light for calf thymus DNA is 4.5 X in aerated solutions. Gurzadyan and Gomer (34) reported the quantum yield of base release from double-stranded calf thymus DNA and this is (€"baserelease (193 nm…”

“…The advent of shortwave UV lasers has enabled the study of the direct ionization processes to be studied without the interaction of the ionization products of water (1). Photoionization of aqueous solutions of DNA with 193 nm laser light results in the formation of a photoejected electron via a monophotonic process (2). The DNA radicals formed by photoionization are proposed precursors of low yields of single strand breaks (ssb)t (1,3,4), which we have recently confirmed using Rayleigh light scattering experiments (5).…”

Site-specific photosensitised modification of nucleic acids with biradical and electrophilic reagents

“…7 This review is devoted to the analysis of the most important photosensitised, oxygen-independent free-radical and electrophilic addition reactions. One-photon 17,18 and two-photon 19,20 photochemical reactions of NA induced by far UV light (190 ± 300 nm), photooxidation of DNA with singlet oxygen 3,7,21 and radical species 22 ± 24 as well as oxidation caused by photoinduced electron transfer 11,25,26 have been studied in sufficiently great detail and will not be considered in this review.…”