SINGLE‐ and DOUBLE‐STRAND BREAK FORMATION IN DOUBLE‐STRANDED DNA UPON NANOSECOND LASER‐INDUCED PHOTOIONIZATION

Abstract

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.

LASER‐INDUCED STRAND BREAK FORMATION IN DNA and POLYNUCLEOTIDES

“…These radicals should then lead to ssb. Other experiments, however, contradict this hypothesis Bothe et al, 1990) since addition of t-butanol as a scavenger of OH radicals does not lead to a smaller yield of strand breakage in deoxygenated solutions (Table 4). Control experiments in the presence of N 2 0 showed that @ssb is significantly larger than under argon since OH radicals, which are formed by reaction of e& with N20, contribute to strand breakage (see Section 5).…”

“…This transfer mechanism is essentially based on ps excitation measurements of thymine . For ns excitation of polynucleotides and DNA using much lower pulse intensities, this mechanism could not be substantiated Bothe et al, 1990). It should be noted that eLq does not lead to ssb, e.g.…”

“…Some values for GC-of nucleic acids are compiled in Table 1. The absorption of the resulting radical cations has not yet been observed with nucleic acids and their constituents since either the extinction coefficients are too low or the lifetimes of the radical cations under those conditions are too short (Wala et at., 1990;Bothe et al, 1990;Gorner, 1990a).…”

Quantitative determination of the selectivities of five different phenyl radicals in hydrogen atom abstraction from ethanol

“…Unlabeled ethanol (chemical purity 99.5%), and three partially labeled forms of ethanol, CD 3 The chemical purities, and the absence of any partially labeled isotopomers, were confirmed via mass spectrometric analyses of these materials. The aryl radicals were generated and isolated, and their reactivities were then studied, as described previously [12,13].…”

“…This experimental method involves studying the reactivities and selectivities of derivatives of the radicals that contain a chemically inert, charged group (distonic ions), which allows them to be manipulated in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. A large number of studies suggest that the action of some potential anti-tumor antibiotics is based on H-atom abstraction by aromatic, ,-biradical intermediates from a sugar moiety in DNA (which ultimately leads to cell death) [1][2][3][4][5][6][7][8][9]. These compounds, however, are also destructive to healthy cells.…”