INFLUENCE OF NUCLEIC-ACID BASE COMPOSITION ON RADIATION-INDUCED STRAND BREAKAGE IN SINGLE-STRANDED-DNA - A TIME-RESOLVED STUDY

The following study investigates the pathways involved in the inductio n of single strand breaks (ssb) in various samples of single stranded (ss) DNA (calf thymus, Micrococcus lysodeikticus, Clostridium perfring ens) with differing nucleic acid base composition. The time scale for the induction of ssb was determined from changes in the light scatteri ng intensity following pulse irradiation of aqueous solutions containi ng these ssDNA samples at pH7.8 under either aerated or deaerated cond itions. The induction of ssb under these conditions is predominantly b y the hydroxyl radical and shows Various kinetically distinct componen ts. The immediate ssb (t< 0.02 s) account for similar to 40-60% of the total yield of ssb. The majority of these ssb are suggested to arise from the 'common' initial attack of the hydroxyl radicals at the sugar phosphate backbone for each of the three DNA samples. Furthermore, sl ower components for ssb formation (t>0.02s) were observed and are sugg ested to occur through base radical mediated H-atom abstraction from t he sugar moiety. The half lives for formation of the majority of ssb, formed through this base radical mediated H-atom abstraction(s), are i n the range of 20-43 ms. The yields of these 'base-mediated' ssb vary markedly (under both aerobic and anaerobic conditions) and reflect the base composition of the DNA sample. It is suggested from these studie s that the OH-induced base radicals of guanine/cytosine are more effec tive precursors for strand breakage than those from adenine/thymine in ssDNA.