The effect of packing and conformation on free radical yields in films of variably hydrated DNA.

This work investigates the direct-type action of radiation (involving electron addition and electron abstraction) on DNA. Specifically, the effects of DNA hydration, conformation and packing on free radical yields are examined. The fact that these variables are interdependent complicates the analysis of how each variable affects free radical yields. The hydration dependence of free radical yields in films of both Li and Na DNA was examined. At low levels of DNA hydration (less than 25 waters per nucleotide), the relatively high free radical yields and the lack of water-derived radicals are evidence that damage transfer from the DNA solvation shell to the DNA molecule occurs. The scatter of measured free radical yields is significant (50-70%) in Li DNA, while in Na DNA it is much less (<25%). Our conclusions hinge upon two known differences between Li DNA and Na DNA: (1) At low DNA hydrations, the conformation of Na DNA undergoes several changes with increasing hydration, while the conformation of Li DNA is relatively constant over the same range. (2) Compared to Na DNA, Li DNA is more prone to self-associate, giving rise to macroscopic and microscopic crystalline domains in Li DNA films. The greater scatter of free radical yields in Li DNA films is therefore attributed to variability in packing. By virtue of the greater reproducibility of free radical yields in Na DNA films, the effects of DNA packing, conformation and hydration can be ascertained. In Na DNA, hydration-dependent changes in free radical yields are attributed primarily to changes in DNA packing.