Charge Migration Phenomena in γ-Irradiated Costacking Complexes of DNA Nucleotides: I. A Computer-Assisted ESR Analysis of dAMP · dTMP Complexes in Frozen Solution

Following earlier studies on the γ-radiolysis of dAMP and dTMP in frozen aqueous solution, several dAMP·dTMP costacking complexes, at different [dAMP]:[dTMP] molar ratios, have now been investigated using the same computer-assisted technique. The purpose of this investigation was to determine whether single-radical reaction kinetics proceed in these complexes in the same way as in the individual constituents when irradiated separately, or whether intermolecular spin-transfer phenomena take place. This work embodies two interrelated parts. The first part deals with radical populations induced in the dAMP·dTMP complexes by photosensitization, these being essentially radical anions and their conversion products, H-addition radicals. The second part deals with the more complex populations induced by γ rays. With γ rays, the simultaneous presence of both the anionic and the cationic kinetics makes the ESR results rather involved and application of the computer technique becomes imperative. In both cases one prominent fact was discovered. When dAMP is allowed to interact by stacking with dTMP, H-addition radicals on dAMP are never formed, even when [dAMP]:[dTMP] molar ratios are as high as 10:1. Conversely, the relative concentrations of H-addition radicals on dTMP are increased by corresponding factors. This is likely to be due to long-range electron transfer phenomena involving adenine anions and intact thymine molecules:${\rm A}^{-}{\rm T}\rightarrow {\rm AT}^{-}$. Additionally, the existence of a short-range hole transfer mechanism was also detected:${\rm AT}^{+}\rightarrow {\rm A}^{+}{\rm T}$. The presence of a stacking conformation in the complexes is a necessary condition for these charge migration phenomena to occur. Indeed, when the constituent molecules are dispersed in a glassy matrix, the overall radical population is simply the sum of the radical populations formed in dAMP and dTMP when irradiated separately.