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

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Abstract
Following earlier studies on the .gamma.-radiolysis of dAMP and dTMP in frozen aqueous solution, several dAMP.cntdot.dTMP costacking complexes, at different [dAMP]:[dTMP] molar ratios, were 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 2 interrelated parts. The 1st part deals with radical populations induced in the dAMP.cntdot.dTMP complexes by photosensitization, these being essentially radical anions and their conversion products, H-addition radicals. The 2nd part deals with the more complex populations induced by .gamma.-rays. With .gamma.-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 1 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: A-T .fwdarw. AT-. The existence of a short-range hole transfer mechanism was also detected: AT+ .fwdarw. A+T. The presence of a stacking conformation in the complexes is a necessary condition for these charge migration phenomena to occur. 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.