On the efficiency of hole and electron transfer from the hydration layer to DNA: An EPR study of crystalline DNA X-irradiated at 4 K

The aim of this project was to gain an improved understanding of how the ef ficiency of hole and electron transfer from the solvation layer to DNA decr eases as a function of distance from DNA. The packing of DNA in crystals of known structure makes it possible to calculate the degree of DNA hydration with a precision that is significantly greater than that achievable for am orphous samples. Previous work on oligodeoxynucleotide crystals has demonst rated that the efficiency of free radical trapping by DNA exposed to ionizi ng radiation at 4 K is relatively insensitive to base sequence, conformatio n, counterion, or base stacking continuity. Having eliminated these confoun ding variables, it is now possible to ascertain the degree of radical trans fer that occurs from ionized water as a function of DNA hydration (Gamma, i n mol water/ mol nucleotide). EPR is used to measure the hydroxyl radical c oncentration in crystals irradiated at 4 K. From a lack of hydroxyl radical s trapped in the inner hydration mantle, we determine that hole transfer to DNA is complete for water molecules located within 8 Angstrom. This corres ponds to Gamma = 9-11 and indicates that hole transfer is 100% (as efficien t as direct ionization of DNA) for water molecules adjacent to DNA. Beyond similar to 8 Angstrom (Gamma > 10), hydroxyl radicals are observed; thus de protonation of the water radical cation is seen to compete with hole transf er to DNA as soon as one water intervenes between the ionized water and DNA . The boundary for 0% hole transfer is projected to occur somewhere between 15 and 20 waters per nucleotide. Electron transfer, on the other hand, is 100% efficient across the entire range studied, 4.2 less than or equal to G amma less than or equal to 15.6. (C) 2000 by Radiation Research Society.