Electron and hole transfer induced by thermal annealing of crystalline DNAx-irradiated at 4 K

Recent models for long range (>2 nm) transfer of electrons and holes throug h DNA suggest a mechanism that is neither a single long distance tunneling event nor a mechanism strictly due to hopping, but a mixture of the two. Fr om results reported here we argue that any complete model of electron or ho le transfer in DNA should include the effects of reversible proton transfer . Reversible proton transfer (primarily between guanine-cytosine base pairs ) influences the ability of DNA to trap free radicals, which in turn affect s the migration of holes and electrons. We present the annealing characteri stics of electrons and holes trapped in crystalline oligodeoxynucleotides i rradiated at 4 K and annealed stepwise to room temperature (RT). The anneal ing profiles are relatively insensitive to DNA conformation, sequence, or b ase stacking continuity. The packing of the DNA duplexes is known, and it i s readily shown that electron and/or hole transfer must be intermolecular. The distances required for tunneling between separate molecules are found t o be comparable to the distances required for tunneling within a DNA duplex . The annealing characteristics of DNA are considerably different than thos e found in crystals of alpha-Me-mannoside, 5'dCMP, and 1-Methylcytosine:5-F luorouracil. This difference is ascribed to a mechanism wherein reversible proton transfer is a rate-limiting step for electron/hole transfer. Reversi ble proton transfer is, thereby, a "gate" for electron/hole transfer (via t unneling). Because reversible proton transfer is thermally activated, it is proposed that the energetics of this transfer is a dominant factor in dete rmining the thermal annealing profile of DNA. The competing reactions that govern electron/hole migration created by annealing samples irradiated at 4 K; are applicable to electron/hole migration at RT. Evidence for this come s from the observation that, in a number of DNA crystals, the free radical species and radical yields are very similar to crystals irradiated at RT co mpared to those irradiated at 4 K followed by annealing to RT. The proposed mechanism for electron and hole migration through DNA is one where short t ransfers (similar to less than or equal to 1 nm) occur by tunneling, and tu nneling is gated by reversible proton-transfer.