Hole size and energetics in double helical DNA: Competition between quantum delocalization and solvation localization

The transition between single step long-range tunneling and multistep hopping transport in DNA electron transfer depends on a myriad of factors including sequence, distance, conformation, solvation and, consequently, hole state energetics. We show that the solvation energetics of hole (radical cation) states in DNA is comparable to the quantum delocalization energetics of the hole. That is, the solvation forces that tend to localize the hole compete with the quantum effects that give rise to hole delocalization. The net result is that the hole states are predicted to be relatively compact (one to three base pairs in length) and that the "trap depth" of these holes is expected to be much shallower than anticipated by gas-phase quantum chemical analysis of base stacks. This analysis predicts guanine oxidation potential dependence on the length of GC runs to be modest (differences < 0.1 V for holes from one to three base pairs). The lowering of the trapped hole binding energy has significant implications for the structure and mobility of hole states in DNA.