Sequence dependent long range hole transport in DNA

A guanine radical cation (G(+.)) was site-selectively generated in double s tranded DNP, and the charge transfer in different oligonucleotide sequences was investigated. The method is based on the competition between a charge transfer from G(+.) through the DNA and its trapping reaction with H2O. We analyzed the hole transfer from this G(+.) to a GGG unit through one, two, three, and four AT base pairs and found that the rate decreases by about 1 order of magnitude with each intervening AT base pair. This strong distance dependence led to a beta-value of 0.7 +/- 0.1 Angstrom(-1). Within the tim e scale of this assay the charge transfer nearly vanished when the G(+.) wa s separated by four AT base pairs from the GGG unit. However, if the second or the third of the four intervening AT base pairs was exchanged by a GC b ase pair, the rate of the hole transfer from the G(+.) to the GGG unit incr eased by 2 orders of magnitude. In addition, a long-range charge transfer o ver 15 base pairs could be observed in a mixed strand that contained AT as well as GC base pairs. Because G(+.) can oxidize G but not A bases, the lon g-range charge transport can be explained by a hopping of the positive char ge between the intervening G bases. Thus, the overall charge transport in a mixed strand is a multistep hopping process between G bases where the indi vidual steps contribute to the overall rate. The distance dependence is no longer described by the beta value of the superexchange mechanism,