DNA MEDIATED ELECTRON TRANSFER REACTIONS

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The overall objective of the proposed research is to delineate how the DNA base pair stack may mediate long range charge transport. The consequences of long range radical migration through DNA are important to consider with respect to understanding carcinogenesis and to developing new chemotherapeutic strategies. Long range electron transfer chemistry in DNA will be probed using well-defined duplex assemblies with well stacked donors and acceptors. Studies in the current grant period with intercalators as probes have shown that DNA- mediated electron transfer can proceed with a shallow dependence on distance but a high sensitivity to base pair stacking. Assemblies will be constructed in which the donor/acceptor pair are (i) both intercalators, (ii) one intercalator, one modified base, or (iii) two modified bases. The tethered intercalators include phenanthrene quinone diimine of rhodium, dipyridophenazine complexes of ruthenium and osmium, and ethidium. The modified bases include 2-aminopurine, deazaguanine, and oxoguanine. Fluorescence and transient absorption experiments will be conducted to determine rates of electron transfer as a function of distance, sequence, and driving force. Ru(II)/Os(II) modified assemblies will be prepared to examine triplet energy transfer, and Ru(II)/Os(III) assemblies will be used to test for intervalence charge transfer. DNA assemblies containing a tethered Ru(III) intercalator and a tethered Lys-Trp-Lys will be constructed as a model to explore protein-DNA electron transfer. By considering double helical DNA not only as a bridge but also as a reactant, in the past grant period, we exploited the DNA base pair stack to carry out chemistry at a distance, where oxidation is promoted from a remote site to damage or repair DNA. Thus we propose to couple spectroscopic and biochemical assays using defined DNA assemblies to examine how oxidative damage to DNA occurs at a distance. Oxidative DNA damage initiated from a remote site using an intercalating oxidant will be characterized through variations in intervening DNA sequence, structure, and conformation. The effects of protein-induced changes in the base pair stack, notably base flipping, on long range oxidative damage will also be examined in an effort to develop solution assays for protein-promoted helix distortions. Additionally, oxidative damage to DNA will be compared on DNA fragments and within nucleosomes to determine how DNA packaging affects long range charge transport. Our long term objective is to establish whether charge transport through DNA is physiologically important in the long range transmission of chemical information.