Showing papers by "Jacqueline K. Barton published in 1996"

Oxidative DNA damage through long-range electron transfer

Abstract: THE possibility has been considered for almost forty years that the DNA double helix, which contains a π-stacked array of heterocyclic base pairs, could be a suitable medium for the migration of charge over long molecular distances. This notion of high charge mobility is a critical consideration with respect to DNA damage. We have previously found that the DNA double helix can serve as a molecular bridge for photo-induced electron transfer between metallointercalators, with fast rates (≥10^(10) S^(−1))^(10) and with quenching over a long distance (>40A)^8. Here we use a metallointercalator to introduce a photoexcited hole into the DNA π-stack at a specific site in order to evaluate oxidative damage to DNA from a distance. Oligomeric DNA duplexes were prepared with a rhodium inter-calator covalently attached to one end and separated spatially from 5′-GG-3′ doublet sites of oxidation. Rhodium-induced photo-oxidation occurs specifically at the 5′-G in the 5′-GG-3′ doublets and is observed up to 37 A away from the site of rhodium intercalation. We find that the yield of oxidative damage depends sensitively upon oxidation potential and π-stacking, but not on distance. These results demonstrate directly that oxidative damage to DNA may be promoted from a remote site as a result of hole migration through the DNA π-stack.

Rates of DNA-Mediated Electron Transfer Between Metallointercalators

Abstract: Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA. In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (>3 × 10^(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (∼10^(10) per second). This rate is independent of the loading of acceptors on the helix, but is sensitive to sequence and π stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.

Luminescence Quenching in Supramolecular Systems: A Comparison of DNA- and SDS Micelle-Mediated Photoinduced Electron Transfer between Metal Complexes

Abstract: Photoinduced electron transfer reactions between photoexcited Ru(phen)2dppz2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and acceptors Rh(phi)2bpy3+ and Rh(phen)2phi3+ (phi = 9,10-phena...

Os(phen)2dppz2+ in Photoinduced DNA-Mediated Electron Transfer Reactions

Abstract: The photoinduced electron transfer chemistry between Os(phen)2dppz2+ and Rh(phi)2bpy3+ bound to DNA has been characterized Os(phen)2dppz2+ serves as an isostructural analogue for Ru(phen)2dppz2+ with a red-shifted emission spectrum, access to a 3+ oxidation state which is stabilized by ∼500 mV relative to the ruthenium complex, and excited-state lifetimes below 10 ns in the presence of DNA Emission from Δ-Os(phen)2dppz2+ bound to calf thymus DNA is efficiently quenched by Δ-Rh(phi)2bpy3+, and a lower limit for the quenching constant is set at 7 × 109 s-1 The quenching profile over a range of quencher concentrations is found to be remarkably similar to that of the ruthenium analogue, despite an increase of ∼200 mV in ΔG for the photoinduced, forward electron transfer reaction Such an observation may indicate the importance of the HOMO energy in the donor excited state, which is similar for both donors Owing to the lack of spectral overlap between Os(phen)2dppz2+ emission and Rh(phi)2bpy3+ absorption,

Recognition of DNA by Octahedral Coordination Complexes

Abstract: Strategies for the site-specific recognition of DNA are described through the design of a family of octahedral metallointercalating complexes. Coordination complexes, which mimic DNA-binding proteins with regard to their affinity and specificity for DNA sites, may be prepared by the specific functionalization of chiral metal complexes so as to achieve the ensemble of non-covalent contacts in the DNA major groove anchored through intercalation.

Time Resolved Electron Transfer Studies Between Metallointercalators in DNA

Abstract: Ultrafast studies on the rates of DNA-mediated forward and reverse electron transfer between photoexcited [Ru(phen)2dppz]2+ and various acceptors are reported. We also present the first results that resolve the ultrafast emission decay of the so-called “light-switch” molecule [Ru(phen)2dppz]2+ in aqueous solution, distinguishing the diverse photophysical mechanisms of [Ru(phen)2dppz]2+.

Effect of guest/host interactions on photoinduced electron

Abstract: The driving force dependence for the photoinduced electron transfer (ET) from *Ru(phen)3 2÷ (phen = 1,10-phenanthroline) to several cationic M (phen) 3" ÷ complexes (M = Co, Ni, Rh, Cr; n = 2, 3) has been measured in water aria in the presence of polyanionic microheterogeneous environments. The anionic hosts utilized in this study are poly(styrenesuifonate) (PSS) and calf-thymus DNA. in water the driving force dependence of the observed rates, plot of Iog(kq) versus A G (Marcus plot ), reaches a plateau corresponding to the rate of diffusion of the reactants. Qualitatively, the Marcus plot is shallower in the case of PSS compared to that measured in DNA and water. This behavior is explained in terms of differences in bimolecular ET kinetics between the DNA and PSS systems.