Multiple DNA binding modes of a metallointercalator revealed by DNA film voltammetry.
Binding and the redox reaction of the metallointercalator Ru(bpy)2(dppz)2+ (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine) with DNA was investigated by DNA film voltammetry. Calf-thymus DNA (CT-DNA) was assembled on a tin-doped indium oxide electrode by layer-by-layer electrostatic adsorption. Voltammetry of Ru(bpy)2(dppz)2+ (Ru-dppz) bound to the DNA film was measured in a redox-free electrolyte and showed strong dependence on the concentration of the metallointercalator. At low Ru-dppz concentrations, a single oxidation peak was observed, the potential of which shifted from 1.25 to 1.1 V with increasing Ru-dppz concentration (peak 1). At high metal chelate concentrations, an additional oxidation peak emerged with a potential of 1.25 V which was unaffected by the Ru-dppz concentration (peak 2). Three experiments were performed to investigate the mechanism and structural basis of the multiple peaks. First, voltammetry of Os(bpy)2(dppz)2+ bound to the CT-DNA film displayed only one peak at its oxidation potential of about 0.75 V. Second, the concentration dependence of Ru-dppz bound to a poly-(AU) film (which does not contain any guanine bases) exhibited only one oxidation peak at about 1.22 V that was independent of the Ru-dppz concentration. Third, when the guanine concentration in a mixed film of CT-DNA and poly-(AU) was changed and the bound Ru-dppz was kept constant, a pre-peak emerged and shifted to 1.1 V with increasing guanines. Based on these results, the appearance of two peaks in the voltammetric measurements of CT-DNA was rationalized by invoking two different DNA binding modes for the Ru-dppz complex: intercalation and electrostatic association. Peak 2 arises from slow oxidation of guanines catalyzed by Ru-dppz electrostatically associated with the DNA film, since the addition of Mg2+ decreases the magnitude of peak 2. Peak 1 was not affected by Mg2+ ions, leading us to conclude that it is due to intercalated Ru-dppz. The intercalation positions the metal complex in close contact with the guanines inside DNA resulting in fast electrocatalytic reaction, giving rise to a catalytic pre-peak.
Guo
Liang-Hong
LH
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, P O Box 2871, 18 Shuangqing Road, Beijing 100085, China. LHGuo@mail.rcees.ac.cn
Wei
Ming-Yuan
MY
Chen
Hao
H
D000818:Animals; D001703:Biophysics / Q000379:methods*; D002417:Cattle; D002614:Chelating Agents / Q000494:pharmacology; D002627:Chemistry, Physical / Q000379:methods*; D004247:DNA / Q000737:chemistry*; D004573:Electrolytes; D007364:Intercalating Agents / Q000494:pharmacology*; D008274:Magnesium / Q000737:chemistry*; D008670:Metals / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011485:Protein Binding; D012413:Rubidium / Q000737:chemistry*; D013950:Thymus Gland / Q000378:metabolism
D002614:Chelating Agents; D004573:Electrolytes; D007364:Intercalating Agents; D008670:Metals; D004247:DNA; D008274:Magnesium; D012413:Rubidium; D010100:Oxygen