2 SUPERHELIX DENSITY-DEPENDENT DNA TRANSITIONS DETECTED BY CHANGES INDNA ADSORPTION DESORPTION BEHAVIOR/

The adsorption behavior of covalently closed circular plasmid DNA at t he mercury/water interface was studied by means of AC impedance measur ements. The dependence of the differential capacitance (C) of the elec trode double layer on the potential (E) was measured in the presence o f adsorbed DNA. It was found that the C-E curves of supercoiled DNA at native and highly negative superhelix densities to), relaxed covalent ly closed circular DNA, and nicked DNA differed from each other. A det ailed study of topoisomer distributions ranging from -sigma of 0 to 0. 11 revealed two supercoiling-dependent transitions, at about -sigma = 0.04 (transition TI) and 0.07 (transition TII). Transition TI was dete cted by measuring the height of the adsorption/desorption peak 1 (at a bout -1.2 V against the saturated calomel electrode) and the decrease of capacitance (Delta C) at -0.35 V. This transition may be due to a s udden change in the ability of the DNA to respond to the alternating v oltage, probably caused by changes in the DNA tertiary and/or secondar y structure. Transition TII was detected by measuring peak 3 (at abou t -1.3 V), which was absent in topoisomers with -sigma less than 0.05. This transition is due to changes in the DNA adsorption/desorption be havior related to increased accessibility of bases at elevated negativ e superhelix density. Opening of the duplex at highly negative superhe lix density was also detected by the single-strand selective probe of DNA structure, osmium tetroxide, 2,2'-bipyridine, Our results suggest that electrochemical techniques provide sensitive experimental analysi s of changes in DNA structure.