Surface charges effects on the 2D conformation of supercoiled DNA
Résumé
We have adsorbed plasmid pUc19 DNA on a supported bilayer. By varying the fraction of cationic lipids inthe membrane, we have tuned the surface charge. Plasmid conformations were imaged by Atomic ForceMicroscopy (AFM). We performed two sets of experiments: deposition from salt free solution on chargedbilayers and deposition from salty solutions on neutral bilayers. Both sets show similar trends: at lowsurface charge density or low bulk salt concentration, the internal electrostatic repulsion forces plasmidsto adopt completely opened structures, while at high surface charge density or higher bulk saltconcentration, usual supercoiled plectonemes are observed. We experimentally demonstrate theequivalence of surface screening by mobile interfacial charges and bulk screening from salt ions. At lowto medium screening, the electrostatic repulsion at plasmid crossings is predominant, leading to anumber of crossovers decreasing linearly with the characteristic screening length. We compare our datawith an analytical 2D-equilibrated model developed recently for the system and extract the DNAeffective charge density when strands are adsorbed at the surface
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