Surface charges effects on the 2D conformation of supercoiled DNA

TL;DR

This study investigates how surface charge variations on supported bilayers influence the 2D conformations of supercoiled DNA, revealing the effects of surface screening and salt concentration on DNA structure.

Contribution

It demonstrates the equivalence of surface charge screening by mobile interfacial charges and bulk salt screening in affecting DNA conformation.

Findings

Supercoiled DNA adopts different conformations depending on surface charge and salt conditions.

Plectonemic structures are observed on charged surfaces or at 30 mM salt on neutral surfaces.

Surface screening effects are equivalent to bulk salt screening in DNA conformation control.

Abstract

We have adsorbed plasmid PuC19 DNA on a supported bilayer. The mobility of the lipids within the bilayer ensured a 2D equilibrium of the DNA molecule. By varying the fraction of cationic lipids in the membrane, we have tuned the surface charge. Plasmids conformations were imaged by Atomic Force Microscopy (AFM).We performed two sets of experiments: deposition from salt free solution on charged bilayers and deposition from salty solutions on neutral bilayers. Plasmids can be seen as rings, completely opened structures, or tightly supercoiled plectonemes, depending on the experimental conditions. The plectonemic conformation is observed either on charged surfaces (in the absence of salt) or at 30 mM salt concentration on a neutral bilayer. We demonstrate the equivalence of surface screening by mobile interfacial charges and bulk screening from salt ions.