Plasmids at oppositely charged surface


Nam-Kyung Lee (Sejong Univ.)



Inspired by recent Atomic Force Microscope images of plasmids deposited on oppositely charged surfaces from salt free solution, we propose a model for strongly adsorbed supercoiled cyclic stiff polyelectrolytes. We discuss how the excess linking number of the deposited cycle is distributed between writhe and twist at equilibrium and obtain the typical number of self-crossings in the deposited cycle as a function of surface charge density. The number of crossings at equilibrium is simply determined by the crossing penalty which is a local quantity and by the excess linking number. The number of crossings is well defined despite of versatile plasmid shapes. For moderate numbers of crossings the loops are rather small and localized along the primary cycle as expected from entropic loops. In the regime of many crossings, the cycle takes the shape of a regular flat ply ruled by local stiffness. The model allows for a semi-quantitative comparison with the AFM images of deposited plasmids which are strongly charged.