Shape of adsorbed supercoiled plasmids: an equilibrium description

Area of Science:

• Polymer Physics
• Biophysics
• Surface Science

Background:

• Atomic force microscopy (AFM) reveals unique adsorption behaviors of plasmids on charged surfaces.
• Understanding the conformational changes of cyclic polyelectrolytes under surface interactions is crucial.

Purpose of the Study:

• To develop a theoretical model for strongly adsorbed supercoiled cyclic stiff polyelectrolytes.
• To investigate the relationship between excess linking number, writhe, twist, and self-crossings.
• To compare model predictions with experimental AFM data of deposited plasmids.

Main Methods:

• Development of a physical model for adsorbed cyclic stiff polyelectrolytes.
• Analysis of the sharing of excess linking number (Lk) between writhe (Wr) and twist (Tw).
• Calculation of self-crossing number as a function of surface charge density.

Main Results:

• The number of self-crossings is determined by a local crossing penalty and the excess linking number.
• Model predicts distinct regimes: small, localized entropic loops for moderate crossings, and flat, regular plectonemic structures for many crossings.
• Semiquantitative agreement between model predictions and AFM images of strongly charged plasmids.

Conclusions:

• The proposed model successfully describes the adsorption and conformational states of supercoiled polyelectrolytes on charged surfaces.
• The interplay between linking number, local stiffness, and surface charge density dictates the observed structures.
• The model provides a framework for understanding complex polymer conformations in confined environments.