Force induced DNA melting in the presence of an attractive surface.

The self avoiding walk (SAW) model of the polymer has been extended to study the equilibrium properties of double stranded DNA (dsDNA) where two strands of the dsDNA are modeled by two mutually attracting self-avoiding walks (MASAWs) in the presence of an attractive surface. We study simultaneous adsorption and force induced melting transitions and explore different phases of DNA. It is observed that melting is entropically dominated, which can be substantially reduced under the application of an applied force. We consider three scenarios, where the surface is weakly, moderately and highly attractive. For both weakly and moderately attractive surfaces, the DNA desorbs from the surface in a zipped form and acquires the conformation of a melted state with the rise in temperature. However, for a strongly attractive surface, the force applied at one end of the strand (strand-II) results in unzipping, while the other strand (strand-I) remains adsorbed on the surface. We identify this as adsorption-induced unzipping, where the force applied on a single strand (strand-II) can unzip the dsDNA if the surface interaction energy exceeds a specific threshold. We also note that at a moderate surface attraction, the desorbed-zipped DNA melts with an increase in temperature and the free strand (strand-I) gets re-adsorbed onto the surface.