  We study the role of cations on the stability of double stranded DNA (dsDNA)
molecules.It is known that the two strands of double stranded DNA(dsDNA) have
negative charge due to phosphate group. Cations in the form of salt in the
solution, act as shielding agents thereby reducing the repulsion between these
strands. We study several heterogeneous DNA molecules. We calculate the phase
diagrams for DNA molecules in thermal as well as in force ensembles using
Peyrard-Bishop-Dauxois (PBD) model. The dissociation and the stacking energies
are the two most important factors that play an important role in the DNA
stability. With suitable modifications in the model parameters we investigate
the role of cation concentration on the stability of different heterogeneous
DNA molecules. The objective of this work is to understand how these cations
modify the strength of different pairs or bases along the strand. The phase
diagram for the force ensemble case (a dsDNA is pulled from an end) is compared
with the experimental results.
