Long-range interactions between ligands bound to a DNA molecule give rise to adsorption with the character of phase transition of the first kind

Influence of long-range interactions between ligands bound to DNA molecule on the character of their adsorption is studied using computer modeling. Fo r this investigation, two calculation procedures are developed. They are ba sed upon the method of the free energy minimum and on the partition functio n method. The both procedures demonstrate that in the case of a strong enou gh attraction between all the bound ligands their binding to DNA has the ch aracter of phase transition of the first kind. There is a break in the bind ing curve e(c(o)) where c - relative concentration of bound ligands, c,- mo lar concentration of free ligands. The break occurs because there is an int erval of central degrees of binding (similar to 50% of the maximum c value) that are prohibited for individual DNA molecules. Such a transition might be caused by some types of DNA condensation. Attraction between the neighbo ring ligands only, adjacent or/and separated by double helix regions, does not cause this effect.