Effect of persistence length on binding of DNA to polyions and overcharging of their intermolecular complexes in aqueous and in 1-methyl-3-octyl imidazolium chloride ionic liquid solutions

Abstract

The effect of persistence length on the intermolecular binding of DNA (200 bp, persistence length l_p = 50 nm, polyanion) with three proteins, gelatin B (GB) (l_p = 2 nm, polyampholyte chain), bovine serum albumin (BSA) (l_p = 7 nm, polyampholyte colloid), gelatin A (GA) (l_p = 10 nm, polyampholyte chain), and a polysaccharide chitosan (l_p = 17 nm, polycation), was investigated in aqueous and in 1-methyl-3-octyl imidazolium chloride ionic liquid ([C8mim][Cl]) solutions. In DNA–GB and DNA–BSA solutions complexation primarily arises from surface patch binding whereas DNA–chitosan and DNA–GA binding was predominantly governed by electrostatic forces. These occurred at well defined pH values: (i) at pH_c associative interactions ensued and soluble complexes were formed, (ii) at pH_Φ soluble complexes coalesced to give rise to liquid–liquid phase separation (coacervation) and (iii) at pH_prep formation of large insoluble complexes drove the solution towards liquid–solid phase separation. A universal phase diagram encapsulating the aforesaid interactions can be made using the persistence length of polyion as an independent variable. DNA formed overcharged intermolecular complexes with all these polyions when the polyion concentration was more than the concentration required to produce charge neutralized complexes (disproportionate binding). In IL solutions maximum binding occurred when 0.075 < [IL] < 0.10% (w/v) and the effect of overcharging was substantially screened. The extent of overcharge was a monotonous increasing function of the polyion persistence length. Results clearly revealed that DNA–polyion binding was hierarchical in polyion concentration and persistence length. Overcharging of the DNA–polyion complex was found to be ubiquitous for the polyions used in the present study.