Capillary Electrophoresis and Dynamic Light Scattering Studies of Structure and Binding Characteristics of Protein−Polyelectrolyte Complexes

Abstract

Complex formation between sodium polystyrenesulfonate (NaPSS) and two proteins, bovine serum albumin and -lactoglobulin, was studied by dynamic light scattering (DLS) and capillary electrophoresis (CE) over a range of polyelectrolyte molecular weights. We found that the dimensions of the polyelectrolyte chain do not appear to change significantly upon complex formation and that the dependence of complex mobility on its composition is in agreement with a free-draining model. Estimates of intrinsic binding constants, sizes of binding site, and cooperativities in complex formation were obtained by fitting experimental data to the "overlapping binding sites" model. It was found that the cooperativities shown by the binding isotherms are consistent with the dependence of binding density upon NaPSS molecular weights. The latter can be predicted by the overlapping binding sites model for known binding cooperativity, but was not observed experimentally before.

Coacervation in Biopolymers

“…An approximate model for quantitative estimation of charge on the soluble complexes, both at pH c and pH φ , was proposed which did imply charge neutralization due to surface patch binding. The qualitative features observed by Regnier et al [67], Gao et al [19] and Park et al [18] are clearly visible in the intermolecular complexation data presented here. These can be summarized as follows: (i) formation of intermolecular soluble complexes were observed with both biopolymers having same kind of net charge, (ii) soluble complex is, actually, the partially charge neutralized DNA and (iii) the interaction is electrostatic and is site specific.…”

“…In a more detailed and quantitative experiment the intermolecular binding between the selected proteins RNAse, lysozyme and BSA, and polyelectrolytes with varying linear charge densities was studied by DLS and turbidimetry [18,19]. These results supported the conclusions of "retention maps" constructed and reported by Regnier et al [67].…”

“…In a small class of systems liquid-liquid phase separation gets initiated by surface selective patch binding [11,18,19] even though both the polyions carry similar net charge. This is often referred to as binding on the wrong side of pH.…”

“…Such bindings are often referred to as binding on the wrong side of pH which has been reported in a variety of systems [18,19,64]. Surface patch controlled attractive electrostatic interactions between proteins and adsorbent having same net charge was studied, by ion-exchange chromatography (IEC), for lysozyme, ribonuclease-A, cytochrome-c and α-chymotrypsinogen systems and it was observed that such interactions were governed by the characteristics of the solvent medium like, the pH, ionic strength etc.…”

“…It has been shown that a polyelectrolyte, DNA and a polyampholyte, gelatin can undergo associative interaction and form complex coacervates with interesting thermal properties [15]. Further, it has been realized that in a class of systems coacervation transition is governed by surface selective patch binding even though both the polyions carry similar net charge [11,18,19]. In particular, in SPB interactions complementary polyions (normally a PA-PE pair) seek oppositely charged patches to bind overcoming the repulsion occurring between similarly charged surface patches.…”