Effects of Surface Charge Distribution of Proteins in Their Complexation with Polyelectrolytes in an Aqueous Salt-Free System

Abstract

Complexation between protein and polyelectrolyte was studied by a combination of three main experimental techniques: turbidimetric titration, quasi-elastic light scattering (QELS), and static light scattering (SLS). Proteins of two sorts, lysozyme (Lyz) and ribonuclease (RNase), were chosen by considering the following characteristics: (i) Both proteins have the same number (19) of basic groups; (ii) their distribution is almost homogeneous on Lyz but not on RNase; (iii) there is little difference in the molar mass between both proteins. Potassium poly(vinyl alcohol) sulfate (KPVS) with different molecular weights (MPE) and various degrees of esterification (De) was used as the polyelectrolyte. We employed a salt-free aqueous medium and adjusted it to pH 2, the level of which forces to completion the protonation of all of the basic groups. As the titration of proteins with KPVS proceeded, the absorbance (A) as an indication of turbidity increased linearly and then rapidly at a certain titrant volume, r...