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

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 ( M̄ PE ) and various degrees of esterification ( D e ) 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, referring to the end point of titration. The slope of the linear plots of A vs titrant volume for Lyz was little dependent on M̄ PE and D e . In the case of RNase, however, the slope increased with decreasing D e and with increasing M̄ PE . From the studies of the hydrodynamic radius by QELS as well as the molecular weight and radius gyration by SLS, it was found that the M̄ PE and D e effects observed in the titration curve correspond to the changes of both size and mass of aggregated “intrapolymer” complexes formed during titration. Thus, we estimated the degree of aggregation (α) through dividing the mass from SLS by the calculated mass of an intrapolymer complex. This showed that α decreases with increasing D e in both protein systems, while increasing M̄ PE decreases α in the Lyz system but does not change it in the RNase system. These results were discussed in detail by considering the polarizability of the intrapolymer complex, the level of which is affected by the complementarity of the spacing between charges on the protein to the uniform spacing between polymer charges.