Complexation of Proteins with a Strong Polyanion in an Aqueous Salt-free System

We studied the formation in a salt-free solution of complexes between potassium poly(vinyl alcohol) sulfate (KPVS), a strong polyelectrolyte, and proteins with known amino-acid sequences: papain, human serumalbumin, lysozyme, ribonuclease, trypsin, and pepsin. Turbidimetric titration, togetherwith quasielastic light scattering (QELS), static light scattering (SLS), and electrophoretic light scattering, was performed at pH 2, in order to completely protonate the basic groups of proteins and so facilitate the determination of their total cationic charge. Even at pH 2, no protein unfolding was observed in all the samples within the precision of QELS. Through complexation with KPVS at pH 2, electrically neutral protein-polyelectrolyte complexes (PPCs) with a uniform size were formed from all the proteins, other than pepsin. Such PPCs can be regarded as aggregates of an intrapolymer complex consisting of a KPVS chainwith the boundproteinmolecules, the formation ofwhich occurs via the stoichiometric neutralization of the polyion charges with the opposite charges of proteins. After most of the proteins have formed PPCs, further addition of KPVS led to the association of the aggregated intrapolymer PPCs to form high-ordered aggregates. However, a decrease in the protein charges due to the characteristics of proteins or pH level appears to alter the nature of the PPC formed.