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 serum albumin, lysozyme, ribonuclease, trypsin, and pepsin. Turbidimetric titration, together with 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 chain with the bound protein molecules, the formation of which 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.

Complexation between human serum albumin (HSA) and poly(ethylene glycol) (PEG) was studied using different experimental techniques:  quasi-elastic light scattering (QELS), static light scattering (SLS), electrophoretic light scattering (ELS), dialysis, and fluorescence spectroscopy. The QELS study for aqueous HSA−PEG mixtures at different levels of pH and ionic strength (NaCl) showed the formation of a water-soluble complex, the size of which varied depending on both the ionic strength and the molecular weight of PEG but remained unaltered when the mixing ratio of PEG to HSA was varied. The study of the complexation in the presence and absence of 1 M urea as a function of pH by QELS and fluorescence spectroscopy strongly suggested that hydrogen bonding plays an important role in the complex formation. A combination of SLS and dialysis at pH 2 and at the ionic strength 0.1 demonstrated that the complexation yielded an “intrapolymer” complex in which several HSA molecules bound to a PEG chain. In addition, ELS indicated that the resulting intrapolymer complex behaves like a free draining coil during electrophoresis.