Light Scattering, CD, and Ligand Binding Studies of Ferrihemoglobin–Polyelectrolyte Complexes

Abstract: Quasi-elastic light scattering (QELS), electrophoretic light scattering (ELS), CD spec-troscopy, and azide binding titrations were used to study the complexation at pH 6.8 between ferrihemoglobin and three polyelectrolytes that varied in charge density and sign. Both QELS and ELS show that the structure of the soluble complex formed between ferrihemoglobin and poly(di-allyldimethylammonium chloride) [PDADMAC] varies with protein concentration. At fixed 1.0 mg/mL polyelectrolyte concentration, protein addition increases complex size and decreases com-plex mobility in a tightly correlated manner. At 1.0 mg/mL or greater protein concentration, a stable complex is formed between one polyelectrolyte chain and many protein molecules (i.e., an intra-polymer complex) with apparent diameter approximately 2.5 times that of the protein-free poly-electrolyte. Under conditions of excess polyelectrolyte, each of the three ferrihemoglobin–polyelec-trolyte solutions exhibits a single diffusion mode in QELS, which indicates that all protein molecules are complexed. CD spectra suggest little or no structural disruption of ferrihemoglobin upon complexation. Azide binding to the ferrihemoglobin–poly(2-acrylamide-2-methylpropanesulfonate