Light scattering, CD, and ligand binding studies of ferrihemoglobin-polyelectrolyte complexes

Quasi-elastic light scattering (QELS), electrophoretic light scattering (EL S), CD spectroscopy, and azide binding titrations were used to study the co mplexation at pH 6.8 between ferrihemoglobin and three polyelectrolytes tha t varied in charge density and sign. Both QELS and ELS show that the struct ure of the soluble complex formed between ferrihemoglobin and poly(diallyld imethylammonium chloride) [PDADMAC] varies with protein concentration At fi xed 1.0 mg/mL polyelectrolyte concentration, protein addition increases com plex size and decreases complex mobility in a tightly correlated manner. At 1.0 mg/mL or greater protein concentration a stable complex is formed betw een one polyelectrolyte chain and many protein molecules (i.e., an intrapol ymer complex) with apparent diameter approximately 2.5 times that of the pr otein-free polyelectrolyte. Under conditions of excess polyelectrolyte, eac h of the three ferrihemoglobin-polyelectrolyte solutions exhibits a single diffusion mode in QELS, which indicates that all protein molecules are comp lexed. CD spectra suggest little or no structural disruption of ferrihemogl obin upon complexation. Azide binding to the ferrihemoglobin-poly(2-acrylam ide-2-methylpropanesulfonate) [PAMPS] complex is substantially altered rela tive to the polyelectrolyte-free protein, but minimal change is induced by complexation with an AMPS-based copolymer of reduced linear charge density. The change in azide binding induced by PDADMAC is intermediate between tha t of PAMPS and its copolymer. (C) 1999 John Wiley & Sons, Inc.