Static light scattering study of complex formation between protein and neutral water-soluble polymer

Complex formation of poly(N-isopropylacrylamide) (PNIPA) having a weight-average molecular weight of 1,720,000 g/mol with human serum albumin (HSA), ovalbumin (OVA) and lysozyme (LYZ) was studied in an aqueous medium containing 0.01 M NaCl and adjusted to pH 3. The polymer–protein mixtures at different molar ratios (rm) were examined by static light scattering (SLS). The analysis of SLS data using our own approach [Kokufuta et al., Langmuir 15 (1999) 940; Biomacromolecules 4 (2003) 728] showed that the molecular weight of each resulting complex is smaller than that of the interpolymer complex composed of two polymer chains plus one protein. This indicates the formation of an intrapolymer complex in all the polymer–protein systems studied. Thus, at each rm we calculated the number of bound proteins per polymer, the value of which was OVA > HSA > LYZ in order. These results were compared with the hydropathy profiles of each protein which are a good tool for obtaining an information about distribution of hydrophobic and hydrophilic segments in a protein. It has become apparent that the hydrophobic interaction between polymer and protein plays an important role in the intrapolymer complex formation.