Complexation between lysozyme and sodium poly(styrenesulfonate): The effect of pH, reactant concentration and titration direction

It is well-known that mixing of aqueous solutions of oppositely charged proteins and polyelectrolytes leads to the formation of aggregates often called protein-polyelectrolyte complexes. In this study, we investigated the interactions between aqueous solutions of lysozyme and sodium poly(styrenesulfonate) (PSS) as the model system for investigating protein-polyelectrolyte complexation processes. The experimental methods used in this study were isothermal titration microcalorimetry, electrophoretic mobility (i.e. zeta potential determination) and particle size (i.e. hydrodynamic radius) measurements. The effect of pH, reactant (protein and polyelectrolyte) concentration and titration direction (addition order) on lysozyme-PSS complexation was investigated at theta =25 degrees C and at ionic strength I-c = 10(-2) mol dm(-3). At all three examined pH values (pH =3.1, 4.6 and 7.5) at low charge ratios a stable colloidal suspension was obtained in which charged colloid particles, whose sign of charge corresponds to that of the titrand, were present. On the other hand, at higher charge ratios large particles (floccules) appeared (with the hydrodynamic radius in the range 2-7 mu m). These floccules were also charged and, at first, their charge sign was equal to that of the titrand (vertical bar zeta vertical bar = 30-50 mV). However, upon further addition of the titrant the charge of the floccules reversed as a result of the surplus of titrant molecules in the corona. The change in pH leads to the significant change in charge ratios at which the onset of flocculation is observed. In all examined cases the charge ratio at which zeta potential starts to change corresponds to the onset of flocculation determined by DLS. The reaction heat effects were determined and it was shown that in all examined cases complexation resulted in measurable negative enthalpy changes. (C) 2015 Elsevier B.V. All rights reserved.