Interaction between lysozyme and colloidal poly(NIPAM-co-acrylic acid) microgels

The interaction between lysozyme and colloidal poly(NIPAM-co-acrylic acid) microgels is investigated in aqueous solutions at neutral pH. Lysozyme binding isotherms, obtained within the ionic strength range 10–220 mM, indicate that the maximum uptake at 10 mM is 2.4 g lysozyme per gram dry gel, and that the uptake capacity decreases with increasing ionic strength to ∼0 at 220 mM. Swelling isotherms, obtained from photon correlation spectroscopy measurements, show that the binding is accompanied by a substantial deswelling of the microgels. The microgel suspension is stable up to a protein-to-polymer charge ratio in the microgels of about 0.6, largely independent of ionic strength, whereas flocculation/sedimentation occurs at higher charge ratios. The charge ratio 0.6 corresponds to a zeta-potential of about −6 mV, as obtained from measurements of electrophoretic mobility. Binding and swelling isotherms are analyzed in detail and compared with predictions of theoretical model calculations. The influence of protein–protein attraction is highlighted, as well as the interplay between electrostatic interactions and network elasticity.

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