Poly(2-(methacryloyloxy)ethyltrimethyl ammonium chloride) (PMATAC) modified magnetic nanopartides (NPs) with a high zeta potential of ca. 50 mV were synthesized by atom transfer radical polymerization (ATRP). The prepared NPs consist of a magnetic core around 13 nm and a PMATAC shell around 20 nm attached on the surface of magnetic nanoparticles. Thermodynamic binding parameters between beta-lactoglobulin and these polycationic NPs were investigated at different ionic strengths by high-resolution turbidimetry, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). Both turbidity and ITC show that binding affinities for BLG display a non-monotonic ionic strength dependence trend and a maximum appears at ionic strength of 50 mM. Such observation should arise from the coeffects of protein charge anisotropy visualized by DelPhi electrostatic modeling and the strong electrostatic repulsion among highly charged NPs at a variety of ionic strengths. (C) 2015 Published by Elsevier Inc.