Core–shell microgels as “smart” carriers for enzymes

We present a thermodynamic study of the adsorption of lysozyme on a negatively charged core–shell microgel at pH 7.2. The carrier particles consist of a polystyrene core onto which a charged poly(N-isopropylacrylamide-co-acrylic acid) network is attached. Isothermal titration calorimetry (ITC) is used to investigate the temperature and salt dependence of lysozyme binding. Our ITC analysis unequivocally shows that the adsorption of lysozyme onto the charged gel is driven by entropy. The addition of salt strongly decreases the binding affinity, indicating significant electrostatic contributions to the adsorption process. However, at high salt concentrations, substantial protein binding with unaltered entropies is still observed pointing to large contributions from hydrophobic interactions. Furthermore, the calorimetric analysis suggests that protonation of lysozyme takes place upon binding. This is directly shown by analysis of the enzymatic activity of adsorbed lysozyme. It was found that the activity is enhanced about ∼3.5 times, indicating that lysozyme has taken up approximately one proton when entering the gel. The entire set of data demonstrates that core–shell microgels present “smart” colloidal carriers for lysozyme that enhance its activity.