Lysozyme uptake by oxidized starch polymer microgels

With the aim of determining suitable conditions for uptake and release of globular proteins on microgels, we studied the interaction between phosphated, highly cross-linked, negatively charged oxidized potato starch polymer (OPSP) microgel particles and lysozyme from hen eggs. Our microgel shows a typical protein-induced deswelling behavior for charged microgels. The protein distributes rather homogenously through the microgel. We found that at low salt concentration the saturation protein uptake Gammasat increases with increasing pH. This is because the binding capacity is mainly determined by charge compensation: with increasing pH, the (positive) charge on the lysozyme molecules decreases, while the (negative) charge of the microgel particles increases. Therefore, more protein molecules are needed to compensate for the charge on the gel and the binding capacity increases. The protein binding affinity, however, decreases sharply with increasing pH, presumably because this affinity is mainly sensitive to the lysozyme charge density. At high pH the binding affinity is relatively low, and by adding salt, the protein can easily be released from the gel. This leads to a maximum in the curves of Gammasat versus pH, and this maximum shifts to lower pH values with increasing ionic strength. We conclude that, for protein uptake and release applications, the present system works best around pH 5 due to a sufficiently high binding affinity and a sufficiently high binding capacity.