Mobility of lysozyme inside oxidized starch polymer microgels

The aim of this paper is to determine the mobility of protein molecules inside oxidized potato starch polymer (OPSP) microgel particles (spherical, 10–20 µm in diameter). This provides relevant information for controlled uptake and release applications of such systems. The mobility of Alexa-488 labelled lysozyme inside the microgel is measured by fluorescence recovery after photobleaching (FRAP) in combination with confocal laser scanning microscopy (CLSM). CLSM images show that the protein molecules distribute quite homogeneously over the microgel particles. By fitting the FRAP data with a model based on exchange between bleached and unbleached protein molecules inside the gel, we identified several protein fractions of different mobility. Increasing the salt concentration (NaCl) or the pH causes a shift in the distribution towards the more mobile fractions. This is consistent with earlier uptake and release measurements, which showed that the binding affinity decreases with increasing salt concentration and pH. At low protein concentrations, at which the microgel is not saturated with protein, the mobility of the bound protein molecules is more restricted than at protein concentrations where the uptake is complete. This is attributed to binding of the protein molecules to multiple binding sites. The model explains reasonably the mechanism of protein mobility inside the microgel, indicating that embedded ingredients with charge properties comparable to those of lysozyme can be protected at low salt concentration and low pH. Increasing the salt concentration or the pH triggers the release