Protein binding to soft polymeric layers: a quantitative study by fluorescence spectroscopy

We present a comprehensive investigation of the adsorption of several proteins, i.e., lysozyme, β-D-glucosidase, and cytochrome c, onto microgels in aqueous solution. The microgels consist of a solid core from polystyrene (PS) onto which a shell of poly(N-isopropylacrylamide) (pNiPAm) together with 10 mol% acrylic acid is grafted. The pronounced fluorescence response observed upon transfer of fluorescent marked lysozyme from the bulk solution to the gel allows us to monitor its uptake by the shell in situ. All data demonstrate directly that full equilibrium is reached rendering the binding of proteins to the particles as a reversible process. We also study the competitive adsorption of lysozyme with cytochrome c or β-D-glucosidase. Here we show that the data obtained for a mixture of lysozyme and cytochrome c can be modeled quantitatively by a competitive Langmuir isotherm. Using time-resolved fluorescence spectroscopy we elucidate the kinetics of protein adsorption. We find that the uptake of proteins by these particles is described by a fast, diffusion-limited binding regime which is followed by a slow binding regime, most probably caused by a slow rearrangement of the proteins within the network. All data demonstrate that charged pNiPAm-microgels present a model system for which the thermodynamics and kinetics of protein adsorption including competitive adsorption can be analyzed in a quantitative manner.