Characterisation of adsorbed layers of a disordered coil protein on polystyrene latex

Abstract

The combined use of small-angle X-ray scattering (SAXS) and photon correlation spectroscopy (PCS) to characterise adsorbed layers of /?-casein at the solid/liquid interface is reported. The protein was adsorbed to polystyrene latex particles at room temperature, low ionic strength and neutral pH and adsorption densities assessed by a solution-depletion technique which showed a plateau in the adsorption. Results from the SAXS experiments were analysed to provide electron-density profiles. These were backed up with results from PCS which provided hydrodynamic thicknesses over the range of the adsorption isotherm. This information, together with calculated hydrophobicity and charge profiles for the protein, yielded a molecular model for the adsorbed layer. Although B-casein in solution has a largely random coil conformation, it appears to adopt a much more compact form when it is adsorbed on polystyrene latex. Most of the protein lies close to the surface, leaving part of the chain extended into the aqueous phase. The most likely candidate for the extended chain is part of the highly charged sequence of 40 or so amino acids at the N terminus of the protein. The hydrodynamic thickness of the protein layers increases with adsorbed concentration of protein. The thicknesses reached are substantially greater than those predicted by theories of self-avoiding walks of the extended chain with volume exclusion interactions included and it is suggested that long-range electrostatic repulsive forces are involved.

Adsorption of Milk Proteins (β-Casein and β-Lactoglobulin) and BSA onto Hydrophobic Surfaces

“…Earlier -casein QCM-D studies reported areal masses in agreement with our results [ 39 , 40 ]. Experimental works based on the batch depletion method [ 41 , 42 ] or ellipsometry [ 43 , 44 ] reported values of 2–3 mg/m 2 for a full-coverage monolayer of -casein. These results also support our interpretation that the value of 6.1 mg/m 2 obtained by QCM corresponds to multilayer formation.…”

Effect of Ethanol on the Structure and Properties of β-Casein Adsorption Layers at the Air/Buffer Interface

“…that this protein is made of about three blocks of one hydrophilic sequence plus one hydrophobic sequence (32)(33). It will be shown elsewhere (34) that the interfacial behavior of a large multiblock macromolecule (phase diagram and surface pressure isotherms) is not very different of the behavior of a simple diblock.…”

Interaction of proteins with spherical polyelectrolyte brushes in solution as studied by small-angle x-ray scattering

“…Thus, if a protein acts as a multivalent counterion within the brush, the release of monovalent counterions will lower the strong osmotic pressure but retain electroneutrality. Therefore, the adsorption of proteins onto SPB cannot be related to forces usually invoked when explaining the experimental findings related to solid surfaces such as, e.g., hydrophobic effects or van der Waals attraction [3,26].…”