THE CONFORMATIONS OF PROTEINS ON SOLID WATER INTERFACES - CASEINS AND PHOSVITIN ON POLYSTYRENE LATTICES

Changes in the hydrodynamic radii of polystyrene latices in the presence of the proteins β-casein, αs1-casein, dephosphorylated β-casein and phosvitin have been measured, using photon correlation spectroscopy, over a range of protein concentrations which varied from zero to 0.3 mg ml-1. The measurements were made in buffer solutions of low ionic strength, and in similar buffers containing 6 M urea or 50 mM NaCl. Both αs1 and β-caseins caused substantial increases in the hydrodynamic radii of the latices, which were very little affected by the presence of urea. Both proteins formed unstable complexes with the latices in the presence of NaCl, as did the dephosphorylated β-casein. For the unmodified caseins, the increase in hydrodynamic radius appeared in parallel with binding of the protein, but a distinct induction stage was observed for dephosphorylated, β-casein: this also was unaffected by the presence of urea. In the presence of 6 M urea, the behaviour of phosvitin showed a considerable change: an induction phase similar to that of dephosphorylated β-casein was introduced, and the overall thickness of the protein layer was markedly decreased. These results were interpreted in terms of the hydrodynamic behaviour of particles with surrounding diffuse layers. It was concluded that, for β-casein, it was unrealistic to visualize the layer as being of constant thickness independent of the amount of protein bound: what appeared to be constant was the segment density of the protein layer, so that the layer became thicker as the amount of bound protein increased. The results of the other proteins were discussed by comparison with β-casein. © 1990.