Interactions between layers of salivary acidic proline rich protein 1 (PRP 1) adsorbed on mica surfaces

Interactions between layers of acidic proline rich protein 1 (PRP 1) films adsorbed on mica surfaces were investigated using the interferometric surface force technique. The influence of the type of electrolyte present (NaCl and CaCl 2 ) as well as the effect of the sequential addition of 10% human mixed saliva was explored. In the presence of NaCl very weak adsorption was observed and the force curves resembled the ones in pure NaCl. Only after long equilibration a thin layer was adsorbed and the forces became entirely repulsive. In the presence of CaCl 2 a substantially thicker layer was adsorbed. After short adsorption times the long-range repulsive forces were replaced by an attractive force below a surface separation of about 120 Å, which caused the surfaces to slide into contact (approximately 40 Å from mica-mica contact). As the layer built up, the attractive force was reduced and the force curve showed an inflection point instead of a jump. Finally, the force became entirely repulsive and no adhesion was observed. The introduction of mixed whole saliva drastically changed the interaction, whose range became longer than expected for electrostatic repulsive forces under the experimental conditions. This indicates that steric forces then prevailed as the force curves were almost identical to those recorded between mica surfaces after adsorption from mixed whole saliva alone.