Supplementary Material for: Strategies for a Direct Characterization of Phosphoproteins on Hydroxyapatite Surfaces

We show in this work how systems formed by phosphoproteins on calcium phosphate surfaces can be <i>directly</i> characterized, in real time, in liquid medium, without the need for elution or labeling. Specifically, we show how this is possible by applying three different techniques: ellipsometry, quartz crystal microbalance with dissipation, and atomic force microscopy-based friction force spectroscopy. We apply these techniques to study two different model systems, i.e. those formed upon the adsorption of two model phosphoproteins (κ- and ß-casein) on hydroxyapatite (HA) surfaces. Information on the kinetics of adsorption, surface excess, viscoelasticity, water content, thickness of the layers, and protein-surface interaction is provided. Results indicate that both phosphoproteins form homogeneous elastic highly hydrated monolayers on the HA surfaces, the strength of ß-casein layers being higher by approximately a factor of 4. Based on the experimental results, models for the conformation of κ- and ß-casein molecules adsorbed on HA surfaces are proposed.