Inhibition of Pathological Mineralization of Calcium Phosphate by Phosphorylated Osteopontin Peptides through Step-Specific Interactions

We present an in situ study of the interaction of osteopontin (OPN) peptide-bearing solutions with brushite (DCPD), CaHPO₄·2H₂O, (010) surfaces using atomic force microscopy. We show that in situ observations of the [1̅00]_Cc step kinetics are consistent with classic Cabrera-Vermilyea model of step pinning combined with adsorption dynamics of phosphorylated OPN peptides, highlighting the effects of supersaturation and peptide concentration on step movement and pinning as a mechanism of inhibitor action. In addition to a kinetic effect, the presence of phosphorylated OPN, preferentially binding to the [1̅00]_Cc steps, may alter mineral interfacial energies, thus delaying the formation of active steps during growth. This is consistent with the bulk nucleation observations. Furthermore, the phosphorylation-deficient form of this segment fails to inhibit DCPD crystallization. These in vitro results may reveal that the dual control of step kinetics and interfacial energy by phosphorylated OPN peptides may have much broader utility for improving our understanding of the mechanisms through which pathological mineralization of calcium phosphate is inhibited.