Effects of Full-Length Phosphorylated Osteopontin and Constituent Acidic Peptides and Amino Acids on Calcite Dissolution

Mineral dissolution events are part of the life history of many protein-rich biomineralized tissues and structures. To assess under dissolution conditions the specificity of interactions between calcite and phosphate moieties in proteins, the (1 0 4) surface of calcite single crystals were etched with aqueous solutions containing osteopontin (OPN) protein and examined by atomic force microscopy. Etch pits were compared to those modified by constituent acidic peptides of OPN as well as by single relevant amino acids. Exposure to phosphorylated OPN and phosphorylated acidic serine- and aspartate-rich motif (ASARM) peptides of OPN produced oval or nearly circular etch pits (rather than the rhombic shape when etched in water alone) in the (1 0 4) calcite surface, with a steep acute [0 1 0] edge, indicating preferential step-specific binding. A carboxyl-group-rich polyaspartic acid peptide, and the nonphosphorylated ASARM peptide, produced pits predominantly elongated parallel to [4 2 (1) over bar]. Etch-pit steps parallel to [0 1 0] were stabilized by the amino acids aspartic acid and phosphoserine, but not by serine. OPN and peptides bearing multiple acidic groups modified pits at concentrations several hundred times lower than did the amino acids alone. Molecule phosphorylation consistently induced step edge rounding, suggesting phosphate-carboxyl cooperative binding to calcium ions at the (1 0 4) calcite surface and step edges.