A chemical model for the cooperation of sulfates and carboxylates in calcite crystal nucleation: Relevance to biomineralization.

Acidic matrix macromolecules involved in regulation of biological crystal growth often contain aspartic acid-rich domains and covalently bound sulfated polysaccharides. We propose that sulfates and beta-sheet structured carboxylates cooperate in oriented calcite crystal nucleation. The sulfates concentrate calcium, creating the supersaturation necessary for nucleation on the structured carboxylate domains. An artificial model, composed of sulfonated polystyrene surfaces and adsorbed beta-sheet poly(aspartate), demonstrates that the two components indeed act cooperatively with respect to two independent assays, both by induction of calcite nucleation off the (001) plane and by calcium association. Evidence is presented that a purified organic matrix acidic glycoprotein from mollusk shells may behave in vitro in a similar way.