Interactions of Various Skeletal Intracrystalline Components with Calcite Crystals

The calcitic skeletal elements of many organisms contain small amounts of proteins and glycoproteins which are incorporated within single crystals of calcite. Extraction and partial purification of the intracrystalline macromolecules allowed the identification of their interactions with synthetic calcite crystals. Specific interactions were inferred from induced morphological modifications associated with the development of new faces on calcite crystals grown in the presence of these macromolecules in solution. Partially purified macromolecules extracted from within sea urchin spines interacted only with faces roughly parallel to the c crystallographic axis of calcite, producing well-developed (01 I} faces. Macromolecules extracted from within mollusk shell prisms separated into two fractions each having a characteristic amino acid composition and displayed distinct specific interactions with growing calcite crystals: A highly acidic fraction interacts with the {OOl} set of faces, while a different fraction, which is only moderately acidic and has an amino acid composition similar to that of the sea urchin spines, interacts specifically with the {Oll} set of faces. Magnesium, a major ionic component of the sea urchin spines, causes the development of the {01 1) set of crystal faces. The existing knowledge on the direction of intercalation of the macromolecules in the biogenic crystals agrees with the interacting planes in the synthetic crystals, suggesting the relevance of our in vitro assay to the system in vivo. We suggest that each separate mode of interaction may infer a specific role in the regulation of biological crystal growth.