CONTROL OF MACROMOLECULE DISTRIBUTION WITHIN SYNTHETIC AND BIOGENIC SINGLE CALCITE CRYSTALS

The ability of organisms to exercise control over crystal growth is wo nderfully exemplified by skeleton formation in echinoderms. A sea urch in spine is a unique composite of a single crystal of calcite and glyc oproteins intercalated inside the crystal during its growth. Here we p erformed a detailed morphological and high-resolution synchrotron X-ra y diffraction study of the textures of synthetic and biogenic calcite crystals. We show that the intracrystalline macromolecules from sea ur chin spines, when allowed to interact with growing calcite crystals in vitro, selectively reduce the coherence lengths and degrees of alignm ent of the perfect domains in specific crystallographic directions. Th ese directions also correspond to the newly-developed stable faces. In contrast, the defect distribution of young sea urchin spines composed entirely of spongy stereomic structure is much more isotropic. In mat ure spines containing secondarily filled-in wedges of calcite, the deg ree of anisotropy is intermediate between that of the synthetic crysta ls and the young spines. The macromolecules extracted from young and m ature spines are, however, very similar. These observations demonstrat e the inherent capability of occluded matrix macromolecules to finely differentiate between crystal planes by stereochemical recognition pro cesses. They also show that in biologically-produced calcite crystals this process can be overridden to produce a more isotropic material.