Interactions of sea-urchin skeleton macromolecules with growing calcite crystals— a study of intracrystalline proteins

Abstract

The exoskeletons of sea urchins are composed of magnesiumbearing calcite. Individual test plates and spines behave as single crystals in polarized light or when examined by X-ray diffraction^1–3. They do not, however, cleave like inorganic calcite crystals along the {104} hexagonal cleavage planes, but have conchoidal fracture surfaces reminiscent of amorphous glass. Discussion of this paradox revolves around whether the phase is monocrystalline^2,4,5, multicrystalline⁶, or some combination thereof⁷, but provides no explanation for the phenomenon. To address this question we grew crystals of calcite in the presence of acidic glycoproteins extracted from within the mineralized hard parts of sea-urchin tests^8–10. As a control we used analogous proteins from the calcitic layer of a mollusc shell which are known to be nucleators of calcite when adsorbed on a rigid substrate, but inhibitors when in solution^11–13. We show that the sea urchin, but not the mollusc macromolecules selectively adsorb onto specific calcite crystal planes and with continued crystal growth are occluded inside the solid phase. These synthetic crystals fracture with a conchoidal cleavage similar to that observed in sea-urchin calcite. Thus intracrystalline proteins may be responsible for this phenomenon in biology and the manner in which they affect the mechanical properties of the crystals may also have interesting implications to the materials sciences.