Phase transitions in biogenic amorphous calcium carbonate

摘要

Crystalline biominerals do not resemble faceted crystals. Current explanations for this property involve formation via amorphous phases. Using X-ray absorption near-edge structure (XANES) spec-troscopy and photoelectron emission microscopy (PEEM), here we examine forming spicules in embryos of Strongylocentrotus pur-puratus sea urchins, and observe a sequence of three mineral phases: hydrated amorphous calcium carbonate (ACC · H2O)→ dehydrated amorphous calcium carbonate (ACC)→ calcite. Unexpectedly, we find ACC • H2O-rich nanoparticles that persist after the surrounding mineral has dehydrated and crystallized. Protein matrix components occluded within the mineral must inhibit ACC · H20 dehydration. We devised an in vitro, also using XANES-PEEM, assay to identify spicule proteins that may play a role in stabilizing various mineral phases, and found that the most abundant occluded matrix protein in the sea urchin spicules, SM50, stabilizes ACC • H2O in vitro.