In vitro regulation of CaCO3 crystal polymorphism by the highly acidic molluscan shell protein Aspein

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Biominerals, especially molluscan shells, generallycontain unusually acidic proteins. These proteins are believedto function in crystal nucleation and inhibition. We previouslyidentified an unusually acidic protein Aspein from the pearloyster Pinctada fucata. Here we show that Aspein can controlthe CaCO3 polymorph (calcite/aragonite) in vitro. While aragoniteis preferentially formed in Mg2+-rich solutions imitating theextrapallial fluids of marine molluscs, Aspein exclusively inducedcalcite precipitation. Our results suggest that Aspein is involvedin the specific calcite formation in the prismatic layer. Experimentsusing truncated Aspein demonstrated that the asparticacid rich domain is crucial for the calcite precipitation.