Chemistry

Bounding Biomineralization

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Science  25 Aug 2006:
Vol. 313, Issue 5790, pp. 1020
DOI: 10.1126/science.313.5790.1020b

Many organisms build skeletons or shells out of calcium carbonate, either by localizing its crystallization or by stabilizing the otherwise short-lived amorphous form. In general, control of this process has been attributed to a mix of proteins, polymers, and magnesium ions on the assumption that each plays roughly the same role in inhibiting the nucleation and growth of crystalline calcite.

DiMasi et al. have used in situ synchrotron x-ray reflectivity to distinguish the roles played by these different substances. Specifically, they monitored the formation of amorphous calcium carbonate films on monolayers of arachidic acid placed against a subphase of saturated aqueous calcium bicarbonate, while varying the concentration of additives, including MgCl2 and the sodium salt of poly(acrylic acid)—a model of naturally occurring aspartate in proteins. The magnesium ions were found to introduce an induction period, delaying the onset of film formation, but had little influence on the subsequent growth of the film. Changing the polymer concentration also did not affect the growth rate of the films but did affect the lifetime of the metastable amorphous phase before it crystallized or redissolved into the solution. Finally, by varying the trough depth, the authors could tune the path length for the diffusion of carbon dioxide during film formation; this component of the study revealed an inverse relation between solution depth and growth rates. Overall, these results clarify the means of controlling the growth of amorphous mineral phases, of interest both in natural processes and in materials fabrication. — MSL

Phys. Rev. Lett. 97, 45503 (2006).

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