Bare bones of crystal growth: Biomolecules enhance metal contents in calcite

BLACKSBURG, Va., Nov. 10, 2008 – From shells to bones, the skeletons of organisms contain small amounts of impurity elements such as magnesium. Because the levels of these elements provide important clues to past environments, a considerable effort has focused on understanding how to relate impurity contents to the ancient environments in which an organism lived.

In the Oct. 31 issue of Science magazine, Allison Stephenson, a Ph.D. candidate in geosciences, and Patricia Dove, professor of geoscience in the College of Science at Virginia Tech, and colleagues* report that a hydrophilic peptide, similar in character to those found in calcifying organisms, significantly enhances the magnesium (Mg)-content of calcite.

"We knew from another study in our group (Elhadj et al., 2006, PNAS) that the chemistry of simple peptides as well as proteins could be tuned to control crystal growth rate and change crystal morphology," said Dove. "From that understanding, we realized that the water-structuring abilities of certain biomolecules could also influence the amount of impurities that can go into minerals."

"All organisms use proteins to grow minerals into complex shapes with remarkable functions," said lead author Stephenson. "But this finding is especially meaningful for geologists because Mg-content in carbonates is used as a 'paleo thermometer'. That is, we know that Mg content increases with temperature, but now we see that certain biomolecules could also affect those 'signatures'. The findings raise questions about the interplay of different factors on metal-contents in biominerals."

The findings also offer new insights for materials synthesis because a high degree of control on impurities is often necessary to give specific properties such as strength or electrical conductivity. By using biomolecules, it may be possible to tune impurities to desired levels, Dove said.

"Also, this basic research suggests new ways of looking at biochemical origins of pathological skeletal mineralization, and whether local biochemistry could influence the uptake of toxic metals into human skeletons," Stephenson said.

*The article, "Peptides enhance magnesium signature in calcite: Insights into origins of vital effects," was written by Stephenson; J. J. DeYoreo of the Chemistry and Materials Science Directorate, Lawrence Livermore National Lab (LLNL), Dove's former student; L. Wu of LLNL and the Department of Applied Science, University of California–Davis; K. J. Wu of LLNL; J. Hoyer of the Department of Biological Sciences, University of Delaware; and Dove. See also related work just published in Nature which cites the “Shell study sheds light on biomineralization” article from naturenews.

The paper is part of Dove's ongoing research into mineral nucleation, growth, and dissolution, aimed towards understanding processes of biomineralization, cementation, global elemental cycling, and climate proxy models. Her research group mimics processes from both biological and inorganic settings to understand underlying reaction mechanisms through direct, nanoscale measurements of mineral-water interactions and their kinetic and surface thermodynamic properties.

Research in Dove's group is supported by the Department of Energy Basic Energy Sciences, the National Science Foundation Division of Earth Sciences program in geobiology and environmental geochemistry, and the National Science Foundation Division of Ocean Sciences program in chemical oceanography. For more information, visit the Biogeochemistry of Earth Processes website.

Contact Dove, corresponding author, at (540) 231-2444.