June 22, 2001

Scientists bone up on calcite research

A Lab team’s new research on the process biological organisms use to modify crystal shape and growth, forming such complex structures as bones, eggshells and seashells, is detailed in the most recent issue of the science journal, Nature.

Christine Orme of Chemistry and Materials Science is the lead author of the article, titled "Formation of Chiral Morphologies through Selective Binding of Amino Acids to Calcite Surface Steps." It details the research and discoveries of a team of physicists, chemists and geologists from the Lab, in collaboration with the University of South Alabama and Viginia Polytechnic.

The results of the team’s research will apply in myriad uses, from potential laboratory growth of human and animal bones to industrial scale formation in pipes to the manufacture of toothpaste – any situation in which calcium-based crystals grow naturally or are utilized.

Orme explains that calcite, the material in eggshells and seashells, is the perfect model system for studying "biomineralization," the organic growth of crystalline structures.

"Pure calcite grows only in a symmetrical, six-sided pyramid-shaped crystal," said Orme. "We’ve wondered how nature controls the growth of the same substance into the intricate shapes found in shells and sea urchin spines. Now, by binding it with the amino acid aspartate, which is a common amino acid found in the protein of shellfish, we’ve been able to skew the growth to form asymmetric crystals, which are mirror-images of one another depending on whether we use right or left-handed amino acids."

The results were seen quite clearly through the Lab’s atomic-level microscopes, in which the team could measure the atomic-step speed and other variables in crystal growth. Surface spectroscopy and molecular modeling confirmed the visual results.

"The exciting outcome," said Orme, "is creating a picture of how these shape changes, on the molecular scale, guide the growth of a crystal into its ultimate form."

Furthermore, Orme reported the geophysical community has responded with great enthusiasm, citing the study results as a means of differentiating between carbonate crystals of biological and non-biological origin, based on the presence or absence of the aspartate.

Also, this discovery may prove a "geochronological tool" to date fossils, since aspartate molecules in living biological crystals tend to be of the L form (left-facing molecule), but upon death begin to change to combinations of L and D (right-facing molecules) forms.

The term "chiral" in the title refers to mirror-image molecules, otherwise identical in structure, therefore "left" or "right" facing as compared to each other.

Finally, Orme said, the results reflect on the origin of life itself. "They support the hypothesis that mineral surfaces (such as calcite) were responsible for the emergence of a single chiral form of the amino acids found in living organisms."

The team is now moving on to research involving calcium phosphate, the material used by animals to grow bones. If ever bones are to be grown in the laboratory, these are the first steps in that process, said Orme.

Other team members participating in this research were Aleksandr Noy, Mary McBride and James de Yoreo of Chemistry and Materials Science; Andrzej Wierzbicki, University of South Alabama; Meg Grantham and Patricia Dove, Virginia Polytechnic Institute and Henry Teng, George Washington University.

For the complete article, see the June 14 issue of Nature, which has both the article as well as a News and Views summary of the research.