MULTIFACETED Hazen displays calcite crystal with mirror-image faces.

The common mineral calcite has crystal surfaces that are mirror images of one another. Robert M. Hazen, staff scientist at Carnegie Institution of Washington's geophysical laboratory; Timothy R. Filley, assistant professor of environmental chemistry at Purdue University; and Glenn A. Goodfriend, research associate professor of geology at George Washington University, find that the l form of certain amino acids adheres selectively to one of calcite's crystal faces and the d form, to the other [Proc. Natl. Acad. Sci. USA, 98, 5487 (2001)].

Although this preference does not appear to hold for every amino acid the researchers have tested, it's true for several, including aspartic acid, glutamic acid, and alanine. These three, along with nonchiral glycine, are the amino acids thought to have been most abundant in a prebiotic world.

"Chiral selection requires three distinct points where you have some sort of bonding between the molecule and the surface," Hazen explains. "I think of it as the 'bowling ball problem.' If you are a left-handed person, you can't use a right-handed bowling ball because the three holes are arranged with the wrong topology."

Earlier studies have considered selective adsorption of d- or l-amino acids onto minerals like quartz, which themselves exist in d and l forms. But calcite, which is not chiral but forms mirror-image crystal surfaces, was also worth examining, the researchers thought. "Calcite crystal surfaces are, and always have been, abundant on Earth," Hazen notes. "We also know that biological systems today take advantage of the strong adsorption between amino acids and calcite."

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