In a series of experiments using an atomic force microscope, De Yoreo's team and that of Patricia Dove, a geoscientist from Virginia Polytechnic Institute and State University, used four different biomolecules to study their effects on the dynamics of atomic steps during crystallization. They set out to test a two-decade-old model of crystal-shape modification believed to be at odds with classic theories of crystal growth. Their results appear in the Nov. 19 issue of the journal, Science.
The main focus of the work was on the mineral calcite, which has more than 300 identified crystal forms that can combine to produce at least a thousand different crystal variations. Crystals can form a thousand different shapes by combining the basic forms of the positive rhombohedron (a prism with six faces, each a rhombus), negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid. De Yoreo and Dove first determined that when combined with magnesium, the corners formed by the intersection of atomic steps flatten and roughen, leading to flattening of the crystal's corners and elongation and roughening of the crystal shape.
When combined with acidic amino acids, both the step and crystal shapes changed to reflect the handedness (whether the molecule was right-handed or left-handed) of the amino acids. Molecular simulations showed that the step edges provided the most favorable binding environment for the acids.
When citrate, a naturally occurring inhibitor and therapeutic agent, was used in the experiment, the change in crystal shape again mimicked the change in step shape, and molecular models also identified the steps as the preferred interaction sites.
In the last experiment, calcite crystals were combined with a protein extracted from abalone nacre, a pearly substance that lines the interior of many shells, and is most perfect in the mother-of-pearl. The changes were step-specific and directly determined the shape of the macroscopic crystals.
"Although crystal growth modifications are diverse, the source of shape changes in these studies is clear," De Yoreo said. "Crystal shape is controlled by step-specific interactions between growth modifiers and individual step edges on pre-existing crystal faces. Through this research, our team has shown that the classic theories of growth merge smoothly with the models proposed to explain shape modification."
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Science