Like so many bricks, identical molecules can often fit together in many patterns. That means a single chemical compound can form several different crystals, each with different properties. But a brief flash of polarized light can force the commingling molecules to chose one crystal structure over another, researchers report in the 21 October print issue of PRL. The effect might someday help materials scientists and drug designers to fine tune their high-tech concoctions.
For more than a century, physicists and chemists have puzzled over how molecules in solution start forming crystals, a process called nucleation. Six years ago a team led by Bruce Garetz of Polytechnic University in New York and Allan Myerson of the Illinois Institute of Technology in Chicago found that a blast of polarized laser light caused crystals to form in a urea solution in just a few nanoseconds, instead of several days. The near-infrared light shifts the electrons in slowly growing random clusters of urea molecules, the researchers hypothesized, leaving the clusters with a slightly lopsided charge distribution. That so-called electric polarization (which is different from light polarization) helps align the molecules in the clusters and accelerates the formation of needle-like crystals, they reasoned. Skeptics countered that contaminants may have provided seeds for the crystals to grow around, or that the light may have triggered subtle chemical reactions within the molecules that boosted crystal formation.
But light polarized in different ways triggers the growth of different crystal structures, report Garetz, Myerson, and Jelena Matic of Polytechnic University in their new work. This time, the researchers shined their laser on solutions of glycine, an amino acid that can form three different crystal structures called alpha, beta, and gamma. The gamma structure consists of many parallel twisting strands of glycine molecules, while the alpha structure is made of stacked planes of molecules. Linearly polarized light–in which the electric field oscillates in a single direction–triggered the formation of the gamma structure; circularly polarized light–in which the electric field rotates like the hand of a stopwatch–triggered the formation of the alpha structure, but never the other way around.
The new observation should dispel doubts about the previous results in urea because the alternative explanations shouldn’t be sensitive to the type of light polarization, says David Oxtoby of the University of Chicago. “Any holes that might have existed in the past have been closed by this very pretty experiment,” Oxtoby says. The polarization effect could provide a useful tool for material scientists and drug manufacturers, he says, as it might be used to reliably select the one desired crystal structure, or even to produce new materials with novel properties.
But first others will have to confirm that the promising effect can be reproduced, says Michael McBride of Yale University in New Haven, Connecticut. Crystal nucleation is so mysterious and fickle, McBride says, that researchers often stumble across new effects that defy explanation. For example, in 1998, drug manufacturer Abbott Laboratories suddenly found that the AIDS drug Norvir insisted on crystallizing in a new, less soluble structure. The company eventually reformulated the drug in a gel capsule form. Of course, that’s the kind of problem that carefully applied polarized light might someday prevent.