A nanopulse of laser light is enough to trigger crystallization.
A technique that creates crystals on demand using laser pulses could make it easier to prepare the high-quality crystals needed to study protein structure.
Chemists and biologists need crystals of proteins and other chemicals to analyse their atomic structure using X-rays, while many industrial processes rely on triggering crystal formation at precisely the right time and place during the production of drugs and other useful compounds.
Yet "crystallization still remains largely a black art," says Stephen Curry, a protein crystallographer at Imperial College London. The trickiest part is controlling the very first step, where molecules begin to aggregate in an ordered way around a nucleation point, such as a seed crystal or speck of dust.
Chemist Andrew Alexander and colleagues from the University of Edinburgh have now shown that pulses of low-energy laser light can trigger the formation of crystals from a solution of a chemical held within a gel. This enables them to control exactly when and where crystals form without the need for an added nucleation point1.
The team exploited an effect called non-photochemical laser-induced nucleation (NPLIN), which was discovered by serendipity a decade ago2. Scientists led by Bruce Garetz at the Polytechnic Institute of New York University in Brooklyn found that shining pulses of laser light into a solution of urea in water triggered crystal formation.
Alexander's team have refined the technique, training 7-nanosecond-long pulses of near-infrared laser light onto a gel of the polymer agarose incorporating a solution of potassium chloride. Agarose gels are increasingly used to grow crystals because they often produce large, high-quality single crystals suitable for structure analysis.
Covering parts of the gel with a mask and then scanning the laser across resulted in potassium chloride crystals forming within minutes in the areas left uncovered (see picture). The laser is not powerful enough to cause chemical changes in the sample, and one pulse triggers the formation of just one crystal. And two beams can be used to induce crystallization only at the point where they cross, controlling the process in three dimensions. Preliminary experiments have proved that the technique can also speed up crystallization of a protein, the enzyme lysozyme, adds Alexander.
The team do not know precisely how the laser light causes crystallization, but believe that it changes the distribution of electrical charges within the molecules so that oppositely charged parts begin to stick together.
"We do not honestly believe that the NPLIN will work for every molecule or compound," says Alexander, but he hopes that the method could eventually be used routinely in the laboratory.
Curry agrees that "this technique gives us another bit of artistry to try", but warns that it is far from clear whether the technique will work on more complicated proteins of biological interest. Potassium chloride and lysozyme are easy to crystallize by conventional means.
"One would want to see this technique demonstrated to work with other proteins, maybe 10 or 15 of them. If it works that would be really impressive, but it's not a revolution just yet."
Duffus, C., Camp, P. J. & Alexander, A. J. J. Am. Chem. Soc. doi:10.1021/ja905232m (2009)
Gartez, B. A. et al. Phys. Rev. Lett. 77, 3475-3476 (1996).
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