July 07, 2011
Brookhaven scientists and lead authors, from left, Marc Allaire, Allen Orville, and Alexei Soares in the National Synchrotron Light Source II (NSLS-II) ring building, now under construction at Brookhaven Lab. The research group’s work on acoustic drop ejection could improve macromolecular crystallography studies at the new facility. Photo: Brookhaven National Laboratory
With a new class of brighter X-rays produced at next-generation light sources like Brookhaven's soon-to-be National Synchrotron Light Source II (NSLS-II), scientists will be able to study crystals measuring only a few micrometers along an edge (like those from membrane proteins)—an ability that’s not possible with conventional macromolecular crystallography.
But there's a problem: As the crystal size is reduced, the diffraction signal weakens due to noise from the surrounding solvent. To address this technological gap, scientists from Brookhaven and Labcyte Inc. (Sunnyvale, Calif.) are incorporating a technique called acoustic drop ejection (ADE) into studies at NSLS and the Advanced Photon Source (APS) at Argonne National Laboratory. The technique uses intense sound waves to launch from a solution of very small droplets containing even smaller protein crystals through the air and to a mounting mesh. There, the crystals are held in the line of X-ray beam.
"The idea that sound waves of great intensity near the surface of a liquid will eject droplets was demonstrated in the 1920s," says Brookhaven biologist Alexei Soares, a beamline scientist at NSLS for the Macromolecular Crystallography Research Resource (PXRR). "Nowadays, ADE is used to transfer living cells and isolated DNA without inducing strand breaks, suggesting that it might be gentle enough for protein crystals. The possibility of using ADE to move droplets of extremely small volume is ideal in minimizing the solvent surrounding microcrystals."
First, the scientists had to make sure that the ultrasonic sound waves—which have a greater frequency than we can hear—produced by ADE don’t damage the crystals. The researchers grew two slurries containing solvent and hundreds of microcrystals of insulin or lysozyme—each crystal measuring about 10 micrometers on a side, about the size of a single cell. With an ADE device built by Labcyte, the group focused acoustic waves through the slurries, the pressure of which eventually expels 2.5 nL drops of the crystal-filled liquid to the mounting mesh.
The individual microcrystals, which are nearly impossible to see even with powerful microscopes, were located within the drops by scanning a very small X-ray beam across the micromeshes. Then, X-ray diffraction was performed at NSLS and APS to determine the crystal structures.
"We were astounded by the results. The fragile microcrystals are not harmed by the novel ADE mounting technique, and therefore they produce high-resolution X-ray structures," says Brookhaven biologist and PXRR beamline scientist Allen Orville. "These developments enable new opportunities in automated microdiffraction and the manipulation of microcrystals."
Their results were published in Biochemistry.
Because ADE is able to transfer an ejected drop to a precise location, the researchers also foresee the possibility of using the technique to load multiple drops of different crystal solutions on a single mesh, speeding up the discovery process.
"Using ADE, a new strategy could be developed for high-throughput structural screening of microcrystals against libraries of chemical compounds for drug development," Allaire says.
(a) Stroboscopic photomicrograph of a single 2.5 nL water droplet launched via ADE from the liquid surface.
(b) Image from NSLS beamline X25 of the insulin microcrystal slurry supported on a micromesh (25 ?m by 25 ?m grid pattern) used to determine the structure. The 20 ?m by 20 ?m X-ray beam is centered at the intersection of the white cross hairs.
(c) Illustration of the concepts for the raster-scanning X-ray diffraction strategy with a microdiffractometer and a several-micrometer-wide X-ray beam. Image: Brookhaven National Laboratory
Read the article in Biochemistry (Acoustically mounted microcrystals yield high-resolution X-ray structures. Soares et al., Biochemistry. 2011, 50(21):4399-401.) Read the poster from Labcyte and Brookhaven National Laboratory