A particle gun that fires liquid droplets less than a millionth of a meter in diameter, faster than hundreds of thousands of times a second, is poised to revolutionize biological imaging. Tested at Berkeley Lab's Advanced Light Source and soon to be installed at SLAC's Linac Coherent Light Source, the sample jet injects a beam of droplets across a tightly focused x-ray beam in single file, each droplet so small it contains only a single protein or virus.
John Spence, a physicist at Arizona State Univ., is a longtime user of the Advanced Light Source at Lawrence Berkeley National Lab, where he has contributed to major advances in lensless imaging. It's a particularly apt propensity for someone who works with x-rays, since they can't be focused with ordinary lenses.
As new light sources evolve to produce brighter x-rays in faster pulses, lensless imaging becomes ever more critical for science. Among the promises of superbright, ultrafast x-ray pulses is the ability to solve the structure of the complicated molecules from which our bodies are made.
The Linac Coherent Light Source (LCLS) will soon begin operation at the SLAC National Accelerator Laboratory in Palo Alto, Calif., using energetic electrons from a linear accelerator to produce coherent x-rays with an instrument called a free electron laser (FEL). The x-rays will be delivered 120 times/sec in pulses only a tenth of a trillionth of a second long. These brief, bright pulses offer a novel approach to the problem of protein structure.
Proteins are usually large molecules containing many thousands of atoms. Drug molecules are much smaller, and do their work by attaching themselves to the larger protein molecules. A knowledge of the arrangement of a protein's atoms is therefore a great help to drug designers, who like to understand how a drug molecule will dock with a protein to promote or inhibit its activity, or cripple the organism of which it is a part.
Until now, the best way to solve the structure of a protein or virus has been with x ray crystallography. The crystal consists of many copies of the protein or virus arranged in regular order. As the crystal rotates in the x-ray beam, x-rays scatter off the atoms and reveal how the electrons, and thus the atoms, are arranged.
But many proteins can't be crystallized, and others are so difficult to crystallize it's impossible to obtain crystals large enough to use in today's light sources.
Ultrafast, ultrabright x-rays offer a way past this dilemma. The idea is that a quick pulse of tightly focused x-rays can be diffracted from a microcrystal or even a single protein or virus in solution. The pulse is so brief that it comes and goes before any of the atoms can move, freezing their orientation like a strobe light. Just as important, a sufficiently brief pulse may terminate before radiation damage effects can start. In this way it can outrun radiation damage, always one of the fundamental limitations to imaging in biology.
Another quick pulse could be diffracted from another copy of the protein in a different orientation. As the process is repeated, diffractions from different angles give the overlapping views needed for the computer to construct a 3-D image of the structure.
It's a great idea, but as Spence notes, there are problems. "So as not to scatter, the x-ray beam has to be in a high vacuum, but a protein or virus in its natural state is usually wet. The answer was what Spence calls a "particle gun, like an ink-jet printer," designed to inject a beam of water droplets across the tightly focused x-ray beam in single file, each droplet so small it contains only a single protein or virus. He and colleagues Bruce Doak and Uwe Weierstall designed a nozzle that can fire liquid droplets, each less than a millionth of a meter in diameter, faster than hundreds of thousands of times a second. The sample jet is designed to shoot droplets right through a pulsed beam of x-rays a billion times brighter than any ever created in a light source before.
In their nozzle, liquid flows through a narrow capillary inside the tube through which the gas flows; the liquid issues from the capillary some distance from the opening in the outer tube, so the gas surrounds it, then increases speed and pressure as it approaches the opening, squeezing and accelerating the thin stream of liquid until it is so small that the proteins or viruses dissolved in the liquid can only fit into the droplets one at a time.
And the nozzle won't clog, because even a particle bigger than the sample protein or virus-bigger than the stream of liquid itself-can still fly through the glass nozzle without hitting the walls and getting stuck.
The frequency at which the droplets emerge can be controlled by an oscillator the researchers call an "acoustic trigger." Tuning the acoustic trigger adjusts the frequency so that each droplet containing a protein or virus meets an incoming pulse of x-rays.
The entire device - which the researchers call a gas dynamic virtual nozzle (GDVN) - is only about a millimeter in diameter (not counting feed lines and cables) and fits to the side of the beamline's vacuum chamber.
Source: Lawrence Berkeley National Lab