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SSRL Stirs Up a Witch's Brew

(Photo - Spider Fang) One might assume the presence of insects or rodents in SSRL is a situation to be altogether avoided. But this summer, beamline 6-2 is experiencing just such a visitation, if not from the vermin themselves, then from a vast number of their individual parts. No it's not a piecemeal plague, but a witch's brew of studies involving spider fangs, fruit fly heads and mouse livers.

SSRL's beamline 6-2 is uniquely qualified to handle this onslaught. The x-ray microprobe housed there is a research destination to scientists from all over the world interested in taking a deeper look at the chemical make up of things, and in particular, what role certain metals play. Three of these recent studies have sought a deeper understanding of how such metals behave in some unusual organisms.

The x-ray microprobe at 6-2 takes advantage of a phenomenon whereby an x-ray, tuned to a specific energy, can knock free an electron from near an atom's nucleus. Other electrons within the atom rush in to fill the gap, emitting an x-ray of their own as they do so. This second x-ray, called fluorescence, is what scientists measure. That fluorescence is unique for every element. By tuning the x-ray microprobe beam to a specific energy, researchers can scan a piece of material, measure the fluorescence that comes out, and produce a detailed map of where specific compounds lie within that material.

Copper, for instance, is a trace metal important for living organisms, but too much of it in the body can cause disease. Wilson disease is a genetic disorder whereby copper accumulates in the liver and destroys its ability to filter the blood. Scientists hope to better understand this disease by working out where that copper accumulates. In June, a team led by Martina Ralle of the Oregon Health & Science University took scans of mouse livers to develop ways of studying Wilson disease using the x-ray microprobe. So far, x-ray fluorescence at SSRL has given Ralle a far more effective way of looking at where copper collects than the traditional method of tissue staining, and could lead to a quicker method of measuring the effects of the disease.

Besides mice, another common specimen used in medicine—the fruit fly—is helping researchers by revealing the location and chemistry of certain metals in the brain. By scanning the brains of fruit flies with x-rays, a team led by Helen Nichol of the University of Saskatchewan used the x-ray fluorescence technique to find copper, iron and zinc. These trace metals play a role in neurological disorders such as Alzheimer's, Parkinson's and Lou Gehrig's diseases. Nichol and her colleagues hope that perfecting a technique for identifying these elements in specimens such as fruit flies will lead to more rapid and accurate tests for new drugs intended to fight these illnesses.

The most unlikely creature under investigation at SSRL may soon provide insight into some of Nature's least-understood engineering principles. Spiders, it turns out, have a knack for concentrating elements such as zinc and bromine into their fangs, which researchers believe make them stronger and more durable. Many creatures with hard chewing mouthparts or stingers, such as ants and scorpions, use this strategy to strengthen their "tools." University of Georgia chemist Robert Scott and his team, including Robert Schofield of the University of Oregon and Michael Nesson of Oregon State University are using x-ray fluorescence at SSRL to figure out how and why a common garden spider known as an orb weaver uses these metals in its fangs.

"These are trace elements, and to use them they must be concentrated by the organism, which takes a lot of energy," said Scott. "These organisms have gone to a lot of trouble to concentrate these elements in their tools."

Scott and his team have also been investigating the hard parts of another arthropod, a common shore crab. This time they looked at the tips of the claws, called the crab claw "spoon," which researchers knew contained concentrations of bromine. But the resolution of the x-ray microprobe at SSRL revealed for the first time that zinc also appears in the crab claws in much the same way it does in spider fangs. Understanding why these two very different creatures have this biological feature in common could lead engineers to develop new and useful materials.

Studies such as Scott's may eventually reveal why Nature seems to favor using zinc and bromine together in animal tools, information that could bode well for manufacturers and materials scientists. And in the same way, using x-rays to look at mouse livers and fly heads could unlock important medical secrets. X-ray fluorescence microscopy, at its most basic level, is a tool that enables researchers to pinpoint the location of metals. But it's the nuanced application of such information that makes possible advances in fields that are as different as spider fangs and crab legs.

—Brad Plummer
    SLAC Today, July 21, 2006

Above image: X-ray scan of a spider fang showing concentrations of zinc (in red).