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Critter Chemistry

October 17, 2003

Bait with Bite

Zinc ions help make clam worm jaws tougher than most synthetic polymers

Victoria Gilman

Humans, like most other vertebrates, rely on calcium to give jaws and teeth the toughness and wear-resistance they need for a lifetime of chomping. However, calcium is not the only element capable of giving life its bite. Ongoing research is uncovering evidence that several species of invertebrates prefer biopolymers based on transition metals to make their teethlike jaws hard and tough.

BRINY BRISTLES The bristle-like parapodia on the clamworm help it swim or crawl along the sea floor in search of food. PHOTOS BY DR. HELGA C. LICHTENEGGER
An international team of researchers based at the University of California, Santa Barbara (UCSB), has found that zinc in the jaws of the marine polychaete worm Nereis limbata is responsible for the jaws' toughness and wear-resistance [Proc. Natl. Acad. Sci. USA, 100, 9144, (2003)]. Nereis worms, commonly known as clam worms, are scavengers found in sediments along both coasts of North America.

"Transition metals in polychaete worm jaws have been a matter of record for 20 years, but then the research didn't progress," says J. Herbert Waite, professor of molecular, cellular, and developmental biology at UCSB. In 1980, two British researchers reported in the U.K.-based Journal of the Marine Biology Association that they found high levels of copper in the jaws of the common polychaete known as the bloodworm and similarly high levels of zinc in the jaws of the clam worm.

The 1980 findings, however, did not indicate why or in what form the metals existed in the worms' jaws, and the research was never followed up. When Waite and fellow UCSB chemistry professor Galen D. Stucky were approached by Austrian postdoctoral fellow Helga C. Lichtenegger to embark on a research project, they figured the worms' metal mouths might finally meet their match.

The researchers began by considering two possibilities: Either the worms are exposed to pollution containing the metals and concentrate the metals in their jaws to detoxify them, or the metals are metabolized from gravel in marine sediment to serve a function in the structure of the jaws. To find an answer, the team began studying the forms the metals took and how they were arranged in the jaw structure.

In a study released last year, the team reported that the copper in the jaws of the bloodworm exists as the mineral atacamite, the first copper-containing biomineral ever discovered (C&EN, Oct. 14, 2002, page 16). The copper is concentrated at the tips of the jaws, giving them the strength and toughness needed for the carnivorous worm to bite its prey and inject venom.

Based on the bloodworm findings, the team began searching for evidence that the clam worm jaws contained a zinc biomineral. The researchers used a combination of advanced techniques to study various aspects of the clam worm jaws, including X-ray diffraction, electron microscopy, and nanoindentation.

JAW DROPPING Zinc fluorescence imaging shows that the metal is concentrated at the tip of the clamworm jaw.
The team found that, like copper in the bloodworm, the highest levels of zinc occur in the tips of the clam worm jaws. In addition, only zinc is bound to the protein matrix with high affinity, which told them it was not being "dumped" into the jaws as a detox method. In trying to identify a zinc biomineral, they found that the zinc always appears in tight correlation with chlorine, but they couldn't detect evidence of a simple inorganic zinc-chlorine compound.

"Every single technique we used indicated the absence of a mineral," Waite says. Instead, the data suggested that zinc and chlorine are both directly coordinated to the protein, with the zinc acting as a surrogate cross-linker. The protein fibers are highly oriented and are wrapped in tight bundles using zinc cations as the binder. "It's like tying together lots of willow branches; one branch will droop, but many wrapped into a bundle are strong," Waite says.

"Cross-linking the histidine groups with the metal atoms gives the biopolymer a very tough structure," Stucky says. In fact, the material that makes up the clam worm jaw can outperform most known synthetic polymers. "The mechanical properties of these biopolymers are at the upper limit of what man can make," he says. The team is continuing to study how chlorine fits into the jaw structure, as well as how the structure and function of zinc in clam worms compares to copper in bloodworms.

By understanding the intricacies of the two worms' biomaterials, scientists could one day use them as the basis for designing longer lasting dental fillings and more environmentally friendly adhesives. Waite stresses, however, that often in fundamental research, scientists are not working with specific applications in mind.

"Nature is selfish," he says. "You can't immediately take the trick it's developed for making worm jaws and apply it to spaceships. You first have to understand the subtleties of the interfaces of these materials; then you can try to apply them to the synthesis and processing of future polymers."

Chemical & Engineering News
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