WORMS WITH A COPPER SMILE
Mineral composite in Glycera jaws imparts strength, abrasion resistance
The rare mineral atacamite--Cu2(OH)3Cl--is named after the Atacama Desert in Chile, one of the few places where it is found. A team of U.S. and Austrian scientists has now found another source: the teethlike jaws of the marine bloodworm Glycera dibranchiata.
The discovery by research associate Helga C. Lichtenegger of the department of chemistry and biochemistry at the University of California, Santa Barbara, and co-workers adds to the relatively short list of about 70 minerals that animals make to form structures such as shells, teeth, and bones. Atacamite is the first of these biominerals known to contain copper [Science, 298, 389 (2002)].
A low degree of Cu2(OH)3Cl mineralization in a protein matrix yields a surprisingly strong material that makes up the 1.5-mm-long jaws of the bloodworm Glycera dibranchiata.
Lichtenegger and her colleagues used a combination of X-ray diffraction, electron microscopy, and other techniques to determine the chemical composition and mechanical properties of the worm's jaws. The carnivorous worm uses the hard, sharp-tipped structures to bite and inject venom into its prey.
The tip region of the jaws contains layers of polycrystalline atacamite fibrils dispersed in a protein matrix, the researchers find. The fibrils, which align with the jaw's outer contour, are about 80 nm in diameter and 1210 mm long. They are concentrated toward the center of the tip, while the base region is devoid of fibrils. The hardness and stiffness of the jaw increases from the base to the tip and from the surface to the interior, which correlates with the increasing degree of mineralization.
Cross-sectional maps of copper and chloride concentration confirm the elemental distribution in the jaw and show that the Cu-Cl ratio is higher than that found in atacamite, suggesting that free copper ions are present. Copper is known to cross-link polymers and protein scaffolds, the researchers point out, and the excess copper in the worm's jaw could be playing that role.
The jaw's impressive structural stability and resistance to abrasion approaches that of human tooth enamel, the team notes--even though atacamite makes up only 4% of the worm's jaw, while hydroxyapatite, Ca5(PO4)3OH, makes up 96% of tooth enamel. This toughness is needed to protect the jaw from wear and tear as the worm burrows through gritty marine sediment where it may chomp indiscriminately on bits of gravel.
G. dibranchiata appears to exhibit strict chemical control to consistently produce only atacamite, rather than any of the other three known Cu2(OH)3Cl polymorphs. The selectivity for copper rather than other elements such as calcium, silicon, and iron commonly found in biominerals suggests that copper may play an additional role, such as activation of the worm's venom during injection, the researchers say.
"The marriage of protein with copper mineral as well as with copper ions is an intriguing concept per se, but may serve as a design prototype for new materials that need to be hard, lightweight, and durable," the researchers write.