The Jaws of Nereis : Microstructure and Mechanical Properties
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The Jaws of Nereis: Microstructure and Mechanical Properties Henrik Birkedal1,4, Chris Broomell2, Rashda K. Khan1, Nelle Slack3, Helga C. Lichtenegger1,5, Frank Zok3, Galen D. Stucky1,3, J. Herbert Waite1,2 1
Department of Chemistry and Biochemistry, 2Department of Molecular, Cellular, and Developmental Biology, and 3Materials Department, University of California, Santa Barbara, CA 93106, U.S.A. 4 Present address: Department of Chemistry, University of Aarhus, 140 Langelandsgade, DK8000 Aarhus, Denmark. 5 Present address: Department of Materials Science and Technology, Vienna University of Technology, Favoritenstrasse 9-11, A-1040 Wien, Austria. ABSTRACT The jaws of the marine worm Nereis sp. are made of protein fibers and are reinforced by zinc. Here we study a transverse section through the jaw using optical microscopy and nanoindentation. Optical microscopy images demonstrate a complex microstructure which includes channels that extend throughout the jaw. We suggest that these channels may be related to jaw remodeling. The mechanical results reveal spatial variations in both indentation hardness and reduced modulus. Specifically, the toothed side of the jaw (used for grasping food) is harder than the remainder of the jaw and the very exterior surface is hardest. INTRODUCTION Nature uses a host of strategies for the design of hard tissues such as teeth and jaws including biomineralization [1] but also protein cross-linking of cuticle [2]. We have previously shown that the related polychaete marine worms Glycera sp. and Nereis sp. use two different designs for their jaw systems: Glycera is partially mineralized [3] while Nereis is not [4]. Glycera dibranchiata presents the first instance of the use of a copper biomineral, namely atacamite (Cu2(OH)3Cl) [3,5]. Nereis in contrast contains non-mineralized zinc [4,6,7,8]. The zinc is concentrated towards the jaw tip, the portion of the jaw that is exposed to the greatest mechanical stress. Indeed, we found a correlation between zinc content and both indentation hardness and modulus [4]. The Nereis jaws also contain the halogens Cl, Br and I [4, 7]. The chlorine concentration is linearly dependent on the Zn concentration suggesting that they occur together in the jaws. The zinc/chlorine is not bound in a detectable crystalline phase. However, the histidine concentration in the protein matrix also increases towards the jaw tip and on the basis of EXAFS data we suggested that a Zn(His)3Cl binding motif is a fair representation of the average zinc environment [4]. The heavier halogens, Br and I, are concentrated in the surface region of the jaw [7,9]. Herein we report a study of a transverse cross-section of a Nereis jaw by optical microscopy and nanoindentation and show that the toothed side of the jaw, in particular its surface, is the hardest.
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EXPERIMENTAL DETAILS Jaws were dissected from recently thawed Nereis virens specimens (Maine Bait Company, ME, USA) that had been put to death by rapid freezing at -80ºC. The jaws were washed three
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