SSRL Science
Highlight - May 2009 ![]() | ||||||||||
![]()
Our results suggest that the greatest advantage of bromine-rich cuticle over calcified cuticle is resistance to fracture (the energy of fracture is about an order of magnitude greater than for calcified cuticle). The greatest advantage relative to unenriched cuticle, was a factor of about 1.5 greater hardness and modulus of elasticity, making the tips harder and more stiff than acrylic glass. The claw tips gain increased fracture resistance from the orientation of the constituent laminae and from the viscoelasticity of the materials. It may not be surprising that smaller organisms would employ fracture-resistant materials in the thin regions of their "tools". Smaller organisms may be subjected to the same forces from the environment, predators, and prey as larger organisms, but the smaller cross sections of their "tools" make them more susceptible to fracture.
The mechanism by which the bromine (and other heavy elements in other organisms) modifies the mechanical properties is not understood. One possibility is that bromine makes the material harder and stiffer by increasing the cross-links between proteins. Another, more speculative, possibility is that the high mass density of the heavy elements improves resistance to fracture. The attachment of many heavy bromine atoms to many phenyl rings along the protein would reduce the resonant frequencies of certain low frequency large-scale molecular motions (such as standing torsional waves over the length of the molecule). If these resonances were lowered to overlap more with the range of frequencies associated with impacts, then bromination might improve damping of impact energy.
A good example of the bromine biomaterial can be investigated the next time you
eat a Dungeness crab. Notice that the sharp tip of the leg is a cap of
translucent material that is very different from the rest of the crab. It is
very difficult to break the tip, even though it is very thin, stiff and hard.
Humans are just starting to try to engineer tiny machines and tools, and we
have a lot still to learn from organisms that have coped with being small for
millions of years.
Primary Citation
R. M. S. Schofield, J. C. Niedbala, M. H. Nesson, Y. Tao, J. E. Shokes, R. A.
Scott and M. J. Latimer, "Br-rich Tips of Calcified Crab Claws are Less Hard
but More Fracture Resistant: A Comparison of Biomineralized and Heavy-element
Biomaterials", J. Struct. Biol. 166, 272 (2009)
| ||||||||||
SSRL is supported by the Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences. |
Last Updated: | 27 May 2009 |
Content Owner: | R. Scott, University of Georgia |
Page Editor: | L. Dunn |