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Nat Mater. 2016 Aug;15(8):903-10. doi: 10.1038/nmat4631. Epub 2016 May 2.

Tuning hardness in calcite by incorporation of amino acids.

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School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
Department of Materials Science and Engineering, 214 Bard Hall, Cornell University, Ithaca, New York 14853, USA.
BioArCh, Departments of Chemistry and Archaeology, University of York, York YO10 5DD, UK.
Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK.
Department of Materials Science and Engineering and the Russell Berrie Nanotechnology Institute, Technion Israel Institute of Technology, Haifa 32000, Israel.
Kavli Institute at Cornell for Nanoscale Science, 420 Physical Sciences Building, Ithaca, New York 14853, USA.


Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unit-mineral single crystals containing embedded macromolecules-remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite. We analysed lattice distortions in these model crystals by using X-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.

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