Researchers, including scientists at York, wanted to understand why in chalk, the mineral calcite is weak, but in mollusc shells, it has evolved into a much harder, tougher material capable of protecting the shell’s occupants from sharp-toothed predators.
In a breakthrough which could pave the way for stronger synthetic materials, they found that the key to calcite’s toughness is linked to the role of amino acids, the basic building block of proteins which make up an integral part of biologically-formed minerals.
Growing calcite crystals
To understand the chemistry behind the process, the team grew synthetic calcite samples in the lab in solutions containing different concentrations of the amino acids. They then tested the chemical and mechanical properties of the crystals formed.
When two of the amino acids (aspartic acid and glycine) were added in controlled amounts, they increased the strength of single crystal calcite to match that of the biogenic calcite found in materials such as seashells, coral and teeth.
The project involved analysis by researchers from our Departments of Chemistry and Archaeology, working with teams from the Universities of Leeds, Cambridge, Sheffield and Cornell University.
Dr Kirsty Penkman, an analytical chemist at York and Dr Beatrice Demarchi from our Department of Archaeology measured the concentration of amino acid within each calcite crystal. The work was carried out in the North East Amino Acid Racemization (NEaar) labs at York.
“The work in the labs at York showed that the amino acids were incorporated directly into the single calcite crystals formed.” said Dr Penkman. “The modelling studies showed that this incorporation meant that to break the crystal apart, the bonds within the amino acids had to be broken too, giving the crystal much greater hardness.”
Mother Nature
Dr Penkman added: “This hardening process has been applied by Mother Nature through over 500 million years of evolution. Now we have managed to replicate the technique in the lab.
“The understanding gained from this research opens up opportunities to apply the same mechanism to other man-made materials, such as synthetic bone implants, to increase their toughness and durability.”
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