Controlling calcium carbonate precipitation in the presence of biological and organic molecules

Biomineralisation arises due to a partnership between the biological and inorganic components of a living system. The final structure and form of the inorganic material is in some way controlled by the nature of the specific organic entities present. This manifests itself in the initiation of the growth, by providing the appropriate matrix in which the inorganic material forms and/or by providing a "defect" base such that the inorganic crystal packing may be appropriately perturbed. Working with proteins is not necessarily the best or easiest way to understand a physical process and over past years people have turned to simple organic molecules, surfactants, small biological moieties and organic substrates in order to determine, at least in part, the import of organic/inorganic interactions during the growth of the inorganic material. As one example of these additives, surfactants, which represent approximately 50% of the cell membrane, display a diverse and vast array of geometrical forms in aqueous solution, many of which bare striking resemblance to biominerals, albeit on considerably smaller length scales. They also have the ability to associate in solution with inorganic material precursors, such as calcium ions. Hence, while they may not be the main driving force in the formation of biominerals, they are certainly present during the process and may, when used as model systems, allow us some way into the world of nanomaterials. Surfactants, a series of simple alcohols and carboxylic acids, and proteins extracted from the spines of adult sea urchins, have been used by our group to study the fori-nation of calcium carbonate based inorganic materials. The growth of the calcium carbonate is significantly affected by the inclusions, with deviations varying from simple stepped growth to the formation of curved surfaces.