A study of calcium carbonate formation in biological systems
This thesis has studied aspects of biomineralisation, covering the inorganic mineral, the organic matrix, the possible use of phospholipid bilayer vesicles to control mineralisation, calcium and other metal binding to an antibiotic ionophore, Lasalocid-A and a study of a known inhibitor of biomineralisation, the glycopeptide antifreeze found in the plasma of fish which live under polar conditions. The mineral systems studied have been calcium carbonate formations in otoconia and otoliths, crystals which form part of the balance organs of the inner ear, and coccoliths, the earliest eukariotic formation of calcium carbonate, from an alga. Both these systems have been studied by ultra-high resolution electron microscopy with the observation that both types of structures grow in a unique manner, quite distinct from their geological counterparts; indeed the coccolith system involves two distinct mechanisms of growth for different parts of its structure, which is only 2 μm in diameter. Mechanisms of growth of both biominerals are proposed. The study of the organic matrix was less successful in that it was not possible to fully characterise an acidic matrix protein, but it has been shown that the soluble matrix consists of many polypeptide chains cross-linked together, which undergo a conformational change on dissolution from the insoluble matrix on which they lie in vivo and consequently give in vitro results which do not mimic the in vivo condition. Equally, the use of vesicles to control the formation of calcium carbonate was shown to be possible on occasion, but lipids are very unstable in the presence of calcium and no means of stabilising the system to produce consistent results was determined. Two studies were made by 1H-nmr, the metal-ion complexes of the ionophore Lasalocid-A and the antifreeze glycopeptide of polar fish, in order to demonstrate principles of the handing of isolated ions and of crystallisation inhibition. In both cases, the biological action of the system was mimicked and followed by nmr and a mechanism for their function proposed.