Biological Control on Calcite Crystallization by Polysaccharides

Polysaccharides control the growth of calcite in coccoliths by adsorbing preferentially onto particular surfaces of the calcite crystal. We chose four units from the coccolith associated polysaccharide (CAP) that is known to promote formation of vicinal faces in Emiliania huxleyi, namely, galactose, mannose, xylose, and rhamnose, and used molecular dynamics simulations to calculate the absorption of oligosaccharide units onto a number of calcite surfaces. The simulations show a wide range of adsorption energies, which depend on the combination of organic molecule and surface. Oligosaccharides on polar surfaces with Surplus negative charge have the strongest adsorption, while those on polar surfaces with surplus positive charge have the weakest. Acute stepped vicinal surfaces have stronger adsorption than planar surfaces, while obtuse stepped surfaces have weaker adsorption than the planar surfaces. On the basis of these simulations, the behavior of two saccharides on the calcite {1 0 (1) over bar 4) surface was observed experimentally with atomic force microscopy (AFM) and shown to be consistent with the simulations. This helps explain why the polysaccharides involved in biomineralization have the chemical composition that they do and also suggests criteria for new molecules to control calcite crystal growth.