engleski

Investigation of calcite crystal growth mechanism in the presence of poly-L-glutamic and poly-L-aspartic acid

Calcium carbonate polymorphs (calcite, aragonite and vaterite) are probably the most common biominerals, at least the two more stable ones (calcite and aragonite). In the case of mollusc shells, regular calcite and/or aragonite mineral layers are associated with organic matrix, thus resulting in hybrid materials having remarkable mechanical properties. Water-soluble components of organic matrix, mostly acidic proteins rich in aspartic and glutamic acid residues, are supposed to be responsible for the formation of a particular polymorph and for arranging its growth into desired patterns by controlling the shape, size and orientation of single crystals. In this work, poly-L-glutamic (pGlu) and poly-L-aspartic acid (pAsp) were chosen as simple analogues to the mentioned soluble acidic proteins. Although both polypeptides have similar chemical composition, they differ in structure when in solution containing calcium ions: pAsp is predominantly in the form of  sheet and pGlu is predominantly in random coil conformation. In order to study the specific polypeptide/mineral surface interactions, the kinetics of calcite seed crystal growth was determined in the precipitation systems containing different concentrations of polypeptides. It was found that, for the whole range of the investigated initial supersaturations, the crystal growth rate of calcite was a parabolic function of supersaturation, indicating that the integration of constituent ions into the spiral step was the rate determining process. Both, pAsp and pGlu inhibited the crystal growth rate of calcite, the effect being more pronounced in the case of pAsp. Moreover, the exponential relationship between the crystal growth rate and supersaturation was found when either pGlu or pAsp was present in the solution. This indicated that the surface nucleation was the controlling mechanism. The obtained results suggest the possible role of the polypeptide structure on the extent and the type of their interactions with the calcite surface.

calcium carbonate; calcite; crystal growth mechanism; poly-L-glutamic acid; poly-L-aspartic acid

nije evidentirano