engleski

Calcite crystal growth kinetics in the presence of charged synthetic polypeptides

Poly-L-glutamic (pGlu) and poly-L-aspartic (pAsp) acids, as analogues of naturally occurring soluble acidic proteins, were used to investigate specific interactions between the acidic polypeptides and calcite as a mineral surface. A basic polypeptide (poly-L-lysine, pLys) served as a negative control for testing the role of conformity between the substrate and the adsorbed polypeptides in biomineralization processes. The kinetics of calcite crystal growth was determined in a simplified precipitation model system by inoculating well defined calcite seed crystals into a moderately supersaturated solution containing one of the polypeptides. From the kinetic data, the mode and extent of the calcite/polypeptide interactions were determined. The parabolic rate law was found to be valid for the calcite crystal growth, the integration of ions into the spiral steps at the calcite crystal surface being the rate-determining mechanism. Small amounts of pGlu or pAsp caused an inhibition of calcite crystal growth, the effect being pAsp > pGlu, and the exponential dependence of the growth rate on supersaturation confirmed that surface nucleation was the growth controlling mechanism in the presence of the two acidic polypeptides. The pLys non-selective, weak, electrostatic adsorption at the crystal surface was probably responsible for increasing the calcite growth rate at low concentrations and for inhibiting it at higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides were observed for the calcite/pAsp systems. They could be accounted for by coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β -pleated sheet) and Ca2+ ions from the calcite surface.

calcium carbonate ; calcite ; polypeptides ; crystal growth mechanism ; biomineralization

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