Calcite Crystal Growth Kinetics in the Presence of Charged Synthetic Polypeptides

Abstract

Poly-l-glutamic (pGlu) and poly-l-aspartic (pAsp) acids, as analogues of naturally occurring soluble acidic proteins involved in biomineralization processes, and poly-l-lysine (pLys), were used to investigate calcite growth kinetics as a function of the interaction between the charged polypeptides and the calcite surface. 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. 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 nonselective, 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 account for 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.