Kinetic Approach to Biomineralization: Interactions of Synthetic Macromolecules with Calcium Carbonate Polymorphs

Biomineralization processes are the subject of numerous investigations. This article gives a review of the study on interactions between the charged polypeptides and the mineral surfaces involved in biomineralization, with an additional kinetic approach. The influence of polypeptides on two types of precipitation processes is discussed: the spontaneous precipitation from supersaturated solution and the growth kinetics of calcite seed crystals. In the first case the phenomenon of the formation and stabilization of metastable phases was found while in the second case the influence of the applied polypeptides on the kinetics and mechanisms of calcite crystal growth was investigated. Calcium carbonate polymorphs, calcite and vaterite, were used as biomineral substrates and acidic polypeptides, poly-L-aspartic (pAsp) acid and poly-L-glutamic (pGlu) acid, as simplified models of naturally occurring soluble acidic proteins. A basic polypeptide, poly-L-lysine (pLys), was also used in experiments in order to find out whether conformity between the crystal surface and the adsorbed polypeptide, or just the electrostatic interactions, have a decisive role in these processes. The addition of a particular polypeptide into the precipitation system caused a significant inhibition of nucleation and growth of vaterite, the extent of inhibition being in the order InhpAsp < InhpGlu InhpLys. In addition to the inhibition of precipitation, the change of the polymorphic composition and the crystal morphology of the precipitate were also achieved. The explanation of such acidic polypeptide behaviour is a consequence of kinetic constraints through the diverse efficiency of inhibition of both calcite nucleation and vaterite growth caused by adsorption of acidic polypeptides. The acidic polypeptides also caused the inhibition of calcite crystal growth, the effect being pAsp < pGlu, and changed the observed mechanism of growth controlled by the integration of ions into the spiral steps, as found for the model systems, to the surface nucleation rate-determining mechanism. Nonselective, weak and electrostatic adsorption of pLys at the crystal surface was probably responsible for increasing the calcite crystal growth rate when pLys was present at low concentrations and for inhibiting it at pLys higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides, observed for the calcite/pAsp system, can account for coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β-pleated sheet) and Ca 2+ ions from the crystal surface.