Polypeptide effect on Mg2+ hydration inferred from CaCO3 formation: a biomineralization study by counter-diffusion

Abstract

The use of a counter-diffusion system allows the evaluation of diverse parameters involved in a crystallization process. In this study, this tool has been used to infer the hydration status of Mg²⁺ during CaCO₃ formation experiments in an agarose highly viscous sol entrapping charged polypeptides. The experimental data allow us to infer that the hydration status of Mg²⁺ is altered by the presence of poly-L-aspartate or poly-L-glutamate. This changes the CaCO₃ polymorphic distribution in favor of Mg-calcite with respect to aragonite, but does not favor the isomorphic substitution of Mg²⁺ with Ca²⁺ within the calcite lattice. The latter may exclude the formation of an amorphous transient form, which leads to a high Mg-calcite, as expected when using a counter-diffusion system set up. The presence of poly-L-lysine does not affect the hydration of Mg²⁺, but favors the formation of aragonite with respect to calcite. In this case an inhibition of calcite formation and an alteration of the hydration sphere of Ca²⁺ could be invoked; both effects are able to increase CaCO₃ supersaturation. In conclusion, this study reveals that charged polypeptides can orchestrate CaCO₃ formation by also controlling the hydration status of cations.