Polypeptide effect on Mg2+hydration inferred from CaCO3formation: a biomineralization study by counter-diffusion

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.