Calcite is one of the anhydrous polymorphs of calcium carbonate. When it is found in biominerals, they often have complex morphologies controlled by associated proteins. Asprich, rich in carboxylic acid groups, is one of the representative proteins. In previous studies, peptides originated from Asprich showed extensive modification abilities of calcite morphologies, especially in the presence of magnesium ions. In the present study, synthetic polymers with carboxylic acid groups were utilized to regulate the morphologies of calcite. Poly(acrylic acid) and poly(methacrylic acid) were able to reproduce the controlling ability of Asprich peptides, but poly(ethylene imine) and poly(N-isopropylacrylamide) could not. The observed phenomenon can be explained through selective binding of the anionic polymers to the {hk0} planes of calcite.
The polymorph-selective crystallization of calcium carbonate has been studied in terms of epitaxial relationship between the inorganic substrates and the aragonite/calcite polymorphs with implication in bioinspired mineralization. EpiCalc software was employed to assess the previously published experimental results on two different groups of inorganic substrates: aragonitic carbonate crystals (SrCO3, PbCO3, and BaCO3) and a hexagonal crystal family (α-Al2O3, α-SiO2, and LiNbO3). The maximum size of the overlayer (aragonite or calcite) was calculated for each substrate based on a threshold value of the dimensionless potential to estimate the relative nucleation preference of the polymorphs of calcium carbonate. The results were in good agreement with previous experimental observations, although stereochemical effects between the overlayer and substrate should be separately considered when existed. In assessing the polymorph-selective nucleation, the current method appeared to provide a better tool than the oversimplified mismatch parameters without invoking time-consuming molecular simulation\.
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