The Role of Residue Acidity on the Stabilization of Vaterite by Amino Acids and Oligopeptides

Nature uses proteins containing acidic residues to stabilize thermodynamically unstable calcium carbonate polymorphs, producing complex composite structures and controlling their final morphology. Stabilization of vaterite, the most thermodynamically unstable polymorph of anhydrous calcium carbonate, is possible in the presence of soluble biogenic molecules. We analyze here, in a systematic manner, the effect of the side-chain acidity of different amino acids on vaterite stabilization by following the kinetics of calcium carbonate crystallization through the free calcium concentration, pH, and turbidity. Acidic residues from aspartic and glutamic acid showed the greatest stabilization of vaterite, while cysteine and serine only partially stabilize this polymorph. Using the method of direct mixing of calcium and carbonate ions, we were able to further tailor the kinetics of calcium carbonate crystal growth by choosing the carbonate source as either sodium bicarbonate or sodium carbonate. Finally, oligopeptides of two oppositely charged amino acids, aspartic acid and histidine, were selected to study the effect of the chain length on the interactions between additive and calcium carbonate. Control over the interactions between ions, crystal faces, and charged additives results in stabilizing vaterite, which provides an insight into biomineralization processes.