Abstract
Biological organisms display sophisticated control of nucleation and crystallization of minerals. In order to mimic living systems, deciphering the mechanisms by which organic molecules control the formation of mineral phases from solution is a key step. We have used computer simulations to investigate the effects of the amino acids arginine, aspartic acid, and glycine on species that form in solutions of calcium carbonate (CaCO3) at lower and higher levels of supersaturation. This provides net positive, negative, and neutral additives. In addition, we have prepared simulations containing hexapeptides of the amino acids to consider the effect of additive size on the solution species. We find that additives have limited impact on the formation of extended, liquid-like CaCO3 networks in supersaturated solutions. Additives control the amount of (bi)carbonate in solution, but more importantly, they are able to stabilize these networks on the time scales of the simulations. This is achieved by coordinating the networks and assembled additive clusters in solutions. The association leads to subtle changes in the coordination of CaCO3 and reduced mobility of the cations. We find that the number of solute association sites and the size and topology of the additives are more important than their net charge. Our results help to understand why polymer additives are so effective at stabilizing dense liquid CaCO3 phases.
Highlights
While there have been major advances in the production of synthetic, bioinspired materials in recent years,[1−3] the degree of control that biological organisms can exercise in engineering hard, mineralized tissues with distinct functionalities and morphologies far surpasses anything available using current technologies
Three crystalline polymorphs of calcium carbonate are found in biology: calcite, aragonite and vaterite, in order of decreasing thermodynamic stability.[4−6] These ordered materials are often formed after the initial deposition of amorphous calcium carbonate (ACC)[7,8]
Computer simulations have provided much insight into the early stages of phase separation in a range of systems.[58−61] Here, we report molecular dynamics (MD) simulations that investigate the effect of three amino acids (AAs) on calcium carbonate aqueous solutions
Summary
While there have been major advances in the production of synthetic, bioinspired materials in recent years,[1−3] the degree of control that biological organisms can exercise in engineering hard, mineralized tissues with distinct functionalities and morphologies far surpasses anything available using current technologies. Proteins and peptides can control nucleation and crystal growth processes.[9,15] Certain proteins, such as the sea urchin matrix protein LSM34, are essential for the formation of calcite spicules.[15−17] Simulations have shown that the avian eggshell protein, Ovocleidin-17, hastens the formation of calcite from initially amorphous nanoparticles.[18] Proteins and peptides adsorbed onto a β-chitin matrix are thought to selectively nucleate nacreous aragonite in mollusk shells.[19,20] Intrinsically disordered proteins (IDPs), which are present in nacre, selfassemble during mineralization and selectively precipitate vaterite or aragonite.[21,22] The inherent conformational freedom associated with IDPs appears to modulate the mechanisms associated with both mineralization and the resulting mineral phases.[23−26] Protein control of the amount of impurities, such as Mg(II), in amorphous phases may be an important step in aragonite polymorph selection.[27] Inspired by these examples in nature, synthetic additives have been designed and applied to control the morphology and rate of crystal growth.[28,29] Other synthetic additives appear to inhibit crystallization. Polyaspartic acid (Poly(ASP)) in solution, for example, retarded the transformation of amorphous liquid and solid phases of calcium carbonate into crystalline ones.[30−32] a wealth of research has been dedicated to understanding how organic
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