Abstract
In many marine organisms, biomineralization—the crystallization of calcium-based ionic lattices—demonstrates how regulated processes optimize for diverse functions, often via incorporation of agents from the precipitation medium. We study a model system consisting of l-aspartic acid (Asp) which when added to the precipitation solution of calcium carbonate crystallizes the thermodynamically disfavored polymorph vaterite. Though vaterite is at best only kinetically stable, that stability is tunable, as vaterite grown with Asp at high concentration is both thermally and temporally stable, while vaterite grown at 10-fold lower Asp concentration, yet 2-fold less in the crystal, spontaneously transforms to calcite. Solid-state NMR shows that Asp is sparsely occluded within vaterite and calcite. CP-REDOR NMR reveals that each Asp is embedded in a perturbed occlusion shell of ∼8 disordered carbonates which bridge to the bulk. In both the as-deposited vaterites and the evolved calcite, the perturbed shell contains two sets of carbonate species distinguished by their proximity to the amine and identifiable based on 13C chemical shifts. The embedding shell and the occluded Asp act as an integral until which minimally rearranges even as the bulk undergoes extensive reorganization. The resilience of these occlusion units suggests that large Asp-free domains drive the vaterite to calcite transformation—which are retarded by the occlusion units, resulting in concentration-dependent lattice stability. Understanding the structure and properties of the occlusion unit, uniquely amenable to ssNMR, thus appears to be a key to explaining other macroscopic properties, such as hardness.
Highlights
Crystalline calcium carbonate is the most widespread[1−4] of minerals synthesized by marine organisms
The two polymorphs are distinguished in the 13C direct excitation (DE) magic angle spinning (MAS) NMR spectra, with calcite showing a single characteristic peak and vaterite two peaks of unequal intensity (Figure 1a,c)[67−69] representing
Accompanying the multitude of terms are a comparable number of questions, on topics as varied as the magnitude of disruption to the surrounding matrix, the specific chemical interactions bridging between the matrix and the occlusion, and the size of the region which is only poorly described by reference to the bulk, as these hybrid structures affect macroscopic properties well beyond our observation of lattice stability with respect to bulk transformations
Summary
Crystalline calcium carbonate is the most widespread[1−4] of minerals synthesized by marine organisms. Bio-organics extracted from biogenic calcium carbonates are typically proteinaceous[22,23] and often rich in those amino acids with acidic side chains (Asp and Glu).[24,25] While local order must inevitably be disrupted so as to incorporate foreign species, the resulting mesocrystalline material diffracts as if it were single crystalline,[26−29] even where upon finer examination it is clear that there are small lattice distortions attributable to the incorporated organic additives.[30,31] As yet, the description of the interaction interface between mineral and occluded bioorganic molecules is unavailable; experimental access is made more difficult as these interfacial regions are scarce within the bulk; their location is unpredictable, and the interfaces are disordered. As such the notion of intracrystalline occlusion, broadly and loosely used, remains obscure and devoid
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