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.

中文翻译：

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弹性晶内闭塞：局部结构的固态 NMR 视图，因为它调整体晶格特性

在许多海洋生物中，生物矿化——钙基离子晶格的结晶——展示了调节过程如何优化各种功能，通常是通过结合沉淀介质中的试剂。我们研究了一个由 l-天冬氨酸 (Asp) 组成的模型系统，当将其添加到碳酸钙的沉淀溶液中时，会使热力学上不受欢迎的多晶型球霰石结晶。虽然球霰石充其量只是动力学稳定，但这种稳定性是可调的，因为在高浓度 Asp 下生长的球霰石在热和时间上都是稳定的，而球霰石在 Asp 浓度低 10 倍时生长，但在晶体中自发地降低了 2 倍转变为方解石。固态 NMR 显示 Asp 被稀疏地包裹在球霰石和方解石中。CP-REDOR NMR 表明，每个 Asp 都嵌入到一个由约 8 个无序碳酸盐组成的扰动闭塞壳中，这些碳酸盐与主体相连。在沉积的球霰石和演化的方解石中，扰动的壳包含两组碳酸盐物质，其特征在于它们与胺的接近程度，并可根据 13C 化学位移进行识别。嵌入外壳和被封闭的 Asp 作为一个整体，直到它最小程度地重新排列，即使主体经历了广泛的重组。这些闭塞单元的弹性表明，大的无 Asp 域驱动球霰石向方解石转变——这被闭塞单元阻滞，导致浓度依赖的晶格稳定性。了解闭塞单元的结构和特性，特别适合 ssNMR，