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
13
C 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.