Investigation of the structure and dynamics of amorphous calcium carbonate by NMR: stabilization by poly-aspartate and comparison to monohydrocalcite

Dense amorphous phases are key intermediates in biomineralization pathways. Structural information is required to understand these pathways, but is, as per the amorphous nature, difficult to obtain. We report an investigation of amorphous calcium carbonate (ACC) with magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Mimicking the use of acidic proteins, we stabilize ACC against crystallization with poly-aspartate (PAsp). Spectra are in agreement with incorporation of PAsp into ACC nanoparticles and show that it forms an α-helix. The pH of the solution, from which PAsp-stabilized ACC is synthesized, affects the 13C chemical shift of carbonate in a way that is identical for additive-free ACC. Generally, we observe that the magnetic properties of the 1H and 13C nuclei in the rigid environment of ACC are similar (though not identical) to those in monohydrocalcite (MHC). This allows us to establish, based on 1H–13C correlation spectra, relaxation properties, and spin dynamics simulations, that the structural water molecules in ACC undergo 180° flips on a millisecond time scale.