Calcium carbonate (CaCO3) is one of the most
significant
biominerals in nature. Living organisms are able to control its biomineralization
by means of an organic matrix to tailor a myriad of hybrid functional
materials. The soluble organic components are often proteins rich
in acidic amino-acids such as l-aspartic acid. While several
studies have demonstrated the influence of amino acids on the crystallization
of calcium carbonate, nanoscopic insight of their impact on CaCO3 mineralization, in particular at the early stages, is still
lacking. Herein, we implement liquid phase-transmission electron microscopy
(LP-TEM) in order to visualize in real-time and at the nanoscale the
prenucleation stages of CaCO3 formation. We observe that l-aspartic acid favors the formation of individual and aggregated
prenucleation clusters which are found stable for several minutes
before the transformation into amorphous nanoparticles. Combination
with hyperpolarized solid state nuclear magnetic resonance (DNP NMR)
and density functional theory (DFT) calculations allow shedding light
on the underlying mechanism at the prenucleation stage. The promoting
nature of l-aspartic acid with respect to prenucleation clusters
is explained by specific interactions with both Ca2+ and
carbonates and the stabilization of the Ca2+–CO3
2–/HCO3
– ion
pairs favoring the formation and stabilization of the CaCO3 transient precursors. The study of prenucleation stages of mineral
formation by the combination of in situ LP-TEM, advanced
analytical techniques (including hyperpolarized solid-state NMR),
and numerical modeling allows the real-time monitoring of prenucleation
species formation and evolution and the comprehension of their relative
stability.
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