Characterization of ACCs stabilized by isolated polymer. The sample was isolated from a titration experiment using 0.1 g/L PAsp at pH 9.8 by soaking the solution in ethanol (see Methods section). A 13C direct excitation (DE) and 1H–1310% C Cross Polarization Spectrum (CP) 13C-carbonate ACC stabilized by PAsp (PAsp_disACC) at a rotational frequency of 10 kHz. Spectra are scaled to Cα-PAsp peak. b TGA (red) and DSC (blue) analysis. The exothermic decomposition of bicarbonate species is highlighted in gray. vs ATR-FTIR spectra of a polymer-stabilized ACC sample, showing significant amounts of polymer incorporation. Pure calcium salt ACC and PAsp (PAsp_Ca) are shown for reference (detailed FTIR spectra are shown in Supplementary Fig. 6). d Normalized QMID for TGA-MS measurement on PAsp_ACC sample using 13Carbonates enriched in C in titrations. Due to the natural abundance of carbonate distribution in the polymer, gases released from the polymer (12CO2; m/z = 44, black) and mineral (13CO2; m/z = 45, red) can be distinguished, showing significant amounts of mineral decomposition below 300 °C (highlighted in gray). e TGA-IR analysis of 13The PAsp_ACC sample enriched in C carbonate confirms the strong 13CO2 release of (bi)carbonate species at approximately 300 °C. Credit: Natural communications (2024). DOI: 10.1038/s41467-023-44381-x
Many organisms can produce minerals or mineralized tissues. A well-known example is mother-of-pearl, used in jewelry because of its iridescent colors. Chemically speaking, its formation begins when a mollusk extracts calcium and carbonate ions from water. However, the exact processes and conditions that lead to nacre, a composite of biopolymers and crystalline calcium carbonate platelets, are the subject of intense debate among experts, and different theories exist.
Researchers agree that non-crystalline intermediates, such as amorphous calcium carbonate (ACC), play a crucial role in biomineralization. Lobsters and other crustaceans, for example, keep a store of ACC in their stomach, which they use to build a new shell after molting. In a recent study published in Natural communicationsresearchers from the University of Konstanz and the Leibniz University of Hannover have succeeded in deciphering the training path of the ACC.
A combination of advanced methods
The researchers led by Denis Gebauer (Leibniz University Hannover) and Guinevere Mathies (University of Konstanz) took advantage of the fact that ACC can be synthesized not only by living organisms, but also in the laboratory. Using advanced methods such as magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), they analyzed tiny ACC particles to determine their structure.
“We had difficulty interpreting the ACC spectra. They suggested dynamics that we were unable to model at first,” says Mathies.
Colleagues from Leibniz University Hannover provided an important clue. Maxim Gindele of the Gebauer group showed that ACC conducts electricity. Since ACC particles are very fragile and only a few tens of nanometers in size, this was not as simple as inserting two probes into them.
Instead, the measurements were made using conductivity atomic force microscopy (C-AFM), in which ACC particles on a flat surface are detected by a tiny cantilever scanning the surface and visualized using a laser beam. When the cantilever is placed on one of the nanoparticles, a current passes through its tip to measure the conductivity.
Two different environments
Informed by the conductivity observation, Sanjay Vinod Kumar of the Mathies group carried out further MAS NMR experiments aimed at probing the dynamics. They indicated two distinct chemical environments in the ACC particles. In the first environment, water molecules are embedded in rigid calcium carbonate and can only undergo 180 degree flips. The second environment consists of water molecules undergoing slow tumbling and translation, with dissolved hydroxide ions.
“The remaining challenge was to reconcile the two environments with the observed conductivity. Solid salts are insulators and so the second mobile environment had to play a role,” explains Mathies. In the new model, mobile water molecules form a network across the ACC nanoparticles. The dissolved hydroxide ions carry the charge.
Researchers can also explain the formation of two chemical environments: In water, calcium and carbonate ions tend to stick together and form dynamic assemblies called pre-nucleation clusters. The clusters can undergo phase separation and form dense liquid droplets, which in turn coalesce into larger aggregates, similar to how soap bubbles coalesce.
“The rigid and less mobile environment arises from the core of the dense liquid nanodroplets. The network of mobile water molecules, on the other hand, remains due to imperfect coalescence of the droplet surfaces during dehydration to a solid ACC,” explains Gebauer .
These results constitute an important step toward a structural model for ACC. At the same time, they provide strong evidence that mineralization begins with pre-nucleation clusters. “This not only brings us closer to understanding the secret of biomineralization, but may also have applications in the development of cementitious materials that bind carbon dioxide and, since we now know that ACC is a conductor, in devices electrochemicals,” concludes Mathies.
More information:
Maxim B. Gindele et al, Colloidal pathways of amorphous calcium carbonate formation lead to distinct aquatic environments and conductivity, Natural communications (2024). DOI: 10.1038/s41467-023-44381-x
Provided by the University of Konstanz
Quote: New research deciphers the biomineralization mechanism (January 12, 2024) retrieved January 12, 2024 from
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