Biomineralization – CaCO3 in nacreous sea shells

Nacre is a natural hybrid tissue made of aragonite platelets and of a layered organic matrix. Aragonite tablets are characterized by a diameter ranging from 5 to 10 µm and a thickness of about 0.5 µm. A thin (~20 nm thick) organic matrix layer composed of proteins, polysaccharides and presumably lipids surrounds the mineral tablets. The aragonite platelets are oriented along organic sheets where the organic matrix forms the scaffolding of the structure. Their three-dimensional organization varies from one species to another, forming brick wall-like disposition for bivalves or a columnar structure in the case of gastropods. Because of its lamellar architecture and hybrid composition, nacre exhibits remarkable mechanical properties especially in terms of fracture resistance which can be 3000 times higher compared to a single crystal of aragonite.

However, aragonite tablets are not perfect single crystals as local disorder exists at the nanoscopic scale either at the surface or within the crystals. Describing such local disorder at the nanometer scale is a challenge and a fundamental to understand the formation of such biomineral. Using European abalone Haliotis tuberculata as key model and advanced solid-state NMR spectroscopy (1D and 2D {1H-echo}-13C CP MAS, {23Na}-1H TRAPDOR, 43Ca DFS), we demonstrated that nacre disordered domains are not strictly amorphous and composed of different divalent (CO32- and Ca2+) as well as monovalent ions (HCO32- and Na+).

a) STEM-HAADF observations of a thin FIB section of Haliotis tuberculata nacre. b) HR-TEM images of the surface of aragonite tablets, revealing the presence of a disordered surface layer. c) Schematic representation of a possible organization within disordered mineral domains in aragonite tablets of Haliotis tuberculata’s nacre.

Moreover, we recently demonstrated the capability of dipolar nuclear polarization (DNP MAS) solid-state NMR to reveal the fine atomic structure of biominerals, and more specifically their surfaces and interfaces. In particular, organo-mineral interactions can be probed at the atomic level with very high sensitivity and . In particular, reliable dipolar-based (1H-31P, 1H-13C HetCOR or {31P}-13C REDOR) experiments are achieved in a few minutes/hours (!), leading to the determination of proximities/distances, molar proportion and binding mode of mineral-bonded organic molecules in native organic tissues. In nacre, we evidenced that the organo-mineral interface is continuous and not mediated by a water layer in opposition to the the bone tissue.

a) Schematic description of the DNP MAS SENS experiment on nacre. b) 1H-13C HetCor DNP SENS spectrum of H. tuberculata‘s nacre displaying the organo-mineral interface mediated by bicarbonate ions in disordered environments.

Collaborators: Dr. Stéphanie Auzoux-Bordenave (UMR Boréa, MNHN), Nicolas Menguy (IMPMC, SU), Axel Gansmüller (CRM2, Nancy), Anne Lesage (High Field NMR Center, Lyon).

Related publications:
Structural description of surfaces and interfaces in biominerals by DNP SENS.
Thierry Azaïs, Stanislas Von Euw, Widad Ajili, Stéphanie Auzoux-Bordenave, Philippe Bertani, David Gajan, Lyndon Emsley, Nadine Nassif, Anne Lesage. Solid state Nucl. Magn. Reson. 2019, 102, 2-11