People

  Tristan Georges (PhD student), 2019-2022

« Understanding the Early Onset of Biomineral Precipitation »

Crystallization of biominerals does not follow classical mineralization pathways whereby the crystal growth ions-by-ions in a saturated aqueous solution (epitaxial mechanism). In opposition, biominerals crystallization follows non-classical nucleation routes involving successive events. (i) Free ions in solution (Ca2+, PO43-, CO32-) are in equilibrium with prenucleation inorganic species (PNIS) described as (meta)stable ionic clusters. The evolution of these PNIS by aggregation or concentration leads to (ii) the nucleation of nano-objects (nanocrystals, amorphous nanoparticles, etc.) that are transient precursors of the mineral phase. Then (iii) the mineral growth often occurs through oriented particle attachment leading to (iv) the final crystalline biomineral.

Importantly, biominerals crystallization is occurring in vivo under a biological control through the action of dedicated proteins. As a result, these processes are extremely complex and the regulation in vivo of these events by mineralizing proteins is largely unknown. This project aims at clarifying molecular mechanisms of apatite formation controlled (or not) by biological agents.
For this purpose, we use a combined approach of cutting edge analytical techniques at multiple length scales on a structural and dynamical point-of-view :
(i) Cryo-TEM
(ii) Liquid-phase TEM
(iii) D-DNP (dissolution dynamic nuclear polarization) solution-NMR
(iv) Cryo-fixed solid-state NMR

 

  •    PhD of Widad Ajili (2015-2018):

« Temporal study of European abalone Haliotis tuberculata shell formation« 

Nacre, the inner part of mollusk shells, is made of a mineral phase (calcium carbonate CaCO3) associated to an organic counterpart. The mineral phase is mainly composed of aragonite tablets surrounded by chitin and mineralizing proteins. This particular composition and structure give to nacre its remarkable mechanical properties. The formation of the mollusks shell is still not entirely understood especially the first steps. Indeed there are still interrogations about the nature of the first deposited mineral phase and about the biological control behind this process.

The originality of my PhD work is based on the use of the European abalone Haliotis tuberculata as a model to study shell and nacre formation through a temporal study of the shell biomineralization from the early stages of the larval development to the adulthood. Our investigations are achieved thanks to various analytic techniques such as solid-state NMR (ssNMR) and electronic microscopies. ssNMR is particularly suitable for the study of biologic material as it is a non-destructive technique. By using 1H and 13C NMR, the different components of nacre can be individually edit namely (i) aragonite, forming the core of the tablets, (ii) the disordered surface of the particles and (iii) the surrounding organic phase. Extraction and analysis of the NMR parameters (including chemical shift anisotropy and cross polarization dynamics parameters) lead us to the conclusion that the aragonite tablets surface is made of a hydrated disordered layer composed of water molecules, carbonate and bicarbonate ions. The presence of inorganic phosphates in the abalone shell was also detected through 13P NMR and the use of the Dynamic Nuclear Polarization (DNP) to enhance the NMR signal allows the record of REDOR experiments showing proximities between phosphate, the carbonate species and the organic fraction.
In parallel, molecular biological immunolocalisation techniques is used trying to better understand the nature and the role of the mineralizing proteins during the shell formation.

PhD director: Dr. Thierry Azaïs, Dr. Nadine Nassif, Dr. Stéphanie Bordenave