Hydrated calcium oxalate phases (CaC2O4.nH2O, n = 1, 2 or 3) are of paramount importance in calcium sequestration in plants (up to 80% of their dry weight) and in human pathological calcifications (kydney stones). The last point impacts 10% of the population in western countries and is considered as a major societal health problem worldwide. Currently, biomineralization processes as well as mechanistic details related to the formation and transformation of calcium oxalates were almost not described and poorly understood. Nevertheless, non traditional routes of precipitation have been proposed recently involving an amorphous hydrated calcium oxalate state, precursor to the final CaC2O4.nH2O phases. In this contribution, we demonstrate that the progressive transformation from amorphous precursor to the final phases is much more subtle than expected. Indeed, we introduce for the first time the concept of "pre-crystalline" phases exhibiting high local order and intermediate range order but a lack of long range order. The detailed in situ NMR study of the transformations of (CaC2O4.nH2O, n = 1, 2 or 3) is presented taking advantage of temperature and pressure effects under fast Magic Angle Spinning (MAS). A combination of variable temperature and variable spinning frequency experiments were implemented in order to disentangle pressure and temperature effects. It was clearly shown that (CaC2O4.nH2O, n = 1, 2 or 3) evolved spontaneously towards CaC2O4.nH2O, n = 1, through a disordered phase (which can be considered as a transient precursor in biomineralization processes). KBr was added in the rotor in order to carelully measure the temperature and as reactive chemical (being able to adsorb water molecules during dehydration processes). The presence of water in interaction with KBr was clearly demonstrated by Br MAS data. Most importantly, the in situ transformation of (CaC2O4.nH2O, n = 1, 2 or 3) occured at T = 40°C. This result can be related to clinical observations demonstrating that kidney stones are mainly composed of (CaC2O4.nH2O, n = 1) (70%) whereas the occurrence of CaC2O4.nH2O, n = 2 or 3 is much reduced. All pre-crystalline phases were clearly detected by 13C CP MAS experiments. The whole sets of resonances were fully assigned by using extensive modeling of ordered/disordered hydrated calcium oxalates (at the DFT level), efficient molecular dynamics and by taking temperature explicitly into account.

In situ description of biomineralization processes and pre-crystalline phases studied by solid state NMR: The study case of hydrated calcium oxalates

ACS Spring Meeting 2021

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