First direct insight into the local environment and dynamics of water molecules in the whewellite mineral phase: mechanochemical isotopic enrichment and high-resolution 17O and 2H NMR analyses

Abstract

Calcium oxalate minerals of general formula CaC2O4.xH2O are widely present in nature and usually associated with pathological calcifications, constituting up to 70 – 80% of the mineral component of renal calculi. The monohydrate phase (CaC2O4.H2O, COM) is the most stable form, accounting for the majority of the hydrated calcium oxalates found. These mineral phases have been studied extensively via X-ray diffraction, IR spectroscopy and, to a lesser extent, using 1H, 13C and 43Ca solid-state NMR spectroscopy. However, several aspects of their structure and reactivity are still unclear, such as the evolution from low- to high-temperature COM structures (LT-COM and HT-COM, respectively), and the involvement of water molecules in this phase transition. Here, we report for the first time a 17O and 2H solid-state NMR investigation of the local structure and dynamics of water in the COM phase. A new procedure for the selective 17O- and 2H-isotopic enrichment of water molecules within the COM mineral is presented using mechanochemistry, which employs only microliter quantities of enriched water, and leads to exchange yields up to ~30%. 17O NMR allows both crystallographically inequivalent water molecules in the LT-COM structure to be resolved, while 2H NMR studies provide unambiguous evidence that these water molecules are undergoing different types of motions at high temperatures without exchanging with one another. Dynamics appear to be essential for water molecules in these structures, which have not been accounted for in previous structural studies on the HT-COM structure due to lack of available tools — highlighting the importance of such NMR investigations for studying the crystallographic structure of biologically relevant minerals like calcium oxalates.

17O solid state NMR as a valuable tool for deciphering reaction mechanisms in mechanochemistry: the case study on the 17O-enrichment of hydrated Ca-pyrophosphate biominerals

“…For both compounds, a broad asymmetric 17 O NMR signal was observed in just a few hours, which is consistent with what one may expect for water molecules within solid environments. 7 This is especially evident when measurements at a lower magnetic eld are considered, as the second order quadrupolar lineshape becomes more visible (see Fig. 4, analyses at 9.4 and 14.1 T).…”

“…The isotropic chemical shi d iso is dened as d iso z À(s À s ref ), where s is the isotropic shielding and s ref is the isotropic shielding for the same nucleus using a hydrate reference system, in a similar way as in our previous work on calcium oxalate monohydrate. 7 For these hydrates, the maximum deviation between experimental and DFT-calculated 17 O isotropic shis was found to be 6 ppm. 7 The quadrupolar moments (Q) used to calculate the C Q of 17 O was À2.558 fm 2 .…”

“…7 For these hydrates, the maximum deviation between experimental and DFT-calculated 17 O isotropic shis was found to be 6 ppm. 7 The quadrupolar moments (Q) used to calculate the C Q of 17 O was À2.558 fm 2 . 22 Calculated values are reported in Table S2 (ESI †).…”

“…The 2 water sites were not resolved at this stage. Further analyses using Multiple-Quantum Magic Angle Spinning (MQMAS) NMR sequences would be needed for this, 7 which is beyond the scope of the present work. Here, only an average isotropic shi d iso and quadrupolar parameter P Q were determined for the 2 sites, by considering the change in peak maximum position as a function of the magnetic eld (see ESI, Fig.…”

“…6 Lastly, in a recent investigation by Goldberga et al, it was shown that the water molecules within calcium oxalate hydrates could be enriched in 17 O, further enabling to investigate the dynamics of the water molecules within these crystalline phases. 7 Moreover, the isotopic labelling of the water molecules in 2 H and 18 O was also performed as part of the same work. This particular isotopic enrichment was used to help rationalise the reaction mechanisms occurring in ball-milling during the isotopic labelling, by showing that the enrichment process predominantly occurred by a dissolution-recrystallisation mechanism.…”