First Direct Insight into the Local Environment and Dynamics of Water Molecules in the Whewellite Mineral Phase:...

Calcium oxalate minerals of the 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 and 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 refining the overall knowledge on biologically relevant minerals like calcium oxalates.