Characterization of Calcium Oxalate Hydrates and the Transformation Process

Abstract

Calcium oxalate can be found in humans as kidney stones and in cultural heritage as films in two crystallographic species, dihydrate (COD/weddellite) and/or monohydrate (COM/whewellite). Due to its instability, COD is transformed into COM. Studying this crystalline conversion provides information about the origin of the monohydrated species, which will help in the assessment of prevention measurements to avoid their formation. In the present study, the synthesis of calcium oxalate hydrate microcrystals has been carefully performed to avoid mixture of phases in the final products; the long and short range order structure of both species have been studied by Xray diffraction (XRD) and X-ray absorption spectroscopy (XAS), respectively. This structural information was considered in the density functional theory (DFT) computational study performed to assign the characteristic vibrational IR and Raman frequencies found. This detailed characterization allows an unambiguous assignment of the vibrations, thus providing the appropriate parameters required to monitor and characterize the transformation process.

Calcium oxalate kidney stones, where is the organic matter?: A synchrotron based infrared microspectroscopy study

“…The PC-1 represents a higher variability on the 1616 cm −1 (PC-1 positive) and 1326 cm −1 (PC-1 negative), which correspond to the ν a (COO) and the ν s (COO), respectively. 10,29 This effect is more related with these peaks ratio and shifts on the calcium oxalate monohydrate species observed on Figure 3 (as explained before). Furthermore, this effect has been also studied by one-way ANOVA test of COM and TRA spectra, distributed along PC-1.…”

“…This has been proven by the results of the one‐way ANOVA test of COM spectra, distributed along PC‐3, which indicates significant differences in the population means, at the 0.05 level, for the peak ratio 1681/1605 between the ones located in PC‐3 positive, and the ones located in PC‐3 negative. The PC‐1 represents a higher variability on the 1616 cm −1 (PC‐1 positive) and 1326 cm −1 (PC‐1 negative), which correspond to the ν a (COO) and the ν s (COO), respectively 10,29 . This effect is more related with these peaks ratio and shifts on the calcium oxalate monohydrate species observed on Figure 3 (as explained before).…”

“…The process used to synthesize the calcium oxalate hydrates references, was previously developed in our research group 10 …”

“…COD samples are located in the PC-3 negative, which is represented with a higher contribution of the wavenumber 1338 cm −1 , while most of the monohydrated species (COM and TRA) are in the PC-3 positive, with a characteristic wavenumber of 1315 cm −1 , vibrations that correspond to the ν s (COO) of the COD and COM references, respectively. 10,29 Moreover, the presence of a high contribution of the wavenumber 1631 cm −1 for the PC-3 positive ( Figure 3) can be attributed to the presence of the second peak observed in that region for the majority of the calcium oxalate monohydrated species (COM and TRA); however, the contribution of the wavenumbers 1605 and 1681 cm −1 for the PC-3 negative can be due to the high ratio variability of these two peaks, indicating a high presence/distribution of the monohydrated species in this score plot section. This has been proven by the results of the one-way ANOVA test of COM spectra, distributed along PC-3, which indicates significant differences in the population means, at the 0.05 level, for the peak ratio 1681/1605 between the ones located in PC-3 positive, and the ones located in PC-3 negative.…”

“…The stone resulting after this transformation (named TRA) has one water molecule being chemically COM, but its nucleation corresponds to a calcium oxalate dihydrated species, COD 9 . It has been previously reported that the crystalline structure of TRA is the same as COM (monoclinic, P2 1 /c), 10 while other works report that it could indicate the presence of amorphous whewellite, which has been recently synthesized 9,11 . The differentiation between COM (being formed as monohydrated) and TRA (coming from the transformation of COD), in any of the transformation stages, is of great importance since their origins are caused by different pathologies, for example, COD nephroliths are related to hypercalciuria, while COM ones are related to hyperoxaluria 9,12,13 .…”