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Lead nitrate is an inorganic salt, commonly used for the accurate temperature determination in the solid state NMR spectroscopy, due to the strong temperature dependence of the 207Pb chemical shift. As the reason for this phenomenon remained unknown, the main purpose of this study was to explain this temperature dependence at the molecular level. To achieve this, combined CASTEP geometry optimization, quantum molecular dynamics at chosen temperatures and GIPAW NMR computations were performed. Due to the previous literature reports on inaccuracy in the calculation of 207Pb NMR parameters using GIPAW, a large emphasis was put on the optimization of computational method. The application of quantum molecular dynamics provided the simulation of the temperature‐dependent vibrational motions and enabled to accurately compute the changes in the value of Pb δiso resulting from them. © 2018 Wiley Periodicals, Inc. This study aims to explain, at the molecular level, the experimentally observed temperature dependence of 207Pb isotropic chemical shift value in the solid state NMR spectra of lead nitrate. For this purpose combined CASTEP geometry optimization, quantum molecular dynamics at chosen temperatures and GIPAW calculations of NMR parameters were performed. The influence of temperature‐depending vibrational motions on the NMR parameters was precisely reflected in the calculations results. [ABSTRACT FROM AUTHOR]

Highlights • Matrix effect does not limit the use of CPE coupled to LC–MS/MS. • Low surfactant concentration and high extraction temperature reduce the risk of ME. • Significant absolute ME (<85% or >115%) was observed only for 25% of the analytes. • Molecular descriptors (HDB, ClogP, PSA) help to identify compounds prone to ME. • A new term – the surfactant effect – was introduced. Abstract Matrix effects are one of the most challenging issues in the analysis of complex samples using liquid chromatography coupled to mass spectrometry (LC–MS). Apart from the instrumental origin, these effects are also related to sample preparation. Cloud-point extraction (CPE) is rarely combined with LC–MS as it requires the use of surfactants which might interfere with droplet evaporation. Thus, they are suspected to cause a significant matrix effect (signal suppression). In this paper, 73 model drugs with different physicochemical properties were screened to analyse how susceptible LC-MS is to the absolute and relative matrix effect (ME) when coupled with CPE for measurements in human plasma. Three combinations of the surfactant Triton X-114 concentration (1.5% or 6%) and extraction temperature (40 or 55 °C) in six pH values gave over 1300 analyte-sample preparation condition pairs. A new term – surfactant effect (calculated for the standard solution) – allowed us to distinguish between the surfactant effect and that related to interferences from human plasma. The screening revealed that CPE combined with LC–MS is not related to a significant ME in the optimal pH of extraction. A significant absolute ME (<85% or>115%) was observed only for 25% of the analytes. Data processing (principal component analysis, classification trees, partial least squares-discriminant analysis) based on the extraction conditions and molecular descriptors helped to identify compounds prone to the matrix effect and speed up method development. A low surfactant concentration and low temperature decreased both the absolute and relative ME. pH of the extraction influenced only the relative ME. Low retention time reduced the risk of relative ME, whereas high polarity and the possibility of hydrogen bond formation minimized the occurrence of the surfactant effect and absolute ME. A significant relative ME (>15%) was observed only for 11% of the compounds, thus CPE merged with LC–MS allowed to measure drug concentrations in a reliable manner for majority of compounds. The presented approach may be further applied to other analytes and matrices. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the comparative structural analysis of a series of compounds (methyl α-d-glucopyranoside, methyl 4,6-O-ethylidene-α-d-glucopyranoside (2), methyl 2,3-di-O-nitro-4,6-O-ethylidene-α-d-glucopyranoside and methyl 3-O-nitro-4,6-O-ethylidene-α-d-glucopyranoside) by way of synthesis leading to methyl 3-O-nitro-α-d-glucopyranoside (5) is reported. The title compound (5) is a novel d-glucosidic mononitrate having potential biological activity against cardiovascular diseases. The structural analysis was supported by single-crystal X-ray diffraction (XRD), 13C CP/MAS NMR spectroscopy and DFT calculations. In the case of 2 and 5, XRD analysis could not be performed due to the fact that 2 is highly hygroscopic and 5 forms improper crystals. However, the molecular structures of 2 and 5 were obtained on the basis of experimental (existing XRD data for similar compounds) and theoretical (DFT optimization) approaches. This showed of very good agreement with the 13C CP/MAS NMR spectral data. [Copyright &y& Elsevier]

Abstract: Single-crystal and powder X-ray diffraction, 13C CP/MAS NMR, and DFT-GIAO calculations of shielding constants were performed for methyl 3,4,6-tri-O-acetyl-2-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-α-d-glucopyranoside (1) and methyl 2,4,6-tri-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-α-d-glucopyranoside (2). 1 crystallizes with three molecules whereas 2 with two molecules in an independent part of the unit cell. The macroscopic sample of 1 consists of one polymorph, while for 2 two or three polymorphs were detected. The existence of different numbers of the molecules in the asymmetric units as well as different numbers of polymorphs of 1 and 2 was verified by solid-state NMR spectroscopy. [Copyright &y& Elsevier]

A reliable method for molecular structure determination, excluding single-crystal X-ray diffraction (SCXRD), has been applied to six methyl glycoside tetrabenzoates. The proposed method is based on a global conformational search using molecular mechanics and subsequent DFT calculations guided by a solid-state NMR experiment. The accuracy of the applied method has been verified on three methyl glycoside benzoates for which the SCXRD analysis has been completed. It appeared that the calculated conformations of unprivileged energy could be found in the solid state. Bulky substituents (benzoates) exerted less energetically favored interactions in crystals in contrast to isolated molecules. Therefore, solid-state NMR was revealed to be an indispensable approach for choosing credible conformations from the calculated conformations. [ABSTRACT FROM AUTHOR]

The main objective of our research was to analyze the structure of the Se-containing polysaccharides and to examine how the selenium is bound to the polysaccharide molecule. During investigation of the biosynthesis of new immunomodulators, we isolated a selenium (Se)-containing polysaccharide-protein fraction containing proteoglycans of molecular weights of 3.9 × 10 6 Da and 2.6 × 10 5 Da, composed of glucose or mannose, nearly 8% of protein and 190 μg Se/g dry weight. X-ray absorption spectroscopy (XAS) data analysis in the near edge region (XANES) confirmed that selenium in the Se-polysaccharides structure is present at the –II oxidation state and that Se is organically bound. The simulation analysis in the EXAFS (extended X-ray absorption fine structure) region suggested that selenium is most likely bound by a glycosidic-link in a β-1,3 or α-1,4-glycosidic bond or substituted for oxygen in a pyranosidic ring. Calculations performed with Gaussian 03 software predicted deformations in the polysaccharide structure caused by the incorporation of the selenium atom including change in bond lengths and torsion angles and, as a result, disappearance of hydrogen bonds in the vicinity of the selenium atoms. [ABSTRACT FROM AUTHOR]

A convenient method for the indirect crystal structure verification of methyl glycosides was demonstrated. Single-crystal X-ray diffraction structures for methyl glycoside acetates were deacetylated and subsequently subjected to DFT calculations under periodic boundary conditions. Solid-state NMR spectroscopy served as a guide for calculations. A high level of accuracy of the modelled crystal structures of methyl glycosides was confirmed by comparison with published results of neutron diffraction study using RMSD method. [ABSTRACT FROM AUTHOR]

Abstract: Comprehensive structural analyses of methyl 2,4,6-tri-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-β-d-galactopyranoside (1) and methyl 3,4,6-tri-O-acetyl-2-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-β-d-galactopyranoside (2) have been performed. The analyses combined experimental and theoretical methods: single-crystal and powder X-ray diffraction and 13C CP/MAS NMR spectroscopy served as experimental techniques, whereas the grid search of the conformers of disaccharides implemented by genetic algorithm was chosen as the theoretical method. One of the calculated conformers of 1 fits the experimental structure very well. Although the two best conformations of 2 are of similar energy, their structures are significantly different. Probably therefore 2 cannot form any crystallographic forms and is amorphous. [Copyright &y& Elsevier]

Abstract: Comprehensive structural analyses were performed for o-, m-, and p-nitrophenyl 2,3,4-tri-O-acetyl-β-d-xylopyranosides. Single-crystal X-ray diffraction data were collected and revealed that meta isomer undergoes temperature-dependent polymorph transition (crystal structures of two polymorphs were obtained). This transition was verified by differential scanning calorimetry. The number of molecules in the independent part of the crystal unit cells of the compounds under investigation was in agreement with the number of resonances in solid-state 13C and 15N NMR spectra. The o- and p-nitrophenyl 2,3,4-tri-O-acetyl-β-d-xylopyranosides exist as single polymorph at room temperature, as confirmed by powder X-ray diffraction measurements. [Copyright &y& Elsevier]