Katarzyna Paradowska

1H, 13C MAS NMR and DFT GIAO study of quercetin and its complex with Al(III) in solid state

Quercetin (Q) as a pentahydroxy flavonoid, has three possible chelating sites competing in complexation processes. (1)H and (13)C MAS NMR spectra were recorded for solid quercetin and its Al(III) complex (AlQ). (1)H MAS spectrum of quercetin shows a broad resonance at ca. 12 ppm that confirms the existence of intramolecular C5-OH … O=C4 hydrogen bond. Such a signal is absent in the spectrum of AlQ, which is in accordance with other spectroscopic data and the suggested model for the solid-state structure of the complex. DFT GIAO calculations were used to verify the experimental (13)C CPMAS NMR data and to suggest the best model structure for the complex AlQ. The calculated shielding constants for different conformers of isolated quercetin molecules, quercetin trimer as taken from the X-ray data, and different model structures for possible Al(III) complexes were compared with the (13)C CPMAS NMR experimental values. The results demonstrate the importance of intermolecular interactions when dealing with structures in solid state and the successful application of the combined DFT GIAO and (13)C CPMAS NMR approach. All data confirm that the chelating site of Q in the solid complex AlQ involves the deprotonated C5-OH and the C4=O groups at ring C, in contrast to the available studies performed in solution.

1H and 13C NMR‐based sugar profiling with chemometric analysis and antioxidant activity of herbhoneys and honeys

BACKGROUND Herbhoneys, relatively new bee products, are expected to have interesting medicinal properties. However, there is still a lack of data concerning their composition and antioxidant properties. ¹H and ¹³C NMR spectroscopy coupled with chemometric analysis (PCA and PLS-DA) and antioxidant assays (DPPH-ESR and ORAC-FL) were used to study 25 samples of Polish herbhoneys and honeys. RESULTS Antioxidant activity varied among the samples. The best properties were exhibited by cocoa and instant coffee herbhoneys. The contents of total polyphenols and total carotenoids in the studied samples were found to be 70-1340 mg GAE kg⁻¹ and 0-28.05 mg kg⁻¹ respectively. No significant differences between herbhoney and honey samples were found in their sugar profiles. The PCA of ¹³C NMR spectra of the samples in DMSO-d6 resulted in sample clustering due to sucrose content. CONCLUSION Herbhoneys have similar antioxidant properties to traditional honeys, being therefore of equal nutritional value. There was a noticeable influence of the extract concentration on the observed antioxidant effect. For samples with high antioxidant activity, polyphenols were responsible for the observed effect. Sample clustering due to sucrose content in the NMR-PCA study allowed effortless detection of adulteration.

Solid-state NMR in the analysis of drugs and naturally occurring materials

This article presents some of the solid-state NMR (SSNMR) techniques used in the pharmaceutical and biomedical research. Solid-state magic angle spinning (MAS) NMR provides structural information on powder amorphous solids for which single-crystal diffraction structures cannot be obtained. NMR is non-destructive; the powder sample may be used for further studies. Quantitative results can be obtained, although solid-state NMR spectra are not normally quantitative. As compared with other techniques, MAS NMR is insensitive and requires a significant amount of the powder sample (2-100mg) to fill the 1.3-7 mm ZrO2 rotor. This is its main drawback, since natural compounds isolated from plants, microorganisms or cell cultures are difficult to obtain in quantities higher than a few milligrams. Multinuclear MAS NMR routinely uses (1)H and (13)C nuclei, less frequently (15)N, (19)F, (31)P, (77)Se, (29)Si, (43)Ca or (23)Na. The article focuses on the pharmaceutical applications of SSNMR, the studies were aimed to control over manufacturing processes (e.g. crystallization and milling) investigation of chemical and physical stability of solid forms both as pure drug and in a formulated product. SSNMR is used in combination with some other analytical methods (DSC, XRD, FT-IR) and theoretical calculations of NMR parameters. Biologically active compounds, such as amino acids and small peptides, steroids and flavonoids were studied by SSNMR methods (part 4) providing valuable structural information. The SSNMR experiments performed on biopolymers and large natural products like proteins, cellulose and lipid layers are commented upon briefly in part 5.

13C CP MAS NMR and GIAO-CHF calculations of coumarins

13C cross-polarization magic-angle spinning NMR spectra were recorded for a series of solid coumarins. Ab initio calculations of shielding constants were performed with the use of GIAO-CHF method. The combined CPMAS NMR and theoretical approach was successful in characterizing solid-state conformations of coumarins; a relationship sigma (ppm) = -1.032 xdelta + 205.28 (R(2) = 0.9845) can be used to obtain structural information for coumarins, for which solid-state NMR or crystal structure data are not available.

13C, 15N CPMAS NMR and GIAO DFT calculations of stereoisomeric oxindole alkaloids from Cat's Claw (Uncaria tomentosa)

Oxindole alkaloids, isolated from the bark of Uncaria tomentosa [Willd. ex Schult.] Rubiaceae, are considered to be responsible for the biological activity of this herb. Five pentacyclic and two tetracyclic alkaloids were studied by solid-state NMR and theoretical GIAO DFT methods. The (13)C and (15)N CPMAS NMR spectra were recorded for mitraphylline, isomitraphylline, pteropodine (uncarine C), isopteropodine (uncarine E), speciophylline (uncarine D), rhynchophylline and isorhynchophylline. Theoretical GIAO DFT calculations of shielding constants provide arguments for identification of asymmetric centers and proper assignment of NMR spectra. These alkaloids are 7R/7S and 20R/20S stereoisomeric pairs. Based on the (13)C CP MAS chemical shifts the 7S alkaloids (delta C3 70-71ppm) can be easily and conveniently distinguished from 7R (deltaC3 74.5-74.9ppm), also 20R (deltaC20 41.3-41.7ppm) from the 20S (deltaC20 36.3-38.3ppm). The epiallo-type isomer (3R, 20S) of speciophylline is characterized by a larger (15)N MAS chemical shift of N4 (64.6ppm) than the allo-type (3S, 20S) of isopteropodine (deltaN4 53.3ppm). (15)N MAS chemical shifts of N1-H in pentacyclic alkaloids are within 131.9-140.4ppm.

Single-crystal and powder X-ray diffraction and solid-state 13C NMR of p-nitrophenyl glycopyranosides, the derivatives of D-galactose, D-glucose, and D-mannose

The X-ray diffraction patterns, (13)C CP MAS NMR spectra, and powder X-ray diffraction analyses were obtained for selected p-nitrophenyl glycosides: alpha- and beta-D-galactopyranosides (1 and 2), alpha- and beta-D-glucopyranosides (3 and 4), and alpha- and beta-D-mannopyranosides (5 and 6). In X-ray diffraction analysis of 1 and 2, characteristic shortening and lengthening of selected bonds were observed in the molecules of 1 due to anomeric effect, and in the crystal lattice of 1 and 2, hydrogen bonds of complex network were detected. In the crystal asymmetric unit of 1 there were two independent molecules, whereas in 2 there was one molecule. For 1 and 3-6 the number of resonances in solid-state (13)C NMR spectra exceeded the number of the carbon atoms in the molecules, while for 2 there were distinct singlet resonances in its solid-state NMR spectrum. Furthermore, the powder X-ray diffraction (PXRD) performed for 1-3 and 5 revealed that 1, 3, and 5 existed as single polymorphs proving that the doublets observed in appropriate solid-state NMR spectra were connected with two non-equivalent molecules in the crystal asymmetric unit. On the other hand 2 existed as a mixture of two polymorphs, one of them was almost in agreement with the calculated pattern obtained from XRD (the difference in volumes of the unit cells), and the subsequent unknown polymorph existed in small amounts and therefore it was not observed in solid-state NMR measurements.

13C CP MAS NMR and crystal structure of methyl glycopyranosides

The X-ray diffraction analysis, (13)C CP MAS NMR spectra and powder X-ray diffraction patterns were obtained for selected methyl glycosides: alpha- and beta-d-lyxopyranosides (1, 2), alpha- and beta-l-arabinopyranosides (3, 4), alpha- and beta-d-xylopyranosides (5, 6) and beta-d-ribopyranoside (7) and the results were confirmed by GIAO DFT calculations of shielding constants. In X-ray diffraction analysis of 1 and 2, a characteristic shortening and lengthening of selected bonds was observed in molecules of 1 due to anomeric effect and, in crystal lattice of 1 and 2, hydrogen bonds of different patterns were present. Also, an additional intramolecular hydrogen bond with the participation of ring oxygen atom was observed in 1. The observed differences in chemical shifts between solid state and solution come from conformational effects and formation of various intermolecular hydrogen bonds. The changes in chemical shifts originating from intermolecular hydrogen bonds were smaller in magnitude than conformational effects. Furthermore, the powder X-ray diffraction (PXRD) performed for 4, 5 and 7 revealed that 7 existed as a mixture of two polymorphs, and one of them probably consisted of two non-equivalent molecules.

1H, 13C MAS NMR and GIAO-CPHF calculations of chloramphenicol, thiamphenicol and their pyrrole analogues

Abstract The 13 C CP MAS and 1 H MAS NMR and ab initio (GIAO-CPHF) calculations were used to obtain structural information on two known antibiotics: chloramphenicol, and thiamphenicol, and two new analogues: dl - threo -1-(1-methyl-4-nitro-pyrrole-2-yl)-2-dichloroacetamidopropane-1,3-diol and dl - threo -1-(1-methylsulfonylpyrrole-3-yl)-2-dichloroacetamidopropane-1,3-diol.

Antioxidant, Cytotoxic, and Antiproliferative Activities and Total Polyphenol Contents of the Extracts of Geissospermum reticulatum Bark

Geissospermum species are medically important plants due to their health-promoting effects. The objective of this study was to determine the antioxidant ability and antiproliferative and cytotoxic effects of infusions, tinctures, and ethanolic extracts of Geissospermum reticulatum barks in relation to the contents of total phenolics and flavonoids. Seven samples of barks were collected in various regions of Peruvian Amazonia. We found that the amount of total phenolics in the studied products varied from 212.40 ± 0.69 to 1253.92 ± 11.20 mg GAE/kg. In our study there is a correlation (R 2 = 0.7947) between the results of antioxidants assays: FRAP and ORAC for tinctures, infusions, and ethanolic extracts of G. reticulatum barks. We have also observed antiproliferative activities of the ethanolic extracts on normal T-cells. These extracts have caused death on malignant cell lines (THP-1 and HL-60) and this data correlates well with their antioxidant capacity measured by ORAC method. Interestingly, the highest concentration of the ethanolic extract was not toxic in the zebrafish embryo developmental assay. Our results indicate that G. reticulatum is rich in antioxidants and have cytotoxic and antiproliferative properties. The data suggests potential immunosuppressive role of the extracts. This is the first study presenting the results of chemical and biological analysis of multiple preparations from G. reticulatum.

Conformational Analysis of Gentiobiose Using Genetic Algorithm Search and GIAO DFT Calculations with 13C CPMAS NMR as a Verification Method

GRAPHICAL ABSTRACT The ALJAR03 software was implemented performing searches in the dihedral angle conformational space of disaccharides. The angles φ, ψ , and ω around glycosidic linkages, as well as the orientations of hydroxymethyl and hydroxyl groups, were considered. The energy maps were calculated using the MM+ force field. The searches were performed using a genetic algorithm (GA) combined with systematic grid search (GS). The application of a genetic algorithm-assisted grid search (GAAGS) approach yielded a significant reduction in the number of structures to be sampled in order to find the best low-energy conformation, as compared with the standard systematic grid search. Thus, time-consuming gauge-independent atomic orbital (GIAO) calculations of shielding constants can be rationalized and limited to some selected conformations. 13C CPMAS NMR chemical shifts for solid gentiobiose were measured and compared with the respective (GIAO DFT) shielding constants calculated for the crystallographic structure and for low-energy conformers. The differences in shielding are associated with the conformational effects (different arrangements of exocyclic groups) and hydrogen bonds. The best agreement (R2 = 0.99) between the experimental and calculated chemical shifts was for the XRD structure and for the best conformer (R2 = 0.98). Their geometries differ in the orientation of OH groups.