Solid-state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside |
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Authors: | Wolniak Michał Oszmiański Jan Wawer Iwona |
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Affiliation: | Department of Physical Chemistry, Faculty of Pharmacy, Medical University of Warsaw, 02-097 Warsaw, ul. Banacha 1, Poland. |
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Abstract: | Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable. |
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Keywords: | solid‐state NMR 13C CPMAS NMR PASS‐2D GIAO shielding calculations baicalein Scutellaria baicalensis |
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