Solid-state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside.

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 TCH and relaxation times TH1ρ 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 δii 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 C5OH···C4 is formed; the existence of baicalein and baicalin with ‘anticlockwise’ orientation of OH groups is more probable. Copyright © 2007 John Wiley & Sons, Ltd.