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 C-13 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 C-13 resonances. The cross-polarization (CP) time constants T-CH and relaxation times T-1P(H) were obtained from the variable-contact time experiments. The principal elements of the C-13 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 C-13 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 center dot center dot center dot O=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable. Copyright (c) 2007 John Wiley & Sons, Ltd.