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 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.
D044950:Flavanones / Q000737:chemistry*; D005419:Flavonoids / Q000737:chemistry*; D005960:Glucosides / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards*; D008958:Models, Molecular; D015394:Molecular Structure; D018517:Plant Roots / Q000737:chemistry; D011789:Quantum Theory; D031364:Scutellaria baicalensis / Q000737:chemistry*
D044950:Flavanones; D005419:Flavonoids; D005960:Glucosides; C038044:baicalin; C006680:baicalein; C473995:wogonoside