Solid-state NMR studies of the molecular structure of Taxol.

Solid-state 13C{1H} cross-polarization/magic angle spinning spectroscopy (CP/MAS) has been utilized to extract the molecular structure information of Taxol, which is an anti-tumor therapeutic medicine extracted from the yew bark. The 13C signals have chemical shift values quite consistent with those measured in solution phase, and the overall chemical shift range is over 200 ppm. Notably, most of the 13C resonances of the taxane ring have two clearly resolved spectral components except the resonance peaks of C-15, C-16 and C-17, which are located at the central part of the taxane ring. On the basis of our NMR data, we propose that these doublets originate from two slightly different molecular conformations of the taxane ring and still the central part of the ring remains structurally similar. Furthermore, it is demonstrated that the 13C chemical shift difference deduced from the doublet splittings can serve as a direct measure of the structural difference between the two conformations, which could possibly correlate with the anti-tumor activity of Taxol.