Structural characterization of an anhydrous polymorph of paclitaxel by solid-state NMR.

The three-dimensional structure of a unique polymorph of the anticancer drug paclitaxel (Taxol) is established using solid state NMR (SSNMR) tensor ((13)C & (15)N) and heteronuclear correlation ((1)H-(13)C) data. The polymorph has two molecules per asymmetric unit (Z' = 2) and is thus the first conformational characterization with Z' > 1 established solely by SSNMR. Experimental data are correlated with structure through a series of computational models that extensively sample all conformations. For each computational model, corresponding tensor values are computed to supply comparisons with experimental information which, in turn, establishes paclitaxel's structure. Heteronuclear correlation data at thirteen key positions provide shift assignments to the asymmetric unit for each comparison. The two distinct molecules of the asymmetric unit possess nearly identical baccatin III moieties with matching conformations of the C10 acetyl moiety and, specifically, the torsion angle formed by C30-O-C10-C9. Additionally, both are found to exhibit an extended conformation of the phenylisoserine sidechain at C13 with notable differences in the dihedral angles centered around the rotation axes of O-C13, C2'-C1' and C3'-C2'.