A novel approach to the study of solution structures and dynamic behavior of paclitaxel and docetaxel using fluorine-containing analogs as probes

Three fluorine-containing paclitaxel and docetaxel analogs, 3'-dephenyl-3'-(4-fluorophenyl)-3'N-debenzoyl-3'N-(4-fluorobenzoyl)paclitaxel (3), 3'-dephenyl-3'-(4-fluorophenyl)docetaxel (4), and 2',10-diacetyl-3'-dephenyl-3'-(4-fluorophenyl)docetaxel (5), are prepared and used as probes for the conformational analysis of paclitaxel and docetaxel in aqueous and nonaqueous solvent systems. The dependence of the F-19 chemical shifts and the J(H2'-H3') values of these fluorinated analogs is examined through F-19 and H-1 variable temperature (VT) NMR measurements. The experiments clearly indicate highly dynamic behavior of these molecules and the existence of equilibrium between conformers, especially in protic solvents, i.e., DMSO-d(6)/D2O, CH3OD/D2O, and CH3OD, which have not clearly been recognized by the previous studies. The analysis of the VT NMR data in combination with molecular modeling including restrained molecular dynamics (RMD) has identified three key conformers, A, B, and C, in which conformer C possesses rather unusual nearly eclipsed arrangements at the C2'-C3' bond. Conformers A and C are essentially the same as those identified by X-ray analysis of docetaxel and paclitaxel, respectively. RMD evaluation of conformer C in a simulated aqueous environment shows substantial stabilization of this conformer in protic solvents as compared to the other conformers. The F-19-H-1 heteronuclear NOE measurements of these fluoro analogs also support the structures of the three conformers. Conformers B and C form a hydrophobic clustering among the 4-fluorophenyl at C-3', the phenyl at the C-2 benzoate, and the methyl at the C-4 acetate moieties. Since conformer C appears to be the predominant molecular structure at ambient temperature in aqueous solvents, this conformer is likely-to be the molecular structure of paclitaxel or docetaxel that is recognized at the tubulin binding site. This study has unambiguously demonstrated the usefulness of these ''fluorine probes'' for the solution structures and dynamic behavior of complex molecules such as paclitaxel and docetaxel.