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, nyl)-3'N-d ebenzoyl-3'N-(4-fluorobenzoyl)paclitaxel (3), 3'-dephenyl-3'-(4-fluoro phenyl)docetaxel (4), and -diacetyl-3'-dephenyl-3'-(4-fluorophenyl)doc etaxel (5), are prepared and used as probes for the conformational ana lysis of paclitaxel and docetaxel in aqueous and nonaqueous solvent sy stems. The dependence of the F-19 chemical shifts and the J(H2'-H3') v alues of these fluorinated analogs is examined through F-19 and H-1 va riable temperature (VT) NMR measurements. The experiments clearly indi cate highly dynamic behavior of these molecules and the existence of e quilibrium between conformers, especially in protic solvents, i.e., DM SO-d(6)/D2O, CH3OD/D2O, and CH3OD, which have not clearly been recogni zed by the previous studies. The analysis of the VT NMR data in combin ation with molecular modeling including restrained molecular dynamics (RMD) has identified three key conformers, A, B, and C, in which confo rmer C possesses rather unusual nearly eclipsed arrangements at the C2 '-C3' bond. Conformers A and C are essentially the same as those ident ified by X-ray analysis of docetaxel and paclitaxel, respectively. RMD evaluation of conformer C in a simulated aqueous environment shows su bstantial stabilization of this conformer in protic solvents as compar ed to the other conformers. The F-19-H-1 heteronuclear NOE measurement s of these fluoro analogs also support the structures of the three con formers. 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 a t the C-4 acetate moieties. Since conformer C appears to be the predom inant molecular structure at ambient temperature in aqueous solvents, this conformer is likely-to be the molecular structure of paclitaxel o r docetaxel that is recognized at the tubulin binding site. This study has unambiguously demonstrated the usefulness of these ''fluorine pro bes'' for the solution structures and dynamic behavior of complex mole cules such as paclitaxel and docetaxel.