Etoposide: Conformational and hydration features

The proton chemical shifts of the anticancer agent etoposide in CD3OD, dry CDCl3 and 'wet' CDCl3 were determined and all proton resonances were assign ed. The general conformational features of etoposide were determined from N MR coupling constants, truncated NOE measurements, NOESY and ROESY experime nts and molecular modeling (MM2, MM+, AMBER). A complete relaxation matrix analysis (CORMA/MARDIGRAS) was used to calculate proton-proton distances fr om ROESY cross-peak intensities and the resulting distance constraints were used for molecular dynamic calculations using AMBER 4.1. Temperature annea ling and water-solute interactions were applied in these simulations. The g lycosidic ring is roughly perpendicular to the polycyclic ring system with the axial protons oriented toward the aromatic ring. The lactone ring is in the half-chair (Cz) form and the 3',5'-dimethoxy-4'-hydroxy aryl ring is d irected beneath the polycyclic ring system. The hydration of etoposide in c hloroform solution containing added water was studied. Etoposide hydroxide protons show up as separate proton resonances but irradiation of the water line or any of the OH lines reveals rapid spin communication among this pop ulation. The chemical shifts of the 2 "- and 3 "-hydroxyl protons of the gl ycosidic ring are strongly dependent an the water-to-etoposide ratio. Selec tive saturation transfer experiments as a function of decoupler power or ir radiation time and non-selective inversion-recovery Tr measurements were ca rried out. Hydration modeling studies showed several water bridges connect the glycosidic hydroxyl groups with the O-16 atom of the epipodophyllotoxin ring system and with the 4'-hydroxyl group of the pendant 2',6'-dimethoxya ryl group. Despite the very clear presence of a 'spine' of hydration, the g eneral conformational features of 'wet' etoposide are the same as those of the non-hydrated etoposide. Copyright (C) 1999 John Wiley & Sons, Ltd.