Determination of solution structure of anti-cancer drug etoposide by NMR and computer simulation

• Main Author: Zhang, Kangling
• Format: Others
• Published: Scholarly Commons 1997
• Subjects: Etoposide; Medicine and Health Sciences
• Online Access: https://scholarlycommons.pacific.edu/uop_etds/2323; https://scholarlycommons.pacific.edu/cgi/viewcontent.cgi?article=3322&context=uop_etds

2.9 mM etoposide in CDCl3 and CDCl3/(D20) were studied at 300 and 500 MHz NMR. One dimensional (1D) NMR spectra and truncated lD Nuclear Overhauser Effect (NOE) buildup experiments were used to find direct information on the structure of etoposide. Results showed that the E-ring rotates freely about the C1’-C4 bond and is oriented to the side where H2’, H4 project. The C and D rings have a "trans" junction at C2-C3 bond. The ethylidene-glucopyranosyl group approaches the D ring allowing close interaction between H1” and H8. Water spin - transfer experiments were done to determine the interaction between water and the etoposide molecule. Chemical shifts of 2" -OH, 3" -OH were found to be affected remarkably by changes in water concentration due to the hydration of these hydroxide groups by H-bonding with water. Relaxation experiments were done to measure the longitudinal relaxation time and the whole molecular tumbling speed in solution (τc is 0.6 ns at 500 MHz). Higher than average T1 differences between wet sample and dry samples were observed for 4'-0H, H8, H3, H5, H2”. It is suggested that some water residues are associated with these nuclei. The 4'-0H is obviously hydrated by water via H-bonding (as above). Two dimensional NOE spectroscopy (NOESY & ROESY) experiments were done to give the distance constraints for running molecular dynamics (MD). Computer simulation and modeling were carried out to build up the molecule with restraint molecular dynamics (rMD) calculations by Amber 4.1. Temperature annealing, time-averaged distance restraint and water-solute interactions were applied to in these simulations. Several water bridges were found by calculation to correspond to the interactions seen to effect hydrogens by NMR. The hydration of etoposide also accounts for the slower relaxation rates in partially aqueous CDCl3 solution.