MOLECULAR-DYNAMICS AND STATIC SOLVATION STUDIES OF AMILORIDE

The GROMOS molecular mechanics and dynamics force field was extended and modified in order to investigate the static and dynamic conformational properties of amiloride conformers in solution. Torsional potential functions, Lennard-Jones parameters, and atomic point charges were derived for the free base and protonated species of amiloride. The effect of solvent on the conformation, energy, and intramolecular hydrogen bonding patterns of the free base (A1 and A4) and protonated (F1) species of amiloride was examined by 25-ps simulations of each species in a bath of SPC water molecules. The large torsional barriers for A1 to A4 and F1 to F4 conversion, determined from 3-21G* molecular orbital calculations, were found to constrain the average structure of each species to a nearly-planar conformation. This suggests that amiloride binds to the ion channel in a planar conformation. In agreement with previous ab initio calculations, the molecular dynamics simulations found the relative internal energy of the Al conformer to be lower than that of A4. However, the solute-solvent interaction energy was lower for A4 than A1, consistent with the larger dipole moment of A4. Combined, these trends still predict the A1 conformer to be more stable in solution than A4. Static solvation studies of amiloride with an induced polarization charge boundary element (IPCBE) continuum solvent method, the Langevin dipole method, and the self-consistent reaction field method gave qualitatively similar results. These results help to clarify the NMR studies of Smith et al. (J. Am. Chem. Sec, 1979, 101, 191), who were unable to distinguish between the A1 and A4 conformers in solution. Calculation of the electrostatic contribution to both the relative hydration enthalpy and the relative hydration free energy of amiloride conformers using the IPCBE method showed that the maximum difference in these quantities is about 4%.