Muscarine and water: Ab initio potentials

Abstract

Ab initio SCF-LCAO-MO (Self Consistent Field-Linear Combination Atomic Orbitals-Molecular Orbital) computations have been carried out on the muscarine ion in different conformations. The rotational barrier around the C3-C2-C1-N dihedral angle has been calculated and the results show that the molecule possesses a fairly rigid structure in this respect. The rotational barrier around the HO2-O2-C4-H4 dihedral angle (τ 2) was also analyzed, and it was found that in vacuum the preferred conformation is the one withτ 2=0. SCF calculations have also been carried out on the muscarine-water system at many different separations and/or orientations. The interaction energies were then fitted to pair potential functions, which allowed makeup of isoenergy maps that explicitly show the interaction minima and the repulsive regions. These potential functions are also necessary in order to carry out statistical simulations of the molecule water solution. In order to assess the role of the ring methyl group in muscarinic activity, the interaction of the demethylmuscarine molecule with water was also studied; the results show that the loss of the terminal methyl leads to a distinct loss of hydrophobic character.