CH/pi interactions involving aromatic amino acids: refinement of the CHARMM tryptophan force field

High-level ab initio calculations have been carried out to study weak CH/pi interactions and as a check of the CHARMM force field for aromatic amino acids. Comparisons with published data indicate that the MP2/cc-pVTZ level of theory is suitable for calculations of CH/pi interaction, including the T-shape benzene dimer. This level of theory was, therefore, applied to investigate CH/pi interactions between ethene or cis-2-butene and benzene in a variety of orientations. In addition, complexes between ethene and a series of model compounds (toluene, methylindole and p-cresol) representing the aromatic amino acids were studied motivated by the presence of CH/pi interactions in biological systems. Ab initio binding energies were compared to the binding energies obtained with the CHARMM22 force field. In the majority of orientations, CHARMM22 reproduces the preferred binding modes, with excellent agreement for the benzene dimer. Small discrepancies found in the calculations involving methylindole along with a survey of published thermodynamic data for the aromatic amino acids prompted additional optimization of the tryptophan force field. Partial atomic charges, Lennard-Jones parameters, and force constants were improved to obtain better intra- and intermolecular properties, with significant improvements obtained in the reproduction of experimental heats of sublimation for indole and free energies of aqueous solvation for methylindole.