ABSTRACT: We have tested a variety of approximate methods for modeling 30 systems containing mixtures of nitrogen heterocycles and exocyclic amines, each of which is studied with up to 31 methods in one or two phases (gaseous and aqueous). Fifteen of the systems are protonated, and 15 are not. We consider a data set consisting of geometric parameters, partial atomic charges, and water binding energies for the methotrexate fragments 2-(aminomethyl)pyrazine and 2,4-diaminopyrimidine, as well as their cationic forms 1H-2-(aminomethyl)pyrazine and 1H-2,4-diaminopyrimidine. We first evaluated the suitability of several density functionals with the 6-31+G(d,p) basis set to serve as a benchmark by comparing calculated molecular geometries to results obtained from coupled-cluster [CCSD/6-31+G(d,p)] wave function theory (WFT). We found that the M05-2X density functional can be used to obtain reliable geometries for our data set. To accurately model partial charges in our molecules, we elected to utilize the well-validated Charge Model 4 (CM4). In the process of establishing benchmark values, we consider gas-phase coupled cluster and density functional theory (DFT) calculations followed by aqueous-phase DFT calculations, where the effect of solvent is treated by the SM6 quantum mechanical implicit solvation model. The resulting benchmarks were used to test several widely available and economical semiempirical molecular orbital (SE-MO) methods and molecular mechanical (MM) force fields for their ability to accurately predict the partial charges, binding energies to a water molecule, and molecular geometries of representative fragments of methotrexate in the gaseous and aqueous phases, where effects of water were simulated by the SM5.4 and SM5.42 quantum mechanical implicit solvation models for SE-MO and explicit solvation used for MM. In addition, we substituted CM4 charges into the MM force fields tested to observe the effect of improved charge assignment on geometric and energetic modeling. The most accurate MM force fields (with or without CM4 charges substituted) were validated against gas-phase and aqueous-phase geometries and charge distributions of a larger set of 16 drug-like ligands, both neutral and cationic. This process showed that the Merck Molecular Force Field (MMFF94) with or without CM4 charges substituted, is, on average, the most accurate force field for geometries of molecules containing nitrogen heterocycles and exocyclic amino groups, both protonated and unprotonated. This force field was then applied to the complete methotrexate molecule, in an effort to systematically explore its accuracy for trends in geometries and charge distributions. The most accurate force fields for the binding energies of nitrogen heterocycles to a water molecule are OPLS2005 and AMBER.
