Two novel approaches to construct empirical schemes for partial atomic charge calculation were proposed. The charge schemes possess important benefits. First, they produce both topologically symmetrical and environment dependent charges. Second, they can be parameterised to reasonably reproduce ab initio molecular electrostatic potential (MEP), which guarantees their successful use in molecular modelling. To validate the approaches, the parameters of the proposed charge schemes were fitted to best reproduce MEP simultaneously on grids around a set of 227 diverse organic compounds. The residual errors in MEP reproduction due to calculated atomic charges were compared to those due to charges from known charge schemes.
Recently several molecular mechanics models of halogen bonding have been published. They describe the electrostatic potential anisotropy near the heavy halogen atom (known as a σ-hole) in different ways, ranging from an all-atom multipole expansion to a single positive extra-point charge. However, the question of a reasonable balance between the accuracy and the simplicity of the model remains open. In this work, we introduce the simplistic RESPQ electrostatics model built on the RESP charges complemented with fixed atomic quadrupoles. We show that it: (1) correctly describes the MEP anisotropy of aromatic halogen atoms, (2) improves the description of the halogen−water interaction energies in both halogen and hydrogen bonding cases, (3) provides an excellent estimation of solvation free energy differences of aromatic halogens, and (4) is compatible (with the help of multipole charge cluster approximation) with contemporary molecular modeling packages.
Halogen bonding (XB) is a new promising interaction pattern in medicinal chemistry. It has predominantly electrostatic nature - high electrostatic potential anisotropy. However to fully unleash the potential of XB in rational drug design fast and robust empirical methods of XB description should be developed. Current approaches rely heavily on ab initio calculation for each molecule studied. Thus fast prediction of electrostatic parameters for description of XB for arbitrary organic molecules is of paramount importance to promptly establish QSAR/QSPR, virtual screening and molecular docking pipelines suitable for today's agile development requirements. The two most promising approaches to describe anisotropic electrostatic models - the extra point (EP) charge model and the multipole expansion (ME) model - were studied on their ability (1) to describe ab initio molecular electrostatic potential (MEP) and (2) to produce parameters that can be predicted for each molecule empirically rather than estimated via ab initio calculations. The reference ab initio MEP was calculated for a set of 730 substituted halobenzenes. Parameters for anisotropic electrostatics of both empirical models (EP and ME) studied were extracted from ab initio MEP. The FreeWilson and Hansch type QSPR models relating XB parameters with aromatic substituents were built and analyzed, providing the guidelines for further development.
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