The study of the formation of microstructures during the interaction of a protonated drug-like compound (API) with a maleic acid monoanion sheds light on the assembly processes in an aqueous solution at the molecular level. Molecular dynamics (MD) simulations coupled with density functional theory (DFT) calculations made it possible to find initial hydrogen bonding motifs during the assembly process, leading to the formation of heterodimers and trimers. The process of trimer formation [protonated API-maleic acid monoanion-protonated API] proceeds through the formation of three intermolecular H-bonds by the CO2- group of the maleic acid monoanion in both systems. The total enthalpy/energy of these H-bonds is more than 70 kJ/mol. Thus, the maleic acid monoanion plays a key role in the processes of association in aqueous solution, and the interaction of the maleic acid monoanion with API is more preferable than the interaction of API molecules with each other. DFT computations in the discrete continuum approximation reveal the spectral features of heterodimers and trimers, and the ATR-IR spectra confirmed these findings. MD simulations followed by DFT calculations made it possible to describe the initial stages of the formation of pharmaceutical cocrystals in an aqueous solution.
Keywords: ATR-IR spectroscopy; DFT computations; classical MD simulations; intra- and intermolecular hydrogen bonds; multicomponent organic crystals.