The thermodynamic and kinetic aspects of midazolam ring closure from benzophenone to benzodiazepine form, its acid–base equilibria and aromaticity: a quantum-chemical study

Abstract

The process of midazolam ring closure was studied from the thermodynamic and kinetic points of view by means of quantum-chemical methods. B3LYP/6–311 + + G(d,p) model was employed for gas phase and water environment (polarizable continuum model) calculations. It was concluded that the reaction rate determining step is the first step—carbinolamine formation from amine and carbonyl ends of the opened benzodiazepine ring. The Gibbs free energy of activation was calculated as 35.1 kcal/mol for gas- phase and 33.9 kcal/mol for water environment. Intrinsic reaction coordinate calculations were performed to verify that the transition state really connects the substrate and product. Thermodynamically, this reaction is endoergic with ΔG = 9.6 kcal/mol for gas phase and 10.0 kcal/mol for water environment. However, the next step—carbinolamine protonation with immediate water molecule loss is expected to be fast and activation barrierless, which enables further progress of the ring closure, despite the positive ΔG of the fist step. Next, the protonated imine undergoes deprotonation to final closed ring, pharmacologically active molecule of midazolam. The whole chain of reaction is exoergic with ΔG equal to − 5.6 kcal/mol for gas phase and − 7.7 kcal/mol for water environment. In order to understand the role of other than benzodiazepine/imidazole molecular fragments on the ring closure process, a model was build which contains only benzodiazepine and imidazole rings. The activation barrier for the carbinolamine formation of the model is similar to midazolam in the gas phase but higher by about 10 kcal/mol for water environment. The most interesting difference is however that for the model, the carbinolamine formation step is exoergic with ΔG equal to − 2.2 kcal/mol for gas phase and − 1.8 kcal/mol for water environment. This difference can be connected to complicated conformational shape of the midazolam molecule, which during the ring closure undergoes unfavorable deformations with accompanying rise of the energy of the molecule. The protonation sites for both midazolam and the model were also studied. In the case of midazolam, the preferred protonation site is the imidazole ring nitrogen atom, but in the case of the model it is the benzodiazepine ring nitrogen atom. The aromaticity of the 5- and 7-membered rings were analyzed using two aromaticity—HOMA and pEDA. It follows that larger stability of the cation protonated at the benzodiazepine ring is accompanied with substantial increase of the 7-ring aromaticity in the model of midazolam. The complexes of midazolam and its model with a water molecule were analyzed because they are needed for evaluation of the energy of the whole process of ring closure. In the case of midazolam, the water molecule preferentially connects to imidazole ring, but in the case of the model, complexes with both imidazole and benzodiazepine rings have similar stability.