Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations

Abstract

In this work we demonstrate a self-consistent effective technique of analyzing the conformational equilibria of active pharmaceutical ingredient (API) molecules dissolved in supercritical carbon dioxide in a wide range of thermodynamic parameters of state. This approach can be useful for pharmaceutics when the crystalline forms of pharmaceuticals with a high purity degree and desirable polymorphism are produced using CO₂-based supercritical fluids technologies. Within this approach we use a combination of quantum chemical calculations and in situ IR spectroscopy. Quantum chemical calculations allow us to perform the initial conformational search and to determine the energy characteristics of the most stable conformers of API and the energy barriers of transitions between them. IR spectroscopy gives the information on the equilibrium of the most stable conformers of pharmaceuticals dissolved in scCO₂ in the thermodynamic parameter range of interest. Finally we validate our approach by applying it to the study of carbamazepine dissolved in scCO₂ being in permanent contact with an excess of crystalline carbamazepine as an example. The conformational search for carbamazepine molecules in scCO₂ was also performed using molecular dynamics simulation for comparison with the results obtained by the technique presented in this paper.

Keywords: Carbamazepine; Conformational equilibria; IR spectroscopy; Quantum chemical calculations; Supercritical CO(2).