Quality by Design, NMR and Molecular Modeling for the investigation of intermolecular affinities and separation mechanisms in capillary electrophoretic methods for assaying drugs and their impurities

Capillary electrophoresis (CE) is a versatile technique that allows a great modulation of separative conditions. Quality by Design principles are recognized as valuable tools for obtaining reliable information about the experimental space. The successful binomium of multivariate strategies and CE leads to obtain: the best working conditions, a multidimensional region where quality is assured and mathematical models that represent a step forward in the comprehension of the electrophoretic behaviour of the analytes. In the analysis of drugs and their related substances, complex CE systems that involved micelles, microemulsions and cyclodextrins (CDs) as pseudostationary phases are often used. With the goal of understanding the fine effects involved in separation using such complex systems, integrated studies of NMR and molecular modeling were carried out. Molecular Dynamics (MD) and NMR studies were used to elucidate separation mechanisms, host-guest interactions and intermolecular affinities. Aspects such as affinity pattern of analytes towards various CDs, as well as the equilibrium constants and the structure of complexes were addressed. The effect of cosurfactants on separation selectivity and the role of the ionic surfactant sodium dodecyl sulfate (SDS) were investigated. Simulations and NMR experiments underlined the ability of CDs of interacting with the SDS monomer forming inclusion complexes and the pivotal role of this surfactant in modulating the different affinities of the analytes for chiral selectors.