Combined approach using capillary electrophoresis, NMR and molecular modeling for ambrisentan related substances analysis: Investigation of intermolecular affinities, complexation and separation mechanism

A comprehensive investigation on the CE separation mechanisms and on the inclusion complexation with CyDs of the chiral drug S-ambrisentan (S-AMB), its R-enantiomer and other impurities was performed by different techniques. A CE method was previously set up allowing the simultaneous determination of the enantiomeric purity and of impurities of S-AMB, based on the addition of SDS micelles and gamma-cyclodextrin (gCyD) to borate buffer. In this study, the electrophoretic behavior of the analytes in terms of selectivity and mobility with respect to the addition of different CyDs was first investigated, evidencing the presence of interactions for all the CyDs, but the unique ability of gCyD for obtaining the separation of all the compounds. By molecular modeling, aggregates between SDS micelles and analytes, and inclusion complexes between CyDs, SDS and/or analytes of different stoichiometries were simulated. The potential and the gain energy of complexes were calculated on the minimized conformations, showing the great tendency of gCyD of forming mixed complexes with one or two SDS molecules and with the analyte, even if with different affinities among the analytes. For 1:1:1 mixed complexes with different CyDs, the highest difference of potential energy between the enantiomers’ complexes was observed for gCyD. Two-dimensional NOE spectroscopy experiments were performed for S-AMB and I1 and pointed out the interactions of the aromatic moiety of the analytes and of SDS aliphatic chain with gCyD protons,confirming the existence of gCyD mixed complexes. The high affinity of SDS for the gCyD cavity was suggested to justify the fundamental role of SDS in modulating and achieving the CE separation, due to its influence both on the stability and on the type of complexes between gCyD and the analytes.