Enhanced binding and conformational selectivity in affinity capillary electrophoresis using a water-soluble resorcin[4]arene as intrinsic buffer and electrokinetic host.

Affinity capillary electrophoresis (ACE) is a widely used technique for quantifying non-covalent molecular interactions that is dependent on the specific buffer conditions selected. In this study, dynamic 1:1 host-guest inclusion complexation involving a charged resorcin[4]arene with a group of neutral corticosteroids was examined by ACE, where the macrocycle serves as both an intrinsic buffer and electrokinetic host. It was determined that over a 200% enhancement in the apparent binding constant (KB) was realized by ACE when usingthe host as an intrinsic buffer at pH 7.5 relative to an extrinsic sodium phosphate buffer system, which was also confirmed by 1H-NMR experiments. This report also revealed improved selectivity mediated by the discrete conformational properties of the complex among similar corticosteroid guests, as reflected by the complex mobility (mu(ep, AC)) or the relative change in complexation-inducedmobility (1-mu(ep, AC)/mu(ep, C)). This latter property is a unique feature of ACE reflective of hydrodynamic size selectivity involving bulky guests or substrates. 1H-NMR and computer molecular modeling provided complementary information regarding the relative orientation, conformation and overall molecular area of the complex. The coupling of thermodynamic (KB) and electrokinetic (mu(ep, AC)) factors associated with complex formation in buffered solutions that minimize the effects of extrinsic electrolytes serves to enhance enthalpy-driven molecular recognition processes by ACE.