Enhanced Binding and Conformational Selectivity in Affinity Capillary Electrophoresis Using a Water-Soluble Resorcin[4]Arene as Intrinsic Buffer and Electrokinetic Host

Abstract

<p> Affinity capillary electrophoresis (ACE) is a versatile technique for assessing non-covalent molecular interactions in free solution provided that there are significant changes in apparent analyte mobility as a result of specific complexation. The thermodynamics of receptor binding are vital for controlling the selectivity in molecular recognition, which are dependent on the electrolyte composition of solution. In addition, the conformational properties of the complex (e.g., size, shape) can also contribute a secondary influence on receptor selectivity that has been relatively unexplored in ACE to date. In this study, dynamic 1:1 host-guest inclusion complexation involving a anionic 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. The tetraethylsulphonate derivative of 2-methylresorcin[4]arene (TESMR) was first synthesized via an acid-catalyzed condensation reaction, which was then fully characterized in terms of its weak acidity (pKa), mobility, UV spectral and buffer capacity properties. TESMR solutions were demonstrated to have stable intrinsic buffer and ion transport properties at pH 7.5 even at low ionic strength. It was determined that over a 200 % enhancement in the apparent binding constant (KB) was realized by ACE when using TESMR as an intrinsic buffer at pH 7.5 relative to an extrinsic sodium phosphate buffer system, which was also confirmed by 1H-NMR. The coupling of thermodynamic (KB) and electrokinetic (μep, AC) factors associated with complex formation in buffered aqueous solutions that minimize the effects of extrinsic electrolytes serves to enhance enthalpy-driven molecular recognition processes by ACE.</p>