Dependence on the Mobile-Phase pH of the Adsorption Behavior of Propranolol Enantiomers on a Cellulase Protein Used as the Chiral Selector

We reported previously on the unusual thermodynamic characteristics of the enantioselective interactions between the enantiomers of the β-blocker propranolol and the protein cellobiohydrolase I immobilized on silica. The adsorption of the more retained enantiomer, ( S )-propranolol, is endothermic while that of the ( R )-propranolol is exothermic. This causes a rapid increase of the selectivity factor with increasing temperature. In this work, we study the complex dependence of the selectivity factor on the pH of the solvent. We determined the equilibrium isotherms of ( R )- and ( S )-propranolol in a wide concentration range (0.25 μM to 1.1 mM) at six different mobile-phase pHs (4.7, 5.0, 5.2, 5.5, 5.7, and 6.0) and fitted the data obtained to the bi-Langmuir model. This gave the saturation capacity and the binding constant of the nonselective contribution for the two enantiomers. It also gave these parameters for the enantioselective contributions of each of them. The dependence of these parameters on the pH is discussed and interpreted in terms of the retention mechanism. Our conclusions are in excellent agreement with recent, independent results on the structure of the protein obtained by X-ray crystallography.