Influence of the solute hydrophobicity on the enantioselective adsorption of β-blockers on a cellulase protein used as the chiral selector

Adsorption isotherm data were acquired at different eluent pH values for the enantiomers of several β-blockers on cellobiohydrolase I on silica gel. They fit well to the biLangmuir model, allowing the determination of the equilibrium constants and the monolayer capacities for chiral and nonselective adsorption. The adsorption of the S-enantiomers (eluted second) is exothermic at low pH, endothermic at high pH, and athermal in a narrow pH range depending on the β-blocker. This transition pH range is lower for S-alprenolol than for the more hydrophobic S-propranolol, although their endothermic adsorption originates from hydrophobic interactions. This surprising observation is explained by the relative values of the isotherm coefficients. S-Alprenolol seems to have a more pronounced endothermic behavior than S-propranolol because the nonselective interactions of both compounds with the stationary phase are exothermic but their contribution to retention, relative to that of the endothermic chiral interactions, is less important for alprenolol. The order of increasing energy of the chiral interactions is the same as that of hydrophobicity, propranolol>alprenolol>metoprolol.