Thermodynamic Study of an Unusual Chiral Separation. Propranolol Enantiomers on an Immobilized Cellulase

Abstract

The thermodynamics of interaction of (R)- and (S)-propranolol between an acetic acid buffer (pH = 4.7 and 5.5) and the protein cellobiohydrolase I immobilized on silica gel was studied between 5 and 45 °C. The equilibrium data were fitted to a biLangmuir adsorption isotherm with excellent agreement. One of the two Langmuir contributions is the same for both enantiomers and accounts for the nonspecific interactions between these compounds and most sites on the surfaces (type-I, nonselective sites). It has a large saturation capacity. The second contribution accounts for the chiral selective interactions (type-II sites). It has a lower monolayer capacity than the first. The interaction enthalpy and entropy on type-I sites are −1.1 kcal/mol and +0.1 cal/(mol K), respectively. For type-II sites, they are −1.9 kcal/mol and −2.6 cal/(mol K), respectively, for (R)-propranolol and +1.6 kcal/mol and + 11.6 cal/(mol K), respectively, for (S)-propranolol at pH = 5.5. This explains why at this pH the retention time o...