On the Relationship Between Enzymatic Enantioselectivity in Organic Solvents and Enzyme Flexibility

The enantioselectivities of subtilisin Carlsberg and α-chymotrypsin suspended in seven anhydrous solvents were determined in the acylation of sec-phenethyl alcohol with vinyl butyrate. In every case, both a dry lyophilized enzyme and one hydrated by a prior exposure to a humid environment were used as asymmetric catalysts in a given solvent. This hydration was found to boost the water content of the enzyme which, in turn, is known to enhance the enzyme's conformational flexibility. These data together resulted in the following correlation: as enzyme flexibility increases, the enantioselectivity decreases or remains the same. This relationship is opposite to that recently claimed for subtilisin by Broos et al. (Broos, J., Visser, A.J.W.G., Engbersen, J.F.J., Verboom, M., van Hock, A. and Reinhoudt, D.N. (1995) J. Am. Chem. Soc., 117, 12657–12663). The basis for their conclusion is critically analyzed herein and found to be questionable.