Hydroxynitrile lyase-catalyzed synthesis of cyanohydrins in organic solvents - Parameters influencing activity and enantiospecificity

(S)-Hydroxynitrile lyases from Hevea brasiliensis, Manihot esculenta, and S orghum bicolor and (R)-hydroxynitrile lyase from Prunus amygdalus have been used as biocatalyst for the enantiospecific addition of hydrogen cyanide t o aldehydes in organic solvents. The effects of the reaction parameters on the enzymatic reaction rate and product enantiomeric excess (e.e.) are pres ented. The reaction rate increased with the solvent hydrophobicity but high ly hydrophobic solvents were not adapted to high hydrogen cyanide concentra tions and provoked loss of activity and product e.e. In the synthesis of 3- phenylpropionaldehyde cyanohydrin catalyzed by (S)-hydroxynitrile lyases fr om H. brasiliensis, an e.e. value of 88 +/- 1% was obtained under optimized reaction conditions. Lower enantiomeric excess values were obtained under conditions where the enzyme was inactivated: high hydrogen cyanide concentr ation, high solvent log P, low enzyme loading. At lower temperature (down t o -5 degrees C) the e.e. was increased for all four enzymes used. Enzymes f rom different sources used under identical optimized reaction conditions we re found to yield cyanohydrins with very different enantiopurities. This in trinsic enantiospecificity is not an effect of spontaneous reactions indepe ndent of the enzyme. (C) 1999 Elsevier Science Inc. All rights reserved.