Thermodynamic and kinetic aspects on water vs. organic solvent as reaction media in the enzyme-catalysed reduction of ketones

The stereoselective reduction of ketones catalysed by alcohol dehydrogenase from Thermoanaerobium brockii was studied in different reaction media, hexane at controlled water activities, hexane with 2.5% water (biphasic) and pure water. The reactions were studied in the temperature range from -1 to 50 degrees C. Increasing the water activity from 0.53 to 0.97 increased the reaction rate 16-fold. The rate was further enhanced in hexane when exceeding the water solubility and in pure water the rates were even higher. This was general for all ketones studied. At controlled water activity the entropy of activation (Delta S double dagger) was the dominating factor. Large negative Delta S double dagger values caused low reaction rates at low a(w). When increasing the carbon chain length of the substrate, for reactions in hexane, the decrease of reaction rate was mainly due to a decrease in Delta S double dagger. In the comparison between hexane and pure water, Delta G double dagger values were higher in hexane due to higher Delta H double dagger values. The enantioselectivity (E value) increased from 2.6 at water activity 0.53 to 4.6 at water activity 0.97. Changing media from hexane (2.5%, v/v water) to pure water was not affecting the enantioselectivity or the specificity for different ketones. (C) 1999 Published by Elsevier Science B.V. All rights reserved.