Stereoinversion of Enantiopure Secondary Alcohols Using a Single Enzymatic Approach

Stereoinversion refers to the predominant formation of one stereoisomer in a chemical reaction through either retention or inversion of configuration. The main objective of this proposal is the use of a single alcohol dehydrogenase to achieve stereoinversion of enantiopure secondary alcohols. This will be achieved through consecutive oxidation and reduction reactions catalyzed by mutant versions of Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase (TeSADH). This enzyme catalyzes the stereospecific oxidation of racemic alcohols and the enantioselective reduction of the corresponding ketones. Therefore, in this study, several previously reported TeSADH mutants will be evaluated in the aforementioned reactions. The conditions associated with the least stereoselective oxidation reaction, which ensure conversion of the undesired enantiomer of an alcohol to the corresponding ketone (i.e., low stereoselectivity, indicated by a low E value) and that of highest stereoselectivity in the ketone reduction are desired. Concentrations of acetone and 2-propanol co-substrates, in the oxidation and reduction reactions, respectively, will be varied to determine the most suitable conditions for non-stereospecific oxidation and highly stereoselective reductions. The non-stereospecific oxidation conditions will facilitate the oxidation of the undesired enantiomer of an alcohol to the corresponding ketone, using the proper volume of acetone as a co-substrate, in high yields. This process will be following by a highly stereoselective reduction reaction to yield a single enantiomeric alcohol, which will be accomplished by adding the proper amount of 2-propanol that ensures conversion of the intermediate ketone to the corresponding optically active alcohol (the opposite enantiomer to the starting material) in high yield and high enantioselectivity. Thus, stereoinversion of enantiopure secondary alcohols will be accomplished in two steps in a one-pot reaction by a single alcohol dehydrogenase. The proposed method is not only environmentally benign, but is also superior to other stereoinversion approaches that require non-selective and selective enzymes or two enzymes with opposite stereoselectivities. This method will provide a sustainable biocatalytic approach that utilizes a single enzyme to obtain pure enantiomeric alcohols from their counterparts in good yields and high enantioselectivities. This is very important to recycle the undesired enantiomer in kinetic resolution.