Biotransformations using lipase enzymes in organic solvents
Enzymatic transformations in organic solvents have received increasing attention over the past 10 years and lipases have become by far the most popular enzymes in this area.The initial aim of the research was to assess the effect of small modifications to the acyl donor in the transesterification reactions mediated by the Candida cylindracea lipase. 2.2.2-Trichloroethyl butyrate (TCEB) was used as a standard for the rate studies. In the event the acyl donor, trichloroethyl methoxyacetate (TCEMA), accelerated the transesterification reaction with hexan-1-ol by an order of magnitude over that with TCEB. This observation, and the absence of an acceleration with trichloroethyl methoxypropionate (and ethyl 2- fluorobutyrate over ethyl butyrate) suggested that the effect is due to the ~oxygen. A solvent activity profile indicated that the most hydrophobic solvents supported faster initial rates. This was attributed to the ability of the hydrophilic solvents to strip the hydrated water from the enzyme surface thus deactivating it. The switch to organic solvents allowed a wider temperature range to be studied. For the reactions between heptan-2-ol and TCEMA the reaction could be conducted in the temperature range -23 C to 80 C. It was of interest to assess how the alcohol moiety effected the transesterification reaction. A series of alcohols were presented to the enzyme and a pattern emerged with substrates containing an acetylene functionality being processed faster than those with a vinyl group, which were faster than those containing a methyl group (all other groups being the same). A series of heterocyclic alcohols were presented to the enzyme and it was observed that the order of reaction was thiophene > furan > pyridyl. The secondary alcohols in this series, 2-thiopheneethan-1-ol and o pyridylethan-1-ol, were resolved at various temperatures from -1soc to sooc with no variation in. enantioselectivity. These are the first resolutions to be accomplished at temperatures below zero degrees.