Influence of water activity on the enantioselective esterification of (R,S)-ibuprofen by Candida antarctica lipase B in solventless media

The lipase-catalyzed enantioselective esterification of ibuprofen has been studied in a media, composed only of substrates. When racemic ibuprofen is used, the alcohol-chain length affects the esterification rates of individual enantiomers, but it does not affect the enantioselectivity. Water activity affects the esterification rates of (R)- and (S)-ibuprofen differently, leading to higher enantioselectivity at lower water activities. Experiments were also conducted at various (R)- to (S)-ibuprofen ratios. It appears that the esterification rate of (R)-ibuprofen is always proportional to its concentration, whereas at low water activity the esterification rate of (S)-ibuprofen shows a saturation at higher concentrations. Other 2-phenyl carboxylic acids were studied, and the increase in apparent enantioselectivity at low-water activity was not observed for the molecules tested.