Selectivity Enhancement by Addition of Water-Miscible Organic Cosolvents

Selectivity Enhancement by Addition of Water-Miscible Organic Cosolvents 

MeOOC COOMe Organic cosolvent HOOC COOMe 

Finally, as an old example of kinetic resolution of racemic mixtures, mention must be made on the report of Kise and Tomiuchi on the significant effect of acetonitrile on the enantioselectivity of different proteases toward the kinetic resolution of aromatic amino acid ethyl esters (5-8). For instance, (l)-DOPA (8) was obtained with 99% ee in the presence of 90% v/v acetonitrile [9]. 

Organic cosolvent, v/v Relative rate of hydrolysis Enantiomeric excesses of (4) 

Several other examples have been reported in the literature and most of them have been already revievred (see, for instance, [11]). Hovrever, it must be mentioned that, of course, the selectivity enhancement via addition of vater-misdble organic cosolvents may not be taken for granted, as sometimes this approach may be unsuccessfij] [11a]. 

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CALB is an exceptionally robust protein which is deactivated only at 50-60°C, and thus also shows increased resistance towards organic solvents. In contrast to many other lipases, the enzyme appears to be rather rigid and does not show a pronounced effect of interfacial activation [430], which makes it an intermediate between an esterase and a lipase. This latter property is probably the reason why its selectivity could be predicted through computer modeling to a fair extent [431], and for the majority of substrates the Kazlauskas rule (Scheme 2.49) can be applied. In line with these properties of CALB, selectivity-enhancement by addition of water-miscible organic cosolvents such as t-butanol or acetone is possible - a technique which is rather common for esterases. All of these properties make CALB the most widely used lipase both in the hydrolysis [432-437] and synthesis of esters (Sect. 3.1.1). 

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