CLEAs and sol-gel HNLs

The Sheldon group [87] prepared aquagels of different HNLs and compared them in the synthesis reaction of different cyanohydrins with the CLEAs and the free enzymes. The activity recovery for the aquagels and CLEAs measured by a photometric assay were quite low. Using the same loadings, the aquagels turned out to be much faster than the free enzyme. This confirms the underestimation of the recovery of activity by fast assays due to diffusion problems, as reported earlier [74, 75]. The stability and the catalytic performance of the immobilized HNLs are strongly influenced by the solvent, the immobilization method, and the enzyme source. 

The conversion of benzaldehyde by the encapsulated HNLs afforded mandelo-nitrile in 96-98% yield and 97-99% for all three enzymes. Free and entrapped HfoHNL catalyzed the conversion of hexanal with 94% , whereas ee-values of only 85 and 87% could be achieved with MeHNL and PaHNL preparations, respectively, limited by the intrinsic enantioselectivity of the enzymes (Table 9.4). Furthermore, the CLEAs from HfoHNL and PaHNL suffered from activity loss under the reaction conditions in contrast to MeHNL CLEA, indicating that the cross-linked aggregates from MeHNL are particularly robust and necessitate only traces of water to keep the catalytic activity [87]. 

The enzymatic synthesis of chiral cyanohydrins has reached a high stage of development. The different reaction systems give the possibility to convert a great 

15 Hickel, A. Graupner, M. Lehner, D. Hermetter, A. Clatter, O. Griengl, H. Enzyme Microh. Technol. 1997, 21, 361-366. 

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