Abstract
The influence of Trp128‐substituted mutants of the hydroxynitrile lyase from Manihot esculenta (MeHNL) on the stereoselectivity of MeHNL‐catalyzed HCN additions to aldehydes with stereogenic centers, which yield the corresponding cyanohydrins, is described. In rac__‐2‐phenylpropionaldehyde (rac‐1) reactions, wild‐type (wtMeHNL) and all MeHNL Trp128 mutants are highly (S)‐selective toward the (R) enantiomer of__ rac__‐1; this results exclusively in (2__S__,3__R__)‐cyanohydrin ((2__S__,3__R__)‐2) with ≥96 %__ de__. The (S) enantiomer of__ rac__‐1, however, only reacts (S)‐selectively with wtMeHNL to give (2__S__,3__S__)‐2 with 80 %__ de__, whereas with Trp128 mutants, (R) selectivity increases with decreasing size of the amino acids exchanged. The MeHNL W128A mutant is exclusively (R)‐selective, resulting in (2__R__,3__S__)‐2 with 86 %__ de__. The reaction behavior of__ rac__‐phenylbutyraldehyde (rac‐5) is comparable with__ rac__‐1, which also inverts the stereoselectivity from (S) to (R) when the enzyme is exchanged from wtMeHNL to the W128A mutant. Stereogenic centers not adjacent to the aldehyde group, as in 7 and 9, do not influence the stereoselectivity of MeHNL catalysis, and (S) selectivity is observed in all cases. Stereoselectivity and inversion of stereoselectivity of MeHNL Trp128 mutant‐catalyzed cyanohydrin formation can be explained and rationalized with crystal‐structure‐based molecular modeling.__