We report a combined quantum methanical/molecular mechanical (QM/MM) study of the effect of mutations of the Phe434 residue in the active site of cyclohexanone monooxygenase (CHMO) on its enantioselectivity toward 4-hydroxycyclohexanone. In terms of our previously established model of the enzymatic Baeyer-Villiger reaction, enantioselectivity is governed by the preference toward the equatorial ((S) selectivity) or axial ((R) selectivity) conformation of the substituent at the C4 carbon atom of the cyclohexanone ring in the Criegee intermediate and the subsequent rate limiting transition state for migration (TS2). We assess the enantiopreference by locating all relevant TS2 structures at the QM/MM level In the wild-type enzyme we find that the axial conformation is energetically slightly more stable, thus leading to a small excess of (R) product In the Phe434Ser mutant, there, is a hydrogen bond between the serine side chain and the equatorial substrate hydroxyl group that is retained during the Whole. reaction, and hence there is pronounced. reverse (S)-enantioselectivity. Another mutant, Phe434Ile, is. shown to preserve and enhance. the (R) selectivity these findings are in accordance with experiment The. QM/MM calculations allow us to explain the effect of point mutations on CHMO: enantioselectivity for the first time at the molecular level by an analysis of the specific interactions between substrate and active site environment in the TS2 structures that satisfy the basic stereoelectronic requirement of anti-periplanarity for the migrating sigma-bond.