32. Stereochemistry of the Inhibition of δ-Chymotrypsin with Optically Active Bicyclic Organophosphates: 31P-NMR Studies

Article abstract of DOI:10.1002/hlca.19970800208

The inhibition of δ-chymotrypsin with optically active, axially and equatorially substituted trans-3-(2,4-dini-trophenoxy)-2,4-dioxa-3λ ⁵-phosphabicyclo[4.4.0]decan-3-ones (= hexahydro-4H-1,3,2-benzodioxaphosphorin 3-oxides) was investigated. Their inhibitory power was determined by kinetic measurements, and the stereochemical course of the reaction of stoichiometric amounts of the enzyme and inhibitor was monitored with ³¹P-NMR spectroscopy at pH 7.8. The irreversible inhibitors show significant enantioselectivity (the (S_p)-enantiomer reacting faster) and yield diastereoisomeric, covalently phosphorylated derivatives of δ-chymotrypsin. ³¹P-NMR Spectroscopic studies of the inhibition by the axially substituted inhibitor revealed for the racemic (±)-2a first a resonance at -4.4 ppm and later, while inhibition proceeded, a second one at -4.5 ppm. The reaction with optically active (+)-2a showed only one signal at -4.4 ppm and its enantiomer (-)-2a only one signal at -4.5 ppm. Using the equatorially substituted racemic epimer (±)-2b, we observed the main resonance at -5.3 ppm and two minor ones at -4.4 and -4.5 ppm. The optically active compound (+)-2b showed two peaks at -4.5 and -5.3 ppm, whereas its antipode (-)-2b revealed two signals at -4.4 and -5.3 ppm. Comparing the ³¹P chemical shifts of the corresponding covalent phosphoserine derivatives 4a ( -5.7 ppm, axial) and 4b (-4.5ppm, equatorial) shows the inhibition with the axial compounds 2a to proceed via neat inversion of the configuration at the P-atom. whereas the equatorial epimers 2b with a higher conformational flexibility seem to follow a different stereochemical pathway which results in both inversion and retention.