Stereochemistry of the inhibition of δ-chymotrypsin with optically active cis-decaline-type organophosphates:: 31P-NMR studies

Investigation of the inhibition of delta-chymotrypsin with the four novel, optically active, axially and equatorially substituted cis-3-(2,4-dinitrophenoxy)-2,4-dioxa-3 lambda(5)-phosphabicyclo[4.4.0]decan-3-ones (= 3-(2,4-dinitrophenoxy)hexahydro-4H-1,3,2-benzodioxaphosphorin 2-oxides) showed only the equatorially substituted enantiomer (-)-4b to be an irreversible inhibitor of the enzyme. P-31-NMR Spectroscopic monitoring of the inhibition of stoichiometric amounts of the enzyme and (-)-4b at pH 7.8 revealed a quickly rising resonance at - 2.49 ppm assigned to the hydrolysis product 8 and later, while inhibition proceeded, a second one at - 4.08 ppm, attributed to the delta-chymotrypsin adduct 7 (Scheme 3). Comparison of the latter signal with the P-31-NMR chemical shifts of the covalent phosphoserine model compounds (-)-6a (-5.67 ppm, axial substitution) and (+)-6b (-4.02 ppm, equatorial substitution) suggests that the inhibition proceeded via near retention of the configuration at the P-atom of (-)-4b yielding the equatorially substituted covalent Ser(195) adduct 7.