engleski

Stereoselective Hydrolysis of Quaternary Quinuclidinium Benzoates Catalyzed by Butyrylcholinesterase

Four chiral, quaternary, N-methyl and N-benzyl derivatives of (R)- and (S)-quinuclidin-3-yl benzoates were synthesized and studied as substrates of horse serum butyrylcholinesterase (BChE). The kcat for the substrates decreased in order (R)-N-methyl > (R)-N-benzyl (2.3-fold slower) >> (S)-N-methyl (70.5-fold slower reaction), while for (S)-N-benzyl ester inhibition of the enzyme was observed. The kinetics of inhibition (Ka=3.3 microM) indicated that the binding to the catalytic site of BChE occurred. From the ratio of the kcat/KM values of both enantiomers, an enantiomeric excess of 95 % was calculated for N-methyl derivatives. Thus, BChE is suitable as an biocatalyst for the resolution of racemic quaternary quinuclidinium esters. In order to explain experimental data, combined quantum chemical (HF/3– 21G*) and semiempirical (PM3) calculations within the ONIOM scheme of the stable species in the acylation step were performed. Geometry optimizations were carried out for all benzoate esters for an assumed active site model of BChE. It was confirmed that hydrolysis is in an appreciable extent affected by a proper geometrical orientation of substrates at the choline subsite. Energies of the optimized systems were in a good agreement with experimental data.

Quaternary Quinuclidin-3-yl benzoates / Enzyme catalysis / Hydrolysis kinetics / Molecular modeling / ONIOM calculations

nije evidentirano