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 k(cat) for the substrates decreased in the order (R)-N-methyl > (R)-N-benzyl (2.3-fold slower) much greater than (S)-N-methyl (70.5-fold slower reaction), while for the (S)-N-benzyl ester inhibition of the enzyme was observed. The kinetics of inhibition (K-a = 3.3 mum) indicated that binding to the catalytic site of BChE occurred. From the ratio of the k(cat)/K-M values of both enantiomers an enantiomeric excess of 95% was calculated for N-methyl derivatives. Thus, BChE is suitable as a biocatalyst for the resolution of racemic quaternary quinu-clidinium esters. In order to explain the 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 affected to an appreciable extent by a proper geometrical orientation of substrates at the choline subsite. The energies of the optimized systems were in good agreement with the experimental data. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).