Molecular recognition between cyclodextrins and beta -lactam antibiotics
Abstract
Penicillinase is a key enzyme responsible for bacterial resistance against β-lactam antibiotics. It specifically catalyzes the hydrolysis of penicillin into penicilloic acids. The hypothesis that β-CD may serve as the biomimetic model of penicillinase was first proposed by Schwartz. This prompted us to study the molecular recognition between β-CD and penicillin since the insight of this specific molecular recognition may contribute not only to the understanding of the natural enzyme, but also to the development of novel artificial enzymes. A systematic study of the molecular recognition involved in the catalytic hydrolysis of penicillin by cyclodextrin has thus been undertaken. Detailed kinetic studies, 1H NMR and FAB-MS analyses have shown that β-CD catalyzed the hydrolysis of penicillin V via a Michaelis-Menten complex. Molecular modeling study indicated that the most stable complex was formed through the recognition between the β-face of penicillin V and the secondary hydroxyl face of β-CD. Furthermore, the intermolecular distances between β-CD and penicillin in the global minimum-energy conformation indicated that 2-OH of β-CD was most likely to be the nucleophile to attack β-lactam to form an intermediate. This presumption was supported by the isolation and characterization of O2A-penicillin-β-CD tetrahedral intermediate and O2A-penicilloyl-β-CD ester intermediate from the reaction mixture. Based on these information, the plausible catalytic mechanism for the β-CD-catalyzed hydrolysis of penicillin was envisioned. In an attempt to design a more effective enzyme model, (2A R ),(3AS )-3 A-amino-3A-deoxy-β-CD along with some related compounds have been synthesized. Detailed kinetic studies have revealed that (2AR ),(3AS )-3A-amino-3A-deoxy-β-CD has a much improved catalytic activity compared with natural β-CD at neutral pH. This specific catalytic effect was applied to the kinetic resolution of phenethicillin diastereomers. It was found that β-CD selectively hydrolyzed the S-phenethicillin. This selectivity exemplifies an advantage of artificial enzymes over the natural enzymes since penicillinase cannot discriminate the diastereomers. Asymmetric synthesis of penicillin R-sulfoxide was also investigated in the presence of CDs and their derivatives. In the presence of α-CD, oxidation of penicillin V using m-chloroperbenzoic acid as an oxidizing agent gave the best yield of R-sulfoxide (R-/S-sulfoxide = 44/56).
Degree
Ph.D.
Advisors
Chang, Purdue University.
Subject Area
Analytical chemistry|Pharmacology|Organic chemistry|Molecular biology
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