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Open AccessArticle

Novel Insights into the Thioesterolytic Activity of N-Substituted Pyridinium-4-oximes

Department of Chemistry and Biochemistry, School of Medicine, University of Zagreb, Šalata 3, HR-10000 Zagreb, Croatia
Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia
Authors to whom correspondence should be addressed.
Dedicated to the recently deceased, long-time associate and mentor, esteemed Professor Nicoletta Burger.
Molecules 2020, 25(10), 2385;
Received: 21 April 2020 / Revised: 15 May 2020 / Accepted: 19 May 2020 / Published: 21 May 2020
The pyridinium oximes are known esterolytic agents, usually classified in the literature as catalysts, which mimic the catalytic mode of hydrolases. Herein, we combined kinetic and computational studies of the pyridinium-4-oxime-mediated acetylthiocholine (AcSCh+) hydrolysis to provide novel insights into their potential catalytic activity. The N-methyl- and N-benzylpyridinium-4-oximes have been tested as oximolytic agents toward the AcSCh+, while the newly synthesized O-acetyl-N-methylpyridinium-4-oxime iodide was employed for studying the consecutive hydrolytic reaction. The relevance of the AcSCh+ hydrolysis as a competitive reaction to AcSCh+ oximolysis was also investigated. The reactions were independently studied spectrophotometrically and rate constants, koxime, kw and kOH, were evaluated over a convenient pH-range at I = 0.1 M and 25 °C. The catalytic action of pyridinium-4-oximes comprises two successive stages, acetylation (oximolysis) and deacetylation stage (pyridinium-4-oxime-ester hydrolysis), the latter being crucial for understanding the whole catalytic cycle. The complete mechanism is presented by the free energy reaction profiles obtained with (CPCM)/M06–2X/6–311++G(2df,2pd)//(CPCM)/M06–2X/6–31+G(d) computational model. The comparison of the observed rates of AcSCh+ oximolytic cleavage and both competitive AcSCh+ and consecutive pyridinium-4-oxime-ester hydrolytic cleavage revealed that the pyridinium-4-oximes cannot be classified as non-enzyme catalyst of the AcSCh+ hydrolysis but as the very effective esterolytic agents. View Full-Text
Keywords: acetylthiocholine; pyridinium-4-oxime-ester; reaction mechanisms; energy profiles; computations acetylthiocholine; pyridinium-4-oxime-ester; reaction mechanisms; energy profiles; computations
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MDPI and ACS Style

Foretić, B.; Damjanović, V.; Vianello, R.; Picek, I. Novel Insights into the Thioesterolytic Activity of N-Substituted Pyridinium-4-oximes. Molecules 2020, 25, 2385.

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