Stereoelectronic requirements for optimal hydrogen-bond-catalyzed enolization |
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Authors: | Pápai Imre Hamza Andrea Pihko Petri M Wierenga Rik K |
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Affiliation: | Department of Theoretical Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, Pusztaszeri ut 59-67, H-1025 Budapest, Hungary. papai@chemres.hu |
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Abstract: | Protein crystallographic analysis of the active sites of enolizing enzymes and structural analysis of hydrogen-bonded carbonyl compounds in small molecule crystal structures, complemented by quantum chemical calculations on related model enolization reactions, suggest a new stereoelectronic model that accounts for the observed out-of-plane orientation of hydrogen-bond donors (HBDs) in the oxyanion holes of enolizing enzymes. The computational results reveal that the lone-pair directionality of HBDs characteristic for hydrogen-bonded carbonyls is reduced upon enolization, and the enolate displays almost no directional preference for hydrogen bonding. Positioning the HBDs perpendicular to the carbonyl plane induces strain in the catalyst-substrate complex, which is released upon enolization, resulting in more favorable kinetics and thermodynamics than the in-plane arrangement of HBDs. |
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Keywords: | catalyst design density functional theory enolization hydrogen bonds oxyanion hole |
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