Steric Effects on the Primary Isotope Dependence of Secondary Kinetic Isotope Effects in Hydride Transfer Reactions in Solution: Caused by the Isotopically Different Tunneling Ready State Conformations?

Binita Maharjan; Mahdi Raghibi Boroujeni; Jonathan Lefton; Ormacinda R. White; Mortezaali Razzaghi; Blake A. Hammann; Mortaza Derakhshani-Molayousefi; James E. Eilers; Yun Lu

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Steric Effects on the Primary Isotope Dependence of Secondary Kinetic Isotope Effects in Hydride Transfer Reactions in Solution: Caused by the Isotopically Different Tunneling Ready State Conformations?

机译：溶液中氢化物转移反应中的二级动力学同位素效应对一级同位素依赖性的立体效应：由同位素不同的隧穿就绪态构型引起？

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The observed 1° isotope effect on 2° KIEs in H-transfer reactions has recently been explained on the basis of a H-tunneling mechanism that uses the concept that the tunneling of a heavier isotope requires a shorter donor-acceptor distance (DAD) than that of a lighter isotope. The shorter DAD in D-tunneling, as compared to H-tunneling, could bring about significant spatial crowding effect that stiffens the 2° H/D vibrations, thus decreasing the 2° KIE. This leads to a new physical organic research direction that examines how structure affects the 1° isotope dependence of 2° KIEs and how this dependence provides information about the structure of the tunneling ready states (TRSs). The hypothesis is that H- and D-tunneling have TRS structures which have different DADs, and pronounced 1° isotope effect on 2° KIEs should be observed in tunneling systems that are sterically hindered. This paper investigates the hypothesis by determining the 1° isotope effect on a- and β-2° KIEs for hydride transfer reactions from various hydride donors to different carbocationic hydride acceptors in solution. The systems were designed to include the interactions of the steric groups and the targeted 2° H/D's in the TRSs. The results substantiate our hypothesis, and they are not consistent with the traditional model of H-tunneling and l°/2° H coupled motions that has been widely used to explain the 1° isotope dependence of 2° KIEs in the enzyme-catalyzed H-transfer reactions. The behaviors of the 1° isotope dependence of 2° KIEs in solution are compared to those with alcohol dehydrogenases, and sources of the observed "puzzling" 2° KIE behaviors in these enzymes are discussed using the concept of the isotopically different TRS conformations.

机译：最近，已经基于H隧穿机理解释了在H转移反应中观察到的1°同位素对2°KIE的影响，该机理采用的概念是，较重的同位素的隧穿需要的供体-受体距离（DAD）短于较轻的同位素与H型隧道相比，D型隧道中较短的DAD可能带来显着的空间拥挤效应，从而加剧2°H / D振动，从而降低2°KIE。这导致了一个新的物理有机研究方向，该方向研究了结构如何影响2°KIE的1°同位素依赖性以及这种依赖性如何提供有关隧穿就绪态（TRS）结构的信息。假设是，H隧道和D隧道的TRS结构具有不同的DAD，在空间受阻的隧道系统中应观察到2°KIE的明显1°同位素效应。本文通过确定溶液中从各种氢化物供体到不同碳负离子氢化物受体的氢化物转移反应，对a-和β-2°KIE的1°同位素效应进行研究，从而研究了这一假设。该系统设计为包括TRS中空间位与目标2°H / D的相互作用。结果证实了我们的假设，并且与传统的H隧穿和1°/ 2°H耦合运动模型不一致，该模型已广泛用于解释酶催化H中2°KIE的1°同位素依赖性。 -转移反应。比较了溶液中2°KIE与乙醇脱氢酶的1°同位素依赖性行为，并使用同位素不同的TRS构象的概念讨论了在这些酶中观察到的“令人费解”的2°KIE行为的来源。

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《Journal of the American Chemical Society》 |2015年第20期|6653-6661|共9页
作者
Binita Maharjan; Mahdi Raghibi Boroujeni; Jonathan Lefton; Ormacinda R. White; Mortezaali Razzaghi; Blake A. Hammann; Mortaza Derakhshani-Molayousefi; James E. Eilers; Yun Lu;
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作者单位

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

Department of Chemistry, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026, United States;

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收录信息美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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入库时间 2022-08-18 03:09:41

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Steric Effects on the Primary Isotope Dependence of Secondary Kinetic Isotope Effects in Hydride Transfer Reactions in Solution: Caused by the Isotopically Different Tunneling Ready State Conformations?

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