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New Insight into the Mechanism of NADH Model Oxidation by Metal Ions in Nonalkaline Media
The Journal of Physical Chemistry B ( IF 3.3 ) Pub Date : 2018-06-28 , DOI: 10.1021/acs.jpcb.8b03453
Jin-Dong Yang 1 , Bao-Long Chen , Xiao-Qing Zhu
Affiliation  

For a long time, it has been controversial that the three-step (e–H+–e) or two-step (e–H) mechanism was used for the oxidation of nicotinamide adenine dinucleotide coenzyme (NADH) and its models by metal ions in nonalkaline media. The latter mechanism has been accepted by the majority of researchers. In this work, 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1-phenyl-l,4-dihydronicotinamide are used as NADH models and ferrocenium (Fc+) metal ion as an electron acceptor. The kinetics for oxidation of the NADH models by Fc+ in pure acetonitrile was monitored by using UV–vis absorption and a quadratic relationship between kobs and the concentrations of NADH models was found for the first time. The rate expression of the reactions developed according to the three-step mechanism is quite consistent with the quadratic curves. The rate constants, thermodynamic driving forces, and kinetic isotope effects of each elementary step for the reactions were estimated. All results supported the three-step mechanism. The intrinsic kinetic barriers of the proton transfer from BNAH+• to BNAH and the hydrogen-atom transfer from BNAH+• to BNAH+• were estimated by using Zhu equation; the results showed that the former is 11.8 kcal/mol and the latter is larger than 24.3 kcal/mol. It is the large intrinsic kinetic barrier of the hydrogen-atom transfer that makes the reactions choose the three-step rather than two-step mechanism. Further investigation of the factors affecting the intrinsic kinetic barrier of chemical reactions indicated that the large intrinsic kinetic barrier of the hydrogen-atom transfer originated from the repulsion of positive charges between BNAH+• and BNAH+•. The greatest contribution of this work is the discovery of the quadratic dependence of kobs on the concentrations of the NADH models, which is inconsistent with the conventional viewpoint of the “two-step mechanism” on the oxidation of NADH and its models by metal ions in the nonalkaline media.

中文翻译:

非碱性介质中金属离子氧化NADH模型的机理的新见解

长期以来,使用三步法(e–H + –e)或两步法(e–H )氧化烟酰胺腺嘌呤二核苷酸辅酶(NADH)及其模型的争议一直存在争议非碱性介质中的金属离子。后一种机制已为大多数研究人员所接受。在这项工作中,将1-苄基-1,4-二氢烟酰胺(BNAH)和1-苯基-1,4-二氢烟酰胺用作NADH模型,并将二茂铁(Fc +)金属离子用作电子受体。通过UV-vis吸收和k obs之间的二次关系来监测纯乙腈中Fc +氧化NADH模型的动力学。并首次发现了NADH模型的浓度。根据三步机理进行的反应速率表达与二次曲线非常吻合。估计反应的每个基本步骤的速率常数,热力学驱动力和动力学同位素效应。所有结果都支持三步机制。从BNAH质子转移的本征动力学障碍+•到BNAH和从BNAH氢原子转移+•到BNAH +•用Zhu方程估算 结果表明,前者为11.8 kcal / mol,后者大于24.3 kcal / mol。正是氢原子转移的巨大内在动力学障碍使反应选择了三步机理,而不是两步机理。进一步研究影响化学反应固有动力学势垒的因素表明,氢原子转移的较大固有动力学势垒源自BNAH +•和BNAH +•之间正电荷的排斥。这项工作的最大贡献是发现了k obs的二次相关性。 关于NADH模型的浓度,这与在非碱性介质中金属离子氧化NADH及其模型的“两步机理”的传统观点不一致。
更新日期:2018-06-30
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