Kinetic deuterium isotope effect in the reaction of methyl iodide with cyanide ion in aqueous solution from 0 to 40.deg.

Willi, A. V.; Won, Chong Min

doi:10.1021/ja01024a010

Cited by 8 publications

(5 citation statements)

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“…The effect was additionally sensitive to the type of quinol/hydrogen donors . Although a definitive mechanism has not been assigned for this and other − unexpected ‘activation' isotope effects, we postulate that this rare effect might reflect an unusual, but not unprecedented, reaction coordinate for UQH 2 oxidation relative to other quinol oxidation reactions based on its observation in two different reaction systems. Using this approach we can test whether the unique activation isotope effect is indeed consistent with the one-electron oxidation of UQH 2 via a net hydrogen-transfer mechanism due to the improbability of both reactions sharing exactly the same reaction coordinate, thus discriminating between Type I and II mechanisms for the cyt bc 1 complex.…”

Section: Introductionmentioning

confidence: 78%

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex

2005

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Current competing models for the two-electron oxidation of quinol (QH2) at the cytochrome bc1 complex and related complexes impose distinct requirements for the reaction intermediate. At present, the intermediate species of the enzymatic oxidation process have not been observed or characterized, probably due to their transient nature. Here, we use a biomimetic oxidant, excited-state Ru(bpy)2(pbim)+ (bpy=2,2'-dipyridyl, pbim=2-(2-pyridyl)benzimidazolate) in an aprotic medium to probe the oxidation of the ubiquinol analogue, 2,3-dimethoxy-5-methyl-1,4-benzoquinol (UQH2-0), and the plastoquinol analogue, trimethyl-1,4-benzoquinol (TMQH2-0), using time-resolved and steady-state spectroscopic techniques. Despite its simplicity, this system qualitatively reproduces key features observed during ubiquinol oxidation by the mitochondrial cytochrome bc1 complex. Comparison of isotope-dependent activation properties in the native and synthetic systems as well as analysis of the time-resolved direct-detection electron paramagnetic resonance signals in the synthetic system allows us to conclude that (1) the initial and rate-limiting step in quinol oxidation, both in the biological and biomimetic systems, involves electron and proton transfer, probably via a proton-coupled electron-transfer mechanism, (2) a neutral semiquinone intermediate is formed in the biomimetic system, and (3) oxidation of the QH*/QH2 couple for UQH2-0, but not TMQH2-0, exhibits an unusual and unexpected primary deuterium kinetic isotope effect on its Arrhenius activation energy (DeltaGTS), where DeltaGTS for the protiated form is larger than that for the deuterated form. The same behavior is observed during steady-state turnover of the cyt bc1 complex using ubiquinol, but not plastoquinol, as a substrate, leading to the conclusion that similar chemical pathways are involved in both systems. The synthetic system is an unambiguous n=1 electron acceptor, and it is thus inferred that sequential oxidation of ubiquinol (by two sequential n=1 processes) is more rapid than a truly concerted (n=2) oxidation in the cyt bc1 complex.

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Section: Introductionmentioning

confidence: 78%

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex

2005

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“…38 The Arrhenius parameters of the KIE are also sensitive to the C-H stretching force constant. 33 The analysis presented in the previous section provides an ab initio test of the conventional model for kH/kD values and whether these result primarily from the out-of-plane bend (i.e., the umbrella mode of the transition state), the H-C-H bends, or the C-H stretch. The CH3 deformations (including the H-C-H bends and the umbrella mode) are modes 4-8 at the n = 0 transition state, 7-11 at the n = 1 transition state, 10-14 at the n = 2 transition state, and 4-8 in CH3C1.…”

Section: Discussionmentioning

confidence: 99%

Solvent and secondary kinetic isotope effects for the microhydrated SN2 reaction of Cl-(H2O)n with CH3Cl

1991

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“…This has been interpreted in zero-point energy terms, assuming that the A H + isotope effect dominates that of A S f , which is true for several SN2 reactions (12)(13)(14). The argument is that a tight SN2 transition state, such as that for a methyl compound leads to an increase in the C-H bending frequencies during activation, which in turn leads to a faster rate for the deuterium compound than for the protium compound (15).…”

Section: -Deuterium Effectsmentioning

confidence: 99%

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

Leffek¹,

Matheson²

1972

Can. J. Chem.

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A survey of kinetic, secondary deuterium isotope effects, for a, ,!I, and y deuterated alkyl halides reacting with pyridine in nitrobenzene solvent has been made. a-Deuterium effects have been measured for eight compounds, P-deuterium effects for four compounds, and one rate ratio for y-deuteration is reported. The possible errors in the rate ratios for b-deuterated compounds, resulting from the elimination side reaction have been determined. The results are discussed in terms of transition state structure.Une etude sur la cinetique des effets isotopiques secondaires du deuterium a kt6 effectuee sur les halogenures d'alkyle a, /I, et y deuteres qui reagissent avec la pyridine dans le nitrobenzine. Les effets du deuterium en a ont ete mesures au moyen de huit composes, les effets du deuterium en au moyen de quatre composes, et on rapporte un rapport de vitesse pour la deuteration en y. Leserreurs possibles dans les rapports de vitesse pour les composes ,!I-deuterts, et qui resultent d'une reaction annexe d'elimination, ont ete determinees. Les resultats sont discutes en fonction de la structure de I'etat de transition.Canadian Journal of Chemistry, 50,986 (1972) Introduction Although many secondary deuterium isotope effects have been measured for nucleophilic substitutions at saturated carbon, systematic studies involving changes in the leaving group, the alkyl group, and the position of deuteration are not common. Robertson and co-workers (1-4) have provided the most complete set of data for sclvolysis reactions in water of alkyl halides and sulfonates, and Brown and McDonald (5) have reported a series of measurements on various substituted deuterated pyridines reacting with alkyl iodides in nitrobenzene solvent. This paper presents a set of isotopic rate ratios for the reaction of a single nucleophile pyridine, with various deuterated alkyl compounds in the same solvent, nitrobenzene.

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Kinetic deuterium isotope effect in the reaction of methyl iodide with cyanide ion in aqueous solution from 0 to 40.deg.

Cited by 8 publications

References 1 publication

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex

Solvent and secondary kinetic isotope effects for the microhydrated SN2 reaction of Cl-(H2O)n with CH3Cl

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

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Kinetic deuterium isotope effect in the reaction of methyl iodide with cyanide ion in aqueous solution from 0 to 40.deg.

Cited by 8 publications

References 1 publication

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc1Complex

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc1Complex

Solvent and secondary kinetic isotope effects for the microhydrated SN2 reaction of Cl-(H2O)n with CH3Cl

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

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Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc₁Complex