Colossal kinetic isotope effects in proton-coupled electron transfer

Abstract

P roton-coupled electron transfer (PCET) reactions are at the heart of important biological processes, including photosynthesis and respiration (1-4). These reactions avoid high-energy intermediates through the concerted transfer of an electron and a proton. PCET is distinguished from H-atom transfer (HAT) in that the transferring electron and proton come from different sites in the reducing agent rather than from a single chemical bond (5-10). Nuclear quantum effects often play an important role, as evidenced experimentally by the kinetic isotope effect (KIE), the ratio of rate constants for the hydrogen and deuterium forms of the reducing agent. Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13-15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).In earlier work, we described a PCET reaction with a k 1 (H 2 O)͞k 1 (D 2 O) KIE of 175 at room temperature based on the reduction of benzoquinone (Q) by an osmium complex containing a phosphorus-hydrogen bond (18). We report here values of 198 and 455 for PCET reactions from sulfur and nitrogenhydrogen bonds based on an extended analysis of reactions reported earlier in preliminary accounts. 3.0 ϫ 10 Ϫ4 M to 8.0 ϫ 10 Ϫ2 M. The temperature of solutions during the kinetic studies was maintained to within Ϯ0.1°C with use of Lauda RM6 and Thermo Neslab RTE 7 circulating water baths and monitored with an Omega HH-51 thermocouple probe. All rate constants cited in this work are reported as the averages of at least three or more independent experiments.Cyclic voltammetric experiments were measured with the use of Princeton Applied Research (PAR) models 263A and 273 potentiostats, and bulk electrolyses were performed with a PAR model 173 potentiostat͞galvanostat. All measurements were conducted in a three-compartment cell in 1:1 (vol͞vol) CH 3 CN͞ H 2 O with 1.0 M NH 4 PF 6 as the supporting electrolyte. A glassy carbon working electrode was used for aqueous measurements. All potentials are referenced to the saturated sodium chloride͞ calomel electrode (SSCE, 0.236 V vs. the normal hydrogen electrode) at room temperature and are uncorrected for junction potentials. In all cases, the auxiliary electrode was a platinum wire. The solution in the working compartment was deoxygenated by N 2 or argon bubbling. All redox couple potentials are the average of four independent measurements and are Ϯ2 mV. Results and DiscussionThe kinetics of the reactions shown in Eqs. 1-3 were studied in 1:1 (vol͞vol) CH 3 CN͞H 2 O mixtures at 25.0 Ϯ 0.1°C in 1.0 M NH 4 PF 6 ͞HPF 6 mixtures. The details of the reaction in Eq. 2 with phosphorus as the proton-donor atom were reported in ref. 18, and preliminary accounts with S and N were reported in refs. 19 and 20. The structures of the Os(V) nitrogen-based electronproton acceptor and Os(IV) sulfur-ba...

Facile hydrogen atom transfer to iron(iii) imido radical complexes supported by a dianionic pentadentate ligand

“…Using a more accurate method, Meyer et al report a “colossal” k H / k D of 439 ± 8 in the transfer of a hydrogen atom from N to a substrate in an osmium amide/imide system. 54 Thus, such reactions do feature very large KIE, and while the value obtained for the conversion of 3·Ar to 2·Ar here likely has a large error associated with it, the key point is that it is much larger than a conventional KIE. We calculate a k H / k D of 5.82 by substituting D for H in the transition state calculation; clearly, the experimental value is much greater than that and supports a HAT mechanism with a substantial tunneling component during the transfer of the hydrogen atom.…”

Dependence of Biocatalysis on D/H Ratio: Possible Fundamental Differences for High-Level Biological Taxons

“…Mass spectra were processed using the Flex Analysis 2.4 software (Bruker Daltonics). Smoothing by the Savitzky-Golay algorithm [ 71 ] (width 0.1 m / z , 1 cycle) and subtraction of the baseline according to the algorithm were applied to the mass spectra, Convex Hull [ 16 ]. The following peak detection parameters were used: peak detection algorithm—SNAP, signal-to-noise ratio—6, spectrum quality threshold—100.…”

“…They play one of the main roles in determining the structures of macromolecular chains and intracellular processes. Using different biological and chemical substances as an example, it was studied that different kinetic isotope effects are observed in deuterium-depleted and deuterated waters [ 16 ]. This effect is proportional to the concentration of deuterium in the medium and is especially pronounced at lower concentrations compared to natural ones [ 17 , 18 ].…”

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

“…KIE of 13,000 was observed in the PCET reaction of the isomerization of 2,4,6-tri-tert-butylphenyl at low temperature [61]. In PCET reduction of benzoquinone to hydroquinone by various osmium complexes "colossal" KIE-s from 180 to 460 were observed [62,63]. Moreover, in PCET reaction catalysed by enzyme soybean lipoxygenase (SLO), wild type KIE of~80 [64] increased to the values from 553 to 760 in the case of various mutants [7,41,65,66].…”