Fundamental aspects of electron transfer: experimental verification of vibronically coupled electron tunneling.

Potasek, M J; Hopfield, J. J.

doi:10.1073/pnas.74.9.3817

Cited by 39 publications

(9 citation statements)

References 15 publications

(14 reference statements)

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“…36 Alternatively, isotopic substitution of the protein could affect the rate or efficiency of reductive electron transfer from aromatic side chains to FMN if protein vibrations mediate the electron transfer. 37,38 In either case, the similar fluorescence of free Fl and Fh FMN ( Figure 6) show that the protein must play a role in the quenching process. Both mechanisms also violate the BornOppenheimer approximation.…”

Section: Resultsmentioning

confidence: 99%

Untangling Heavy Protein and Cofactor Isotope Effects on Enzyme-Catalyzed Hydride Transfer

et al. 2016

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'Heavy' (isotopically-labeled) enzyme isotope effects offer a direct experimental probe of the role of protein vibrations on enzyme-catalyzed reactions. Here we have developed a strategy to generate isotopologues of the flavoenzyme pentaerythritol tetranitrate reductase (PETNR) where the protein and/or intrinsic flavin mononucleotide (FMN) cofactor are isotopically labeled with 2 H, 15 N and 13 C. Both the protein and cofactor contribute to the enzyme isotope effect on the reductive hydride transfer reaction, but their contributions are not additive and may partially cancel each other out. However, the isotope effect specifically arising from the FMN suggests that vibrations local to the active site play a role in the hydride transfer chemistry, while the protein-only 'heavy enzyme' effect demonstrates that protein vibrations contribute to catalysis in PETNR. In all cases, enthalpy-entropy compensation plays a major role in minimizing the magnitude of 'heavy enzyme' isotope effects. Fluorescence lifetime measurements of the intrinsic flavin mononucleotide show marked differences between 'light' and 'heavy' enzymes on the ns-ps timescale, suggesting relevant timescale(s) for those vibrations implicated in the 'heavy enzyme' isotope effect on the PETNR reaction.

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Section: Resultsmentioning

confidence: 99%

Untangling Heavy Protein and Cofactor Isotope Effects on Enzyme-Catalyzed Hydride Transfer

et al. 2016

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“…The fastest component is instrument-limited (instrument response function ≈200 fs), whilst the slowest is 14 ps, which may be assigned to the lifetime of the relaxed, "thermalised", lowest CT state. 1,[78][79][80] The effect of excess excitation energy on excited state dynamics was further investigated using different pump wavelengths. 6 Expanded fingerprint region of the TRIR data for 1 (Fig.…”

Section: Early Time Dynamics Observed In 1 and 2 With Trirmentioning

confidence: 99%

“…76,77 Vibrational-electronic (vibronic) interactions in these systems are therefore expected to play an important role in ET rates. 1,[78][79][80] The effect of excess excitation energy on excited state dynamics was further investigated using different pump wavelengths. Fig.…”

Section: Early Time Dynamics Observed In 1 and 2 With Trirmentioning

confidence: 99%

Electron transfer dynamics and excited state branching in a charge-transfer platinum(ii) donor–bridge-acceptor assembly

et al. 2014

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A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ∼50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ∼1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 μs and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state.

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“…It is tempting to relate such a rate to the distance between the two groups pertaining to two distinct structural domains, the flavodehydrogenase moiety and the cytochrome b2 core (Gervais et al, 1983). From electrontunnelling theories (De Vault, 1980Vault, , 1984 an edge-to-edge distance of 0.95 nm between the haem and flavin planes can be estimated by using Hopfield's treatment (Potasek & Hopfield, 1977). At such a close distance spin-spin interactions between the paramagnetic species ferric haem and semiquinone can be predicted.…”

Section: Sx-enzymementioning

confidence: 99%

Comparison of the processes involved in reduction by the substrate for two homologous flavocytochromes b2 from different species of yeast

Capeillère‐Blandin¹,

Barber²,

Bray³

1986

Biochemical Journal

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A detailed study of the electron exchanges involved between FMN and haem b2 groups within flavocytochrome b2 of yeast Hansenula anomala (H-enzyme) was performed. The results were compared with those for the homologous enzyme of yeast Saccharomyces cerevisiae (Sx-enzyme) re-investigated at 5 degrees C. The mid-point reduction potentials of FMN and haem were determined by two complementary methods: potentiometric titration with substrate, L-lactate, in the presence of dye mediators with quantification of the reduced species performed by spectrophotometry at suitable wavelengths; anaerobic titration of the enzyme by its substrate by monitoring the e.p.r. signals of the semiquinone and Fe3+ species. Values of Em,7 = -19, -23 and -45 V were determined respectively from the data for the three redox systems Ho/Hr, Fo/Fsq and Fsq/Fr in the H-enzyme instead of +6, -44 and -57 mV respectively in the Sx-enzyme [Capeillère-Blandin, Bray, Iwatsubo & Labeyrie (1975) Eur. J. Biochem. 54, 549-566]. Parallel e.p.r rapid-freezing and absorbance stopped-flow studies allowed determination of the time courses of the various redox species during their reduction by L-lactate. The flavin and the haem reduction time courses were biphasic. In the initial fast phase the reduction of flavin monitored by absorbance measurements is accomplished with a rate constant kF = 360 s-1. The reduction of the haem lags the reduction of flavin with a rate constant kH = 170 s-1. The appearance of flavin free radical is slower than the reduction in flavin absorbance and occurs with a rate constant close to that of the reduction of the haem. At saturating L-lactate concentration the initial rapid phase (up to 15 ms) involved in the overall turnover can be adequately simulated with a two-step reaction scheme. The main difference between the enzymes lies especially at the level of the first step of electron exchange between bound lactate and flavin, which for the H-enzyme is no longer the rate-limiting step in the haem reduction and becomes 8-fold faster than in the Sx-enzyme. Consequently in the H-enzyme for the following step, the intramolecular transfer from flavin hydroquinone to oxidized haem, a reliable evaluation of the rate constants becomes possible. Preliminary values are k+2 = 380 s-1 and k-2 = 120 s-1 at 5 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

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Fundamental aspects of electron transfer: experimental verification of vibronically coupled electron tunneling.

Cited by 39 publications

References 15 publications

Untangling Heavy Protein and Cofactor Isotope Effects on Enzyme-Catalyzed Hydride Transfer

Untangling Heavy Protein and Cofactor Isotope Effects on Enzyme-Catalyzed Hydride Transfer

Electron transfer dynamics and excited state branching in a charge-transfer platinum(ii) donor–bridge-acceptor assembly

Comparison of the processes involved in reduction by the substrate for two homologous flavocytochromes b2 from different species of yeast

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