Influence of hydrogen bonds on the electronicg-tensor and13C-hyperfine tensors of13C-labeled ubiquinones — EPR and ENDOR study

Nimz, Olaf; Lendzian, Friedhelm; Boullais, Claude; Lubitz, Wolfgang

doi:10.1007/bf03161893

Cited by 31 publications

(42 citation statements)

References 25 publications

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“…The g tensor components of quinone radical anions ate good indicators for the polarity of the radical's surrounding [20,21,[26][27][28][29]. The g values obtained for A i-in PSI show that the environment must be very hydrophobic in agreement with the information deduced from the X-ray crystallographic structure [2,4].…”

Section: Introductionsupporting

confidence: 63%

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

Teutloff

Bittl

Lubitz³

2004

Appl. Magn. Reson.

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The secondary acceptor A~ of the electron transport chain(s) of photosystem (PS) I i s a phylloquinone (vitamin K~, VK~). Pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments at X-band frequencies were performed on the photoaccumulated acceptor radical A i-and the radical pair state P);oA~-in PSI of Fhermosynechococcus elongatus. The data obtained were compared with data from the respective radical anion of VK I in organic solvents. The unusual g tensor magnitude of A~-is explained by the hydrophobic binding pocket of this radical. The hyperfine couplings and the spin (and charge) density distribution is very different for A;-in PSI and VK;-in frozen alcoholic solution. This is attributed to a rather strong one-sided hydrogen bond to A;-. The presence of a hydrogen bond to A~-has only a minor effect on g. The hyperfine coupling constants of A;-determined from the radical pair spectra deviate only slightly from those derived from photoaccumulated A;-in PSI treated with dithionite at high pH. ENDOR resonarices of the proton in a H bond were detected and ah estimate of the strength and geometry of this bond to A;-was obtained. The significance of the hydrogen bond and other (hydrophobic) interactions of A l with the surrounding are briefly discussed.

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Section: Introductionsupporting

confidence: 63%

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

Teutloff

Bittl

Lubitz³

2004

Appl. Magn. Reson.

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“…At Q-band, the rhodosemiquinone EPR signal displays a characteristic g anisotropy with principal g values of 2.0058 ( g x ), 2.0049 ( g y ) and 2.0017 ( g z ). These values are different to those corresponding to the ubisemiquinone anion radical (UQ •− ) in nonprotic solvents (see Table 1) [19]. The difference in g values of RQ •− and UQ •− shows that the spin density distribution in the rhodosemiquinone is different than that in the ubisemiquinone.…”

Section: Resultsmentioning

confidence: 94%

“…2). The rhodoquinone radical anion in vitro (RQ •− , 1 mM) was generated electrochemically [19]. The coulometry of the quinone was performed using tetrabutylammonium fluoroborate (0.2 mol/l) as supporting electrolyte under high vacuum conditions in a home-built electrolysis cell.…”

Section: Methodsmentioning

confidence: 99%

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EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

et al. 2009

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In photosynthetic bacteria, light-induced electron transfer takes place in a protein called the reaction center (RC) leading to the reduction of a bound ubiquinone molecule, Q B, coupled with proton binding from solution. We used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to study the magnetic properties of the protonated semiquinone, an intermediate proposed to play a role in proton coupled electron transfer to Q B . To stabilize the protonated semiquinone state, we used a ubiquinone derivative, rhodoquinone, which as a semiquinone is more easily protonated than ubisemiquinone. To reduce this low-potential quinone we used mutant RCs modified to directly reduce the quinone in the Q B site via B-branch electron transfer (Paddock et al. in Biochemistry 44:6920-6928, 2005). EPR and ENDOR signals were observed upon illumination of mutant RCs in the presence of rhodoquinone. The EPR signals had g values characteristic of rhodosemiquinone (g x = 2.0057, g y = 2.0048, g z * 2.0018) at pH 9.5 and were changed at pH 4.5. The ENDOR spectrum showed couplings due to solvent exchangeable protons typical of hydrogen bonds similar to, but different from, those found for ubisemiquinone. This approach should be useful in future magnetic resonance studies of the protonated semiquinone.

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“…[14] Calculated methyl proton hfccs reported in the table are average hfccs for the three methyl protons and experimental isotropic hfccs for methyl protons are reported for experiments in aprotic solvent, [15] then protic solvent [16] and, in parentheses, for experiments in cyt bo3. [7] Principal components of 13 C hyperfine tensors for carbons 1 and 4 are likewise reported from published experiments conducted in aprotic solvent, [17] then protic solvent [18] and, in parentheses, in cyt bo3. [6] [b] Geometry optimizations were performed by using the B3LYP/6-31G(d) method/basis set [19,20] and Berny's optimization algorithm [21] in the Gaussian 03 program package.…”

mentioning

confidence: 85%

Isotropic ¹³C Hyperfine Coupling Constants Distinguish Neutral from Anionic Ubiquinone‐Derived Radicals

Boesch¹,

Wheeler²

2009

ChemPhysChem

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Ubiquinol oxidation to ubiquinone and/or the reverse reaction, ubiquinone reduction to ubiquinol (Scheme 1), is critical in the actions of several enzymes, including cytochrome (cyt) bc 1 [1][2][3] and cyt bo 3 ubiquinol oxidase, [4] as well as photosynthesis in bacterial photosynthetic reaction centers.[5] For example, cyt bo 3 from E. coli reportedly contains two ubiquinol/ubiquinone binding sites, a low-affinity site (QL), where ubiquinol is oxidized and a high-affinity site (QH) where a bound ubiquinone species acts as a conduit for electrons. The QH site binds a quinone tightly and stabilizes a transient intermediate, the one-electron reduced quinone (or semiquinone), through hydrogen bonding by the protein. The best available evidence provides conflicting reports about the exact identity of the intermediate. Multi-frequency EPR experiments with 13 C-labeled ubiquinone-2 [6] implies binding of an anionic semiquinone radical to the protein via a one-sided hydrogen bond or a strongly asymmetric hydrogen-bonding network. In contrast, two-dimensional electron spin-echo envelope modulation experiments with the exchangeable protons implies "significant covalent OÀH binding of carbonyl oxygen O1 that is characteristic of a neutral radical". [7,8] Computed isotropic hyperfine coupling constants (hfccs) for a model of the ubiquinone-derived radicals presented herein show that 13 C hfccs for the carbonyl carbon atoms can be more than twenty times larger for the protonated, neutral radical (UQHC, Scheme 2) than for the anionic semiquinone radical (UQC À ) and confirm that the signs of 13

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Influence of hydrogen bonds on the electronicg-tensor and13C-hyperfine tensors of13C-labeled ubiquinones — EPR and ENDOR study

Cited by 31 publications

References 25 publications

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

Isotropic ¹³C Hyperfine Coupling Constants Distinguish Neutral from Anionic Ubiquinone‐Derived Radicals

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Influence of hydrogen bonds on the electronicg-tensor and13C-hyperfine tensors of13C-labeled ubiquinones — EPR and ENDOR study

Cited by 31 publications

References 25 publications

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

Isotropic 13C Hyperfine Coupling Constants Distinguish Neutral from Anionic Ubiquinone‐Derived Radicals

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Isotropic ¹³C Hyperfine Coupling Constants Distinguish Neutral from Anionic Ubiquinone‐Derived Radicals