An Improved TM110 ENDOR Cavity for the Investigation of Transition Metal Complexes

Zweygart, W.; Thanner, R.; Lubitz, Wolfgang

doi:10.1006/jmra.1994.1151

Cited by 41 publications

(27 citation statements)

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“…EPR, ENDOR, and electron-nuclear-nuclear triple resonance (special TRIPLE) spectra were recorded with a Bruker ESP 300E spectrometer (Billerica, MA) as described (23), using ENDOR accessories and an ENDOR TM110 cavity of local design (24). The cation radical of the BChl dimer was generated in the cavity by continuous wave illumination with a tungsten halogen lamp (100 W, 700-950 nm using filters) (23).…”

Section: Methodsmentioning

confidence: 99%

Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

Artz

Williams

Allen

et al. 1997

Proc. Natl. Acad. Sci. U.S.A.

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124

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The primary electron donor in bacterial reaction centers is a dimer of bacteriochlorophyll a molecules, labeled L or M based on their proximity to the symmetryrelated protein subunits. The electronic structure of the bacteriochlorophyll dimer was probed by introducing small systematic variations in the bacteriochlorophyll-protein interactions by a series of site-directed mutations that replaced residue Leu M160 with histidine, tyrosine, glutamic acid, glutamine, aspartic acid, asparagine, lysine, and serine. The midpoint potentials for oxidation of the dimer in the mutants showed an almost continuous increase up to Ϸ60 mV compared with wild type. The spin density distribution of the unpaired electron in the cation radical state of the dimer was determined by electron-nuclear-nuclear triple resonance spectroscopy in solution. The ratio of the spin density on the L side of the dimer to the M side varied from Ϸ2:1 to Ϸ5:1 in the mutants compared with Ϸ2:1 for wild type. The correlation between the midpoint potential and spin density distribution was described using a simple molecular orbital model, in which the major effect of the mutations is assumed to be a change in the energy of the M half of the dimer, providing estimates for the coupling and energy levels of the orbitals in the dimer. These results demonstrate that the midpoint potential can be fine-tuned by electrostatic interactions with amino acids near the dimer and show that the properties of the electronic structure of a donor or acceptor in a protein complex can be directly related to functional properties such as the oxidation-reduction midpoint potential.The reaction center is the site of the primary process in photosynthesis, which is the conversion of light energy into a charge-separated state (for reviews, see ref. 1). The electron transfer process in the reaction center has the remarkable aspect that the quantum yield is near unity, that is, for every photon absorbed, one electron is transferred. The reaction center isolated from the purple bacterium Rhodobacter sphaeroides is particularly useful for the study of electron transfer because this complex has been well characterized biochemically and spectroscopically, and the three-dimensional structure has been determined by x-ray diffraction (2-5). The reaction center has two core subunits, L and M, that form the binding sites for the cofactors and are related by an Ϸ2-fold symmetry axis. The primary electron donor (P) consists of two symmetry-related bacteriochlorophyll (BChl) a molecules labeled P L and P M that overlap at the ring I position and are separated by Ϸ3.5 Å (Fig. 1). Light absorption causes the excitation of P, and within 3.5 ps, an electron is transferred to a bacteriopheophytin acceptor. Electron transfer proceeds to the primary quinone acceptor, Q A , in Ϸ200 ps and to the secondary quinone, Q B , in Ϸ200 s. After reduction of the donor by cytochrome c 2 , absorption of a second photon leads to the transfer of a second electron to Q B , which then leaves the protein carry...

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Section: Methodsmentioning

confidence: 99%

Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

Artz

Williams

Allen

et al. 1997

Proc. Natl. Acad. Sci. U.S.A.

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124

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“…Experiments at 35 GHz were performed using an ENDOR cavity similar to that described in [32]. X-band EPR/ENDOR experiments at Technische Universität Berlin were carried out on a Bruker ESP 300E instrument with homebuilt ENDOR accessories described in [33,34]. The X-band electron spin echo envelope modulation (ESEEM) experiments were performed on a Bruker ESP 380E FT-EPR instrument as described in [35].…”

Section: Epr/endor Spectrometersmentioning

confidence: 99%

The primary and secondary acceptors in bacterial photosynthesis III. Characterization of the quinone radicals Q A − ⋅ and Q B − ⋅ by EPR and ENDOR

1999

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Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques were used to investigate the electronic structure of the primary (Qp') and secondary (QB') ubiquinone electron acceptors in reaction centers (RCs) of the photosynthetic bacterium Rhodobacter sphaeroides. To reduce the EPR linewidth, the high-spin Fe e present in native RCs was replaced by diamagnetic Zn2 *. Experiments were performed both on frozen solutions and single crystals at microwave frequencies of 9, 35 and 94 GHz. Differences in the EPR/ENDOR spectra were observed for Qp' and QR', which are attributed to different environments of the quinones in the RC. The differences exhibited themselves in: (i) the g-tensors, (ii) the "O and 13 C hyperfine coupling (hfc) constants of the quinones labeled at the carbonyl group, (iii) the 'H-hfcs of the quinone ring and (iv) the exchangeable protons hydrogen bonded to the carbonyl oxygens. From these results and from H/D exchange experiments, the following conclusions were drawn: both Qp' and QB' have at least two hydrogen bonds of different strengths to the carbonyl oxygens. The hydrogen bonds for Qp' are stronger and more asymmetric than for Q. For Q the stronger bond (to 04) was assigned to His(M219) and the weaker (to 0,) to Ala(M260). For Q the stronger bond (to 0 4) was assigned to His(L190), with several weaker bonds (to 0,) to Ser(L223), lle(224) and Gly(L225). From the temperature dependence of the hfcs of the exchangeable protons some dynamic properties of the RC were deduced. Hfcs with more distant nitrogens were observed by electron spin echo envelope modulation (ESEEM). For Q they were assigned to N 8 of His(M219) and to the peptide backbone nitrogen of Ala(M260) and for QB' to N d of His(L190). These interactions indicate the extent of the electron wave function, which is important for the understanding of the electron transfer mechanism. Based on the maghetic resonance results, the function of the quinone acceptors in the reaction center is discussed.

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“…X-band EPR/ENDOR spectra were measured on a Bruker ESP-300E spectrometer using a laboratory-built ENDOR upgrade adapted to a nitrogen gas flow system for temperature control, described previously [12]. The extension for ENDOR consists of a Rhode and Schwarz radio frequency (rf) synthesizer (SMG) and an ENI solid state rf amplifier (A200L).…”

Section: Instrumentationmentioning

confidence: 99%

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

et al. 1998

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Selectively 13 C-labeled ubiquinone anion radicals in protic and aprotic solvents are investigated by EPR and ENDOR spectroscopy, yielding information about the effect of hydrogen bonds on the electronic g-tensor and the carbonyl carbon 13 C-hf tensors. Formation of the hydrogen bonds alter the g-tensor significantly to lower values and increases the A. component of the 13 C-hf tensor. Both effects can be explained by electrostatic interactions between the positively charged hydrogen and the electrons at the carbonyl oxygen leading to a redistribution of charge and rr-spin density. Two different hydrogen bonds were obtained for UQo' which are in agreement with the results of DFT (density functional theory) calculations.

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An Improved TM110 ENDOR Cavity for the Investigation of Transition Metal Complexes

Cited by 41 publications

References 0 publications

Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

The primary and secondary acceptors in bacterial photosynthesis III. Characterization of the quinone radicals Q A − ⋅ and Q B − ⋅ by EPR and ENDOR

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

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