A tunable general purpose Q-band resonator for CW and pulse EPR/ENDOR experiments with large sample access and optical excitation

An improved coupling design for high-frequency TE011 electron paramagnetic resonance cavities

In high-frequency electron paramagnetic resonance (EPR) spectroscopy the sample is usually accommodated in a single-mode cylindrical TE(011) microwave cavity. This cavity stands out in terms of flexibility for various types of EPR experiments due to convenient control of its resonance frequency and easy waveguide-to-cavity microwave coupling. In continuous wave and in pulsed EPR it is, however, essential to be able to vary the coupling efficiency over a large range. We present a new mechanical design to vary the microwave coupling to the cavity using a movable metal sphere. This coupling sphere is shifted in the plane of the iris wall inside the coupling waveguide. The design allows for a compact and robust construction of the EPR probehead that can be easily accommodated inside a limited space of helium flow cryostat. The construction details and characterization of the coupling element for 95 GHz (W-band) EPR as well as for 34 GHz (Q-band) are presented.

EPR/ENDOR, Mössbauer, and quantum-chemical investigations of diiron complexes mimicking the active oxidized state of [FeFe]hydrogenase

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.

Pulse Q-band EPR and ENDOR spectroscopies of the photochemically generated monoprotonated benzosemiquinone radical in frozen alcoholic solution

Quinones are essential cofactors in many physiological processes, among them proton-coupled electron transfer (PCET) in photosynthesis and respiration. A key intermediate in PCET is the monoprotonated semiquinone radical. In this work we produced the monoprotonated benzosemiquinone (BQH(•)) by UV illumination of BQ dissolved in 2-propanol at cryogenic temperatures and investigated the electronic and geometric structures of BQH(•) in the solid state (80 K) using EPR and ENDOR techniques at 34 GHz. The g-tensor of BQH(•) was found to be similar to that of the anionic semiquinone species (BQ(•-)) in frozen solution. The peaks present in the ENDOR spectrum of BQH(•) were identified and assigned by (1)H/(2)H substitutions. The experiments reconfirmed that the hydroxyl proton (O-H) on BQH(•), which is abstracted from a solvent molecule, mainly originates from the central CH group of 2-propanol. They also showed that the protonation has a strong impact on the electron spin distribution over the quinone. This is reflected in the hyperfine couplings (hfc's) of the ring protons, which dramatically changed with respect to those typically observed for BQ(•-). The hfc tensor of the O-H proton was determined by a detailed orientation-selection ENDOR study and found to be rhombic, resembling those of protons covalently bound to carbon atoms in a π-system (i.e., α-protons). It was found that the O-H bond lies in the quinone plane and is oriented along the direction of the quinone oxygen lone pair orbital. DFT calculations were performed on different structures of BQH(•) coordinated by four, three, or zero 2-propanol molecules. The O-H bond length was found to be around 1.0 Å, typical for a single covalent O-H bond. Good agreement between experimental and DFT results were found. This study provides a detailed picture of the electronic and geometric structures of BQH(•) and should be applicable to other naturally occurring quinones.

The basic properties of the electronic structure of the oxygen-evolving complex of photosystem II are not perturbed by Ca2+ removal

Ca(2+) is an integral component of the Mn(4)O(5)Ca cluster of the oxygen-evolving complex in photosystem II (PS II). Its removal leads to the loss of the water oxidizing functionality. The S(2)' state of the Ca(2+)-depleted cluster from spinach is examined by X- and Q-band EPR and (55)Mn electron nuclear double resonance (ENDOR) spectroscopy. Spectral simulations demonstrate that upon Ca(2+) removal, its electronic structure remains essentially unaltered, i.e. that of a manganese tetramer. No redistribution of the manganese valence states and only minor perturbation of the exchange interactions between the manganese ions were found. Interestingly, the S(2)' state in spinach PS II is very similar to the native S(2) state of Thermosynechococcus elongatus in terms of spin state energies and insensitivity to methanol addition. These results assign the Ca(2+) a functional as opposed to a structural role in water splitting catalysis, such as (i) being essential for efficient proton-coupled electron transfer between Y(Z) and the manganese cluster and/or (ii) providing an initial binding site for substrate water. Additionally, a novel (55)Mn(2+) signal, detected by Q-band pulse EPR and ENDOR, was observed in Ca(2+)-depleted PS II. Mn(2+) titration, monitored by (55)Mn ENDOR, revealed a specific Mn(2+) binding site with a submicromolar K(D). Ca(2+) titration of Mn(2+)-loaded, Ca(2+)-depleted PS II demonstrated that the site is reversibly made accessible to Mn(2+) by Ca(2+) depletion and reconstitution. Mn(2+) is proposed to bind at one of the extrinsic subunits. This process is possibly relevant for the formation of the Mn(4)O(5)Ca cluster during photoassembly and/or D1 repair.