Short-range ENDOR distance measurements between Gd(III) and trifluoromethyl labels in proteins.

The measurement of distances in proteins can be challenging in the 5-20 Å range, which is outside those accessible through conventional NMR and EPR methods. Recently it was demonstrated that distances in this range could be measured between a nitroxide as a paramagnetic spin label and a nearby fluorine atom (19F) as a nuclear spin label using high-field (W-band/3.4 T) ENDOR spectroscopy. Here we show that such measurements can also be performed using a gadolinium ion (Gd3+) as the paramagnetic tag. Gd3+ has two advantages. (i) A greater electronic spin (S = 7/2) and fast electronic spin-lattice (T1) relaxation, improving sensitivity by allowing data to be collected at lower temperatures. (ii) A narrow EPR signal for the -½ ↔ ½ transition, and therefore no orientation selection artefacts. Signal intensities can be further enhanced by using a trifluoromethyl (C19F3) group instead of a single 19F atom. Using the protein calbindin D9k with a Ca2+ ion replaced by a Gd3+ ion and a trifluoromethylphenylalanine in position 50, we show that distances up to about 10 Å can be readily measured. Longer distances proved more difficult to measure due to variable electronic TM relaxation rates, which lead to broader Lorentzian ENDOR lineshapes. Gd3+ complexes (Gd3+ tags), which reliably display longer TM times, allow longer distances to be measured (8-16 Å). We also provide preliminary evidence that the intensity of ENDOR signals follows the predicted 1/r6 dependence, indicating that distances r > 20 Å can be measured by this method.

Quintet Dinitrenes with Negative Zero-Field Splitting: Effect of Spin-Orbit Coupling on the Sign of Magnetic Anisotropy.

We report on W-band EPR and quantum chemical investigation of novel organic tetraradicals with negative axial zero-field splitting (ZFS) parameter D. These belong to the class of quintet 1,3,5-tribromophenylene-2,4-dinitrenes bearing different substituents in position 6 of the benzene ring (1b, N3; 1c, F; 1d, CN; 1e; Cl; 1f, Br). Analysis of the W-band EPR spectrum of dinitrene 1c reveals its large negative ZFS parameter D = -0.27 cm-1. Quantum chemical calculations show that negative D gradually grows in the row of 1c(F) < 1b(N3) < 1d(CN) < 1e(Cl) < 1f(Br) dinitrenes due to decreasing of the through-space distance between the nitrene units and neighboring bromine atoms. Shorter steric N···Br distance results in the stronger contribution of the spin-orbit coupling (SOC) to the total ZFS. The sign of D depends on the interplay of three factors: (i) the angle θ between the "easy" z-axes of the dipolar spin-spin (DSS) and spin-orbit (DSOC) interaction tensors, (ii) the ratio of DSOC/DSS values, and (iii) the rhombicity parameters ESS/DSS and ESOC/DSOC. The study demonstrates in which cases organic quintet tetraradicals may have negative ZFS owing to the presence of heavy atoms at appropriate sites nearby the nitrene units and, thus, possess the bistability property as single-molecule magnets.

Characterization of a Triplet Vinylidene.

Singlet vinylidenes (R2C═C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm-1 and |E|/D = 0.028) were determined, and the 13C hyperfine coupling tensor was obtained by 13C-ENDOR measurements. Most strikingly, the isotropic 13C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Sequential Hydrogen Tunneling in o-Tolylmethylene.

o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniques were used to identify two conformers of 1 (anti and syn) in noble gas matrices and in frozen organic solutions. Conformer-specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti 1 and syn 1 rapidly interconvert via efficient quantum tunneling forming a rotational pre-equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti 1 to 2, is rate-limiting for the formation of 2. We here present an efficient strategy for the study of such tunneling equilibria.

Intermolecular background decay in RIDME experiments.

The relaxation-induced dipolar modulation enhancement (RIDME) technique allows the determination of distances and distance distributions in pairs containing two paramagnetic metal centers, a paramagnetic metal center and an organic radical, and, under some conditions, also in pairs of organic radicals. The strengths of the RIDME technique are its simple setup requirements, and the absence of bandwidth limitations for spin inversion which occurs through relaxation. A strong limitation of the RIDME technique is the background decay, which is often steeper than that in the double electron electron resonance experiment, and the absence of an appropriate description of the intermolecular background signal. Here we address the latter problem and present an analytical calculation of the RIDME background decay in the simple case of two types of randomly distributed spin centers each with total spin S = 1/2. The obtained equations allow the explaination of the key trends in RIDME experiments on frozen chelated metal ion solutions, and singly spin-labeled proteins. At low spin label concentrations, the RIDME background shape is determined by nuclear-driven spectral diffusion processes. This fact opens up a new path for structural characterization of soft matter and biomacromolecules through the determination of the local distribution of protons in the vicinity of the spin-labeled site.

Atomic-Scale Explanation of O2 Activation at the Au-TiO2 Interface.

By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes.

Photoredox-Switchable Resorcin[4]arene Cavitands: Radical Control of Molecular Gripping Machinery via Hydrogen Bonding.

Semiquinones (SQ) are generated in photosynthetic organisms upon photoinduced electron transfer to quinones (Q). They are stabilized by hydrogen bonding (HB) with the neighboring residues, which alters the properties of the reaction center. We designed, synthesized, and investigated resorcin[4]arene cavitands inspired by this function of SQ in natural photosynthesis. Cavitands were equipped with alternating quinone and quinoxaline walls bearing hydrogen bond donor groups (HBD). Different HBD were analyzed that mimic natural amino acids, such as imidazole and indole, along with their analogues, pyrrole and pyrazole. Pyrroles were identified as the most promising candidates that enabled the cavitands to remain open in the Q state until strengthening of HB upon reduction to the paramagnetic SQ radical anion provided stabilization of the closed form. The SQ state was generated electrochemically and photochemically, whereas properties were studied by UV/Vis spectroelectrochemistry, transient absorption, and EPR spectroscopy. This study demonstrates a photoredox-controlled conformational switch towards a new generation of molecular grippers.

Reactions of Cyclopentadienylidenes with CF3I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom.

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF3I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF3I at 3 K. Bimolecular reactions between the carbenes and CF3I were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF3 group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.

Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes.

Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a ( D = 0.9930 cm-1, E = 0.0261 cm-1), 1c ( D = 1.244 cm-1, E = 0.030 cm-1), and 1d ( D = 1.369 cm-1, E = 0.093 cm-1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin-spin interactions ( DSS ∼ 1 cm-1), but drastically differ in the spin-orbit coupling parameter (from DSOC = 0.087 cm-1 for 1a to DSOC = 0.765 cm-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of DSOC of nitrenes 1a-f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a-1f, the smaller the N--Br distance and the larger DSOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.

Thermally Activated Delayed Fluorescence in a Y3 N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies.

The endohedral fullerene Y3 N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y3 N@C80 exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3 N@C80 /PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).

Local water sensing: water exchange in bacterial photosynthetic reaction centers embedded in a trehalose glass studied using multiresonance EPR.

Using isotope labeled water (D2O and H217O) and pulsed W-band (94 GHz) high-field multiresonance EPR spectroscopies, such as ELDOR-detected NMR and ENDOR, the biologically important question of detection and quantification of local water in proteins is addressed. A bacterial reaction center (bRC) from Rhodobacter sphaeroides R26 embedded into a trehalose glass matrix is used as a model system. The bRC hosts the two native radical cofactor ions (primary electron donor) and (primary electron acceptor) as well as an artificial nitroxide spin label site-specifically attached to the surface of the H-protein domain. The three paramagnetic reporter groups have distinctly different local environments. They serve as local probes to detect water molecules via magnetic interactions (electron-nuclear hyperfine and quadrupole) with either deuterons or 17O nuclei. bRCs were equilibrated in an atmosphere of different relative humidities allowing us to control precisely the hydration levels of the protein. We show that by using oxygen-17 labeled water quantitative conclusions can be made in contrast to using D2O which suffers from proton-deuterium exchange processes in the protein. From the experiments we also conclude that dry trehalose operates as an anhydrobiotic protein stabilizer in line with the "anchorage hypothesis" of bio-protection. It predicts selective changes in the first solvation shell of the protein upon trehalose-matrix dehydration with subsequent changes in the hydrogen-bonding network. Changes in hydrogen-bonding patterns usually have an impact on the global function of a biological system.

Computing distance distributions from dipolar evolution data with overtones: RIDME spectroscopy with Gd(iii)-based spin labels.

Extraction of distance distributions between high-spin paramagnetic centers from relaxation induced dipolar modulation enhancement (RIDME) data is affected by the presence of overtones of dipolar frequencies. As previously proposed, we account for these overtones by using a modified kernel function in Tikhonov regularization analysis. This paper analyzes the performance of such an approach on a series of model compounds with the Gd(iii)-PyMTA complex serving as paramagnetic high-spin label. We describe the calibration of the overtone coefficients for the RIDME kernel, demonstrate the accuracy of distance distributions obtained with this approach, and show that for our series of Gd-rulers RIDME technique provides more accurate distance distributions than Gd(iii)-Gd(iii) double electron-electron resonance (DEER). The analysis of RIDME data including harmonic overtones can be performed using the MATLAB-based program OvertoneAnalysis, which is available as open-source software from the web page of ETH Zurich. This approach opens a perspective for the routine use of the RIDME technique with high-spin labels in structural biology and structural studies of other soft matter.

Jim Hyde and the ENDOR Connection: A Personal Account.

In this minireview, we report on our year-long EPR work, such as electron-nuclear double resonance (ENDOR), pulse electron double resonance (PELDOR) and ELDOR-detected NMR (EDNMR) at X-band and W-band microwave frequencies and magnetic fields. This report is dedicated to James S. Hyde and honors his pioneering contributions to the measurement of spin interactions in large (bio)molecules. From these interactions, detailed information is revealed on structure and dynamics of macromolecules embedded in liquid-solution or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultra-fast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy and its multi-frequency extensions to new horizons concerning sensitivity of detection, selectivity of molecular interactions and time resolution. Among the most important advances is the upgrading of EPR to high magnetic fields, very much in analogy to what happened in NMR. The ongoing progress in EPR spectroscopy is exemplified by reviewing various multi-frequency electron-nuclear double-resonance experiments on organic radicals, light-generated donor-acceptor radical pairs in photosynthesis, and site-specifically nitroxide spin-labeled bacteriorhodopsin, the light-driven proton pump, as well as EDNMR and ENDOR on nitroxides. Signal and resolution enhancements are particularly spectacular for ENDOR, EDNMR and PELDOR on frozen-solution samples at high Zeeman fields. They provide orientation selection for disordered samples approaching single-crystal resolution at canonical g-tensor orientations-even for molecules with small g-anisotropies. Dramatic improvements of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Thus, unique structural and dynamic information is revealed that can hardly be obtained by other analytical techniques. Micromolar concentrations of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems-offering exciting applications for physicists, chemists, biochemists and molecular biologists.

Light and Temperature Control of the Spin State of Bis(p-methoxyphenyl)carbene: A Magnetically Bistable Carbene.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV-vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy. UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light shifts the photostationary equilibrium toward the singlet state. Upon annealing at higher temperatures (>10 K), the triplet is converted to the singlet; however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and irradiation conditions, matrices containing predominantly the triplet or singlet carbene can be generated. Controlling the magnetic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such as information storage and radical-initiated polymerization processes.

EPR characterization of Mn(ii) complexes for distance determination with pulsed dipolar spectroscopy.

The four Mn(ii) complexes Mn-DOTA, Mn-TAHA, Mn-PyMTA, and Mn-NO3Py were characterized by electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and relaxation measurements, to predict their relative performance in the EPR pulse dipolar spectroscopy (PDS) experiments. High spin density localization on the metal ions was proven by ENDOR on 1H, D, 14N, and 55Mn nuclei. The transverse relaxation of the Mn(ii) complexes appears to be slow enough for PDS-based spin-spin distance determination. Rather advantageous ratios of T1/Tm were measured allowing for good relaxation induced dipolar modulation enhancement (RIDME) performance and, in general, fast shot repetitions in any PDS experiment. Relaxation properties of the Mn(ii) complexes correlate with the strengths of their zero field splitting (ZFS). Further, a comparison of Mn(ii)-DOTA and Gd(iii)-DOTA based spin labels is presented. The RIDME technique to measure nanometer-range Mn(ii)-Mn(ii) distances in biomolecules is discussed as an alternative to the well-known DEER technique that often appears challenging in cases of metal-metal distance measurements. The use of a modified kernel function that includes dipolar harmonic overtones allows model-free computation of the Mn(ii)-Mn(ii) distance distributions. Mn(ii)-Mn(ii) distances are computed from RIDME data of Mn-rulers consisting of two Mn-PyMTA complexes connected by a rodlike spacer of defined length. Level crossing effects seem to have only a weak influence on the distance distributions computed from this set of Mn(ii)-Mn(ii) RIDME data.

Möbius-Hückel topology switching in an expanded porphyrin cation radical as studied by EPR and ENDOR spectroscopy.

The symmetry of the arrangement of objects has fascinated philosophers, artists and scientists for a long time, and still does. Symmetries often exist in nature, but are also created artificially, for instance by chemical synthesis of novel molecules and materials. The one-sided, non-orientable Möbius band topology is a paradigm of such a symmetry-based fascination. In the early 1960s, in synthetic organic chemistry the interest in molecules with Möbius symmetry was greatly stimulated by a short paper by Edgar Heilbronner. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them with a 180° phase change into the Möbius symmetry of their hydrocarbon skeleton. In 2007, the group of Lechoslaw Latos-Grazynski succeeded in synthesizing the compound di-p-benzi[28]hexa-phyrin(1.1.1.1.1.1), compound 1, which can dynamically switch between Hückel and Möbius conjugation depending, in a complex manner, on the polarity and temperature of the surrounding solvent. This discovery of "topology switching" between the two-sided (Hückel) and one-sided (Möbius) molecular state with closed-shell electronic configuration was based primarily on the results of NMR spectroscopy and DFT calculations. The present EPR and ENDOR work on the radical cation state of compound 1 is the first study of a ground-state open-shell system which exhibits a Hückel-Möbius topology switch that is controlled by temperature, like in the case of the closed-shell precursor. The unpaired electron interacting with magnetic nuclei in the molecule is used as a sensitive probe for the electronic structure and its symmetry properties. For a Hückel conformer with its higher symmetry, we expect - and observe - fewer ENDOR lines than for a Möbius conformer. The ENDOR results are supplemented by and in accordance with theoretical calculations based on density functional theory at the ORCA level.

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting.

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm(-1), ET = 0.093 cm(-1), and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = - 0.306 cm(-1), EQ = 0.0137 cm(-1), and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = - 0.203 cm(-1), ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

Preparation of cysteine-34-nitroxide spin labeled human α1-microglobulin.

α(1)-Microglobulin (α(1)m) is a protein of yet unresolved function occurring in blood plasma and urine. It consists of a lipocaline type of fold with two cysteine residues forming a disulfide bridge and the third cysteine-34 remaining a free, somewhat reactive thiol. A number of investigations point to an interaction with heme and we have recently reported, that heme binding triggers the formation of a stable α(1)m trimer upon modification of cysteine-34 with 2-iodoacetamide, i.e., [α(1)m(heme)(2)](3) [J.F. Siebel, R.L. Kosinsky, B. Åkerström, M. Knipp, Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin-formation of a [(heme)(2)(α(1)-microglobulin)](3) complex, ChemBioChem 13 (2012) 879-887]. For further structural and functional investigations, an improved purification protocol for α(1)m was sought, in particular yielding an untagged amino acid sequence. The method reported herein improves the speed and the yield of the protein production even when an expression plasmid without tag was applied. Furthermore, for the purpose of future structural studies using electron paramagnetic resonance (EPR) techniques, in accordance to the modification with 2-iodoacetamide (α(1)m(AM)), the protein was modified with 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (3-(2-iodoacetamido)-PROXYL) yielding the nitroxide spin labeled α(1)m(N-O). The extinction coefficient of the protein was calibrated using magnetic circular dichroism (MCD) spectroscopy of tryptophan (ε(280nm)=40,625M(-1)cm(-1)). The parallel quantification by absorbance spectroscopy (protein) and cw-EPR spectroscopy (radical spin) determined the degree of spin labeling to 90%. Characterization of the protein by circular dichroism (CD) spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) upon tryptic digestion further demonstrated the similar fold of α(1)m(AM) and α(1)m(N-O), but also established the modification of cystein-34 as well as the formation of the cysteine-72-cysteine-169 disulfide bond.

Variable bandwidth, high efficiency microwave resonator for control of spin-qubits in nitrogen-vacancy centers.

Nitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and quantum information. Realization of this potential requires effective tools for controlling the spin degree of freedom by microwave (mw) magnetic fields. In this work, we present a planar microwave resonator optimized for microwave-optical double resonance experiments on single NV centers in diamond. It consists of a piece of wide microstrip line, which is symmetrically connected to two 50 Ω microstrip feed lines. In the center of the resonator, an Ω-shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07 × 0.1 mm3. It can be operated at 2.9 GHz in both transmission and reflection modes with bandwidths of 1000 and 400 MHz, respectively. The high power-to-magnetic field conversion efficiency allows us to produce π-pulses with a duration of 50 ns with only about 200 and 50 mW microwave power in transmission and reflection, respectively. The transmission mode also offers capability for efficient radio frequency excitation. The resonance frequency can be tuned between 1.3 and 6 GHz by adjusting the length of the resonator. This will be useful for experiments on NV-centers at higher external magnetic fields and on different types of optically active spin centers.