- Jan Behrends
- Robert Bittl
- Enrica Bordignon
- Klaus Lips
- Thomas Risse
- Alexander Schnegg
- Christian Teutloff
- Lapo Bogani
- Martin Dressel
- Petr Neugebauer
- Philipp Neumann
- Joris van Slageren
- Jörg Wrachtrup
- Andrea Zappe
Uwe Gerstmann: Ab initio calculation of EPR parameters for extended periodic systems – functionalization of surfaces and interfaces
Magnetic resonance spectroscopy comprises some of the most powerful analytic tools in materials science available to date. The ongoing development of this experimental technique will significantly widen the scope of the method and the information that can be extracted. In this project, we want to enhance the accuracy of calculated EPR parameters, necessary for a complete analysis of the experimental spectra. The systems to be investigated will range from silicon-related surfaces and interfaces, as e.g. realized in solar cells, to metal-organic macrocycles like porphyrins adsorbed on various substrates.The geometry and electronic structure of the molecules are strongly influenced by the substrate. Hence, the substrate has to be included explicitely into the theoretical modelling, e.g. by periodic boundary conditions. This necessitates serious methodological extensions. In tight collaboration between theory and experiment, our new concepts will lead to a more reliable interpretation of the experimental data. The resulting better understanding of the physics behind the magnetic fingerprint of the spin systems will lead to optimized sample preparation and, by this, to increased experimental sensitivity.
Dr. Uwe Gerstmann
Dr. Eva Rauls
Vladimir Dyakonov, Andreas Baumann, Andreas Sperlich: Development of an unconventional electron paramagnetic resonance detection technique for selective probing of photogenerated and extracted charge carriers inoptelectronic and photovoltaic devices by means of pulsed field induced extraction
In the first funding period we successfully developted the envisioned EPR method OTRACE-EDMR for a selective probing of photo-generated and extracted charge carriers in opto-electronic and photovoltaic devices by means of pulsed, field-induced extraction under EPR conditions. Our method determines quantitatively the impact of spin-dependend recombination on state-of-the-art thin film organic solar cells. In the second funding period, we will widen our focus and apply this method additionally to a variety of material systems and devices, such as organic-inorganic hybrid solar cells and OLEDs.
Prof. Dr. Vladimir Dyakonov
Dr. Andreas Sperlich
Dr. Andreas Baumann
High-resolution electron paramagnetic resonance (EPR) spectroscopy has reached a level of sophistication that allows for a multitude of small hyperfine and quadrupole couplings to be experimentally determined. In principle, this data contains a wealth of selective geometric structure information about the paramagnetic center and its surrounding. Two severe problems arise: (a) proper data reduction through computer simulation of the experimental spectra and (b) interpretation of the results in terms of molecular geometric and electronic structure. Both problems need to be thoroughly addressed in order for EPR spectroscopy to develop into an even more powerful structure determination tool than it is today. Quantum chemistry is a powerful partner of experiment in approaching both fundamental problems. Quantum chemical calculations provide full sets of spin-Hamiltonian (SH) parameters for anygiven structure (or structural proposal) that can be used as starting points for the computer simulation of the actual spectra. One arrives in this way at a reliable assignment of individual hyperfine and quadrupole tensors to individual nuclei. This is of critical importance as the number of parameters required to completely fit high-resolution spectra quickly becomes unmanageable. Secondly, once it is established that the calculations provide a realistic picture of the spin distribution, the results can be interpreted in terms of geometric and electronic structure. Hence, molecular level insight can be obtained that serves as a basis for understanding molecular reactivity or molecular properties. Todays quantum chemical approaches to EPR spectroscopy are strongly dominated by density functional theory (DFT).However, there are still significant method inherent errors in these calculations that arise from unknown shortcomings of the density functionals used. Rather than trying to engage in the endeavor of empirically trying new functionals, we propose an ab initio wavefunction based route to EPR parameters that is based on the extremely powerful and accurate coupled cluster (CC) theory. It is well known that CC theory converges quickly to the exact solution of the molecular Schrödinger equation. Already at the level of only incorporating single- and double- excitation operators the results for molecular properties, especially hyperfine couplings are known to be excellent. Yet, the computational effort to obtain these results is unmanageable for larger molecules and grows as the sixth power of the system size. In this proposal we propose to combine coupled cluster linear response theory with the concept of local pair natural orbitals in order to arrive at a new and systematically accurate theory for the calculation of EPR properties. Collaborative applications inside the SPP are evident and will initially focus on the experimental group Prof. Marina Bennati.
Prof. Dr. Frank Neese
This project aims to establish electrically detected magnetic resonance (EDMR) on nano-scale devices. The EDMR method is sensitive to spin-dependent transport processes in electronic devices. Its sensitivity is much higher than that of traditional, microwave-photon detected, electron spin resonance (ESR). Using nano-devices will lead to even higher sensitivity by confining the transport paths to one dimension. It will also help distinguish between bulk and interface effects and open new ways to control EDMR signals, e.g., using electrostatic gates. This collaborative project will combine the expertise of two groups with prior experience in EDMR (JGU Mainz) and nano-devices (FZ Jülich). We will focus on supramolecular fullerene nanowires. Research will progress from network structures containing inner contact interfaces between overlapping wires to individually contacted wires. Furthermore, controlled magnetic doping will be introduced to EDMR to pinpoint locations at which the spin-dependent processes occur. Given sufficient sensitivity, few or even single deliberately engineered magnetic impurities may be detectable using this approach.
Dr. Wolfgang Harneit
Dr. Carola Meyer
The proposal describes the initiation of a collaboration between the groups of Edward Reijerse (MPI-Mülheim), Anton Savitsky (MPI-Mülheim) and the group of Dieter Suter (University of Dortmund) to boost the sensitivity of Electron Paramagnetic Resonance (EPR) spectroscopy for very small samples. The expertise of the “Suter group” in the design and construction of planar micro-resonators is combined with the experience at the MPI to design and construct EPR probeheads for “special applications”, i.e. experiments for which commercial probeheads are not suited. The new microresonators will be used to investigate very small samples, in particular protein micro-crystals (typical dimensions: 0.2x0.2x0.4 mm3) of hydrogenases and photosystems, at X- and Q-band frequencies (i.e. 9.5 and 34 GHz) using CW and pulsed EPR. A prototype 244 GHz probe-head at the MPI will be further developed so that the same micro-crystals can also be studied at very high microwave frequency. Step by step the resonators will be expanded to allow double resonance measurements (ENDOR) and light access. Several “spin-off” developments in this project are anticipated: 1) An accurate EPR Gaussmeter based on the micro-resonator design. 2) The integration of a cryogenic preamplifier and further electronics in the micro-probehead to boost its sensitivity. 3) A micro-strip resonator for the detection of paramagnetic monolayers, e.g. on a gold electrode. Since the design and production of micro-resonators is also relevant for other applicants in the priority program this task will be described in a “central application” by the group of Dieter Suter. The developed micro-resonator probeheads will open completely new possibilities to study very small proteins crystals which were inaccessible to EPR up to now. The micro-resonator based EPR Gaussmeter will solve the long standing problem of accurate and dynamic field monitoring in superconducting magnets.
Dr. Edward Reijerse
Dr. Anton Savitsky
Dr. Hideaki Ogata
The aim of this project is to further improve the sensitivity of high frequency electron paramagnetic resonance (HFEPR) techniques. The improvements will enable frequency domain measurements on samples with small numbers of spins, allowing investigation of real-life samples such as metalloenzymes, (nanostructured) thin layers and heterogeneous catalyst nanoparticles. We will build on the achievements from the first funding period, where we were able to improve the sensitivity of terahertz frequency domain magnetic resonance spectroscopy by three orders of magnitude.
This was accomplished by successfully implementing a field-modulation scheme, as well as by largely removing standing waves from the terahertz beam. In the continuation of this project, we expect to gain a further two orders of magnitude sensitivity improvement. This will be realized by further elimination of standing waves, as well as incorporation of a broadband tunable Fabry-Pérot resonator. We aim to achieve a 106-spin-sensitivity.
Secondly, we will implement terahertz rapid scan EPR both in field and frequency domains for the first time. In the frequency domain, this will speed up measurements by four orders of magnitude, compared to conventional field-swept high-frequency EPR techniques. In addition, these measurements will permit us to access the spin dynamics of the systems at terahertz frequencies, which is otherwise not easily accessible. Thirdly, we will perform measurements on real life samples such as thin layers of molecular quantum bits and molecular nanomagnets, as well as metal- oxide catalyst nanoparticles. Finally, we will implement the novel method of terahertz optically detected EPR, where the optical detection will be implemented through measurement of the magnetic circular dichroism.
Dr. Petr Neugebauer
Prof. Dr. Joris van Slageren
Martina Havenith, Erik Schleicher, Stefan Weber: Light-induced magnetization detected by force microscopy: from basic concept to first applications
The aim of this proposal is to extend the existing magnetic-resonance force microscopy detection scheme by application to light-generated, short-lived triplet states as paramagnetic probes. For this purpose, initially diamagnetic chromophores are immobilized on a surface and are photo-excited into their triplet state (S = 1). This novel detection scheme potentially has the following advantages: (i) generation of triplet states by photo-excitation results in strong initial electron-spin polarization, and hence strong magnetization, on a microsecond to millisecond time scale even at comparatively high temperatures; (ii) the scheme allows repetitive photo-excitation thus leading to significantly enhanced signal-to-noise ratio; (iii) the combination of light-excitation and cantilever-detection minimizes unwanted perturbations due to light scattering or background fluorescence, and (iv) stable paramagnetic probes that are susceptible to secondary chemistry are not needed. After successful implementation of the method, we will test two applications: First, the setup will be used (i) to quantify aptamer–substrate binding constants and (ii) to scan triplet-labeled cells for protein localization purposes. With the results, we aim towards all-purpose applications.
Prof. Dr. Stefan Weber
Dr. Erik Schleicher
Prof. Dr. Martina Havenith
Martin Brandt, Martin Stutzmann: New concepts for high-sensitivity pulsed electrically detected magnetic resonance
The electrical detection of magnetic resonance (EDMR) has recently developed into a highly versatile tool which allows the detailed investigation of spin-dependent charge transport processes and paramagnetic defects in semiconductor materials and devices down to the nanometer scale with demonstrated sensitivities of as few as 50 spins. In this project, we combine the latest advances of pulsed EDMR with pulse shaping and optimal control theory to increase the sensitivity and spectroscopic application range of pulsed EDMR. Furthermore, we develop new broadband to deliver pulsed microwaves to the sample, which are expected to lead to a significantly more efficient use of microwave power and to a reduction of the complexity and cost of pulsed magnetic resonance. The improved methods shall be applied to the study of spin-dependent recombination in materials used in thin-film photovoltaics and at silicon/silicondioxide interfaces relevant to microelectronic devices.
Prof. Dr. Martin S. Brandt
Prof. Dr. Martin Stutzmann
While composite and shaped pulses are basic building blocks in state of the art NMR experiments, virtually all pulsed EPR experiments are still performed using simple rectangular pulses. Optimal control theory makes it possible to explore the physical limits of pulse sequence performance and to develop practical sequences that approach these limits with the required robustness with respect to experimental imperfections. The application of such sophisticated and powerful modulated pulses in EPR spectroscopy has become only recently possible due to the availability of fast hardware for microwave pulse shaping. We will develop optimized amplitude and phase modulated pulses that take relaxation and experimental limitations of typical EPR settings into account. Initially, we will focus on applications such as excitation and inversion pulses, where spin-spin coupling effects can be neglected during the pulses. However, as we progress to more sophisticated experiments, such as PELDOR, DQEPR, HYSCORE or electron-nuclear polarization transfer, also the coupling between spins has to be taken into account and the novel concept of cooperative pulses will be exploited, which are designed not only to compensate their own imperfections but also each other's imperfections.
Prof. Dr. Steffen Glaser
This project follows two main approaches: (i) the improvement of the EPR detection sensitivity by employing ultra-sensitive microwave analysis techniques as well as optimized low-loss and downsized microwave detection circuits adapted from the field of circuit quantum electrodynamics, and (ii) an improved control over the spin ensembles by employing optimized pulse sequences. Harnessing these new developments is expected to yield increased detection sensitivity, as well as higher signal-to-noise ratio corresponding to shorter acquisition times. These improvements shall be applied to solid state EPR at millikelvin temperatures, and in particular to defects in silicon.
Dr. Hans Hübl
As our primary goal within this project, we will study efficient readout schemes of the electron spin of single NV centers in diamond, aiming at projection noise limited spin readout in nanoscale magnetic resonance applications. The NV center in diamond is routinely employed as an atomic-size magnetic field sensor to read out NMR and EPR signals of nanoscale spin ensembles such as single biomolecules. Its electron spin is traditionally read out by non-resonant fluorescence, an approach which is technically easy to implement, but highly inefficient, requiring as much as 1000 experimental repetitions for a single spin readout. To improve on this issue, we will implement and study different readout techniques, such as readout assisted by local nuclear spin quantum memories or advanced multi-color laser pulse schemes. Although we will focus on the application of these schemes in nanoscale magnetic resonance applications, we expect our results to be of relevance for several adjacent fields, such as quantum information processing or electric readout of electron spins in other materials than diamond (EDMR).
Dr. Friedemann Reinhard
Microwave pulse excitation bandwidth is of crucial importance for pulse electron paramagnetic resonance spectroscopy. For commonly used rectangular pulses the excitation bandwidth is limited by the accessible microwave power. Unfortunately, for many interesting applications, the spectral width by far exceeds the achievable excitation bandwidth. Therefore, sensitivity and significance of such pulse experiments are reduced. One possibility to further increase the excitation bandwidth is via sideband generation, achieved by amplitude- and phase modulation of the microwave. Only recently, fast enough arbitrary waveform generators have become available. We implemented such a versatile pulse shaping unit into our EPR spectrometer. First broadband pulse sequences have been developed in tight collaboration with the group of Prof. Steffen Glaser (TU Munich), implemented and tested in our group. Optimum control theory methodology has been used to generate such broadband excitation pulses. Within this priority program we want to develop new pulse sequences for inversion pulses. This will be done in collaboration with Prof. Steffen Glaser (TU Munich). The pulses will be tested, evaluated and optimized on model systems and finally applied to biological questions.
Prof. Dr. Thomas Prisner
Andreas Pöppl: Paramagnetic adsorption sites in microporous crystalline solids studied by electron paramagnetic resonance spectroscopy – from single crystals to oriented thin films
Technological applications of microporous materials in molecular sieving and catalytic processes rely more and more on thin films and membranes instead of widely used pellets made from pressed powders. This trend in engineering sets new challenges for the spectroscopic investigation of aluminosilcate and aluminophosphate thin films and small single crystals including the characterization of paramagnetic ions and their interaction with adsorbates and reactants by EPR spectroscopy. Such information is of uttermost importance for the understanding of the adsorptive and catalytic properties of the porous materials. In order to meet this challenge, X-band cw EPR probe heads with higher spin sensitivity will be developed for the studies of such oriented thin films and small crystals in vacuum and at low temperatures. The probe head design will be based on two strategies: (A) the adoption of the planar micro resonators recently introduced by D. Suter and (B) the use of dielectric resonators with high permittivity accommodated together with the samples in sealed quartz containers. By extending EPR spectroscopy to small single crystals and oriented thin films information about the orientation of the magnetic interaction tensors with respect to the crystallographic axes frame will become available and in that way will offer significantly improved opportunities to determine the cation sites and the structure and dynamics of the adsorption complexes formed in zeolite and AlPO porous materials.
Prof. Dr. Andreas Pöppl
Lapo Bogani; Martin Dressel, Joris van Slageren: Pushing the limits of torque-detected electron-spin resonance
This interdisciplinary project aims to develop and employ a highly sensitive, broadband, highfrequency electron spin resonance spectrometer based on torque magnetometric detection. Starting from our recent preliminary work, we will first extend the applicability of torque-detected ESR, and optimize the spectrometer as well as the measurement protocol. The implementation of zero-deflection measurement capabilities will increase the dynamic range, while soft cantilevers and alternative detection schemes improve the sensitivity. This will enable us to carry out highly sensitive frequency-, field-, and angle-dependent measurements at high-frequencies on “real life” samples with limited numbers of spins. These samples include surface arrays of spin centers, and high-spin, high-anisotropy centers in the active sites of metalloproteins.
Prof. Dr. Martin Dressel
Prof. Dr. Joris van Slageren
Dr. Lapo Bogani
Marina Bennati: Sensitivity enhancement in EPR and electron nuclear double resonance by pulsed polarization schemes
In this proposal we plan to examine pulse polarization schemes to enhance the sensitivity of EPR and specifically electron nuclear double resonance (ENDOR) experiments. This latter technique is fundamental in mechanistic studies of enzyme catalysis, such as in ribonucleotide reductase (RNR), to elucidate the structure of formed radical intermediates obtainable only in very small volumes and concentrations (<< 100 µL and << 100 µM). Currently, these investigations are considerably aggravated by the well-known low sensitivity of pulsed ENDOR experiments, which is about 1 to 2 orders of magnitude smaller than in the respective electron spin echo (ESE) experiment. To address this issue, pulsed polarization schemes derived from nuclear magnetic resonance and dynamic nuclear polarization, such as the cross-polarization (CP) technique, will be extensively tested to optimize coherent polarization transfer between electron and nuclei. In a second step, these schemes will be extended to detect the generated nuclear polarization in an ENDOR-type of experiment that measures the hyperfine spectrum of nuclei interacting with the electron spin. The overall performance (sensitivity and resolution) of a so-called CP-ENDOR experiment will be compared with traditional Davies and Mims ENDOR in applications on real samples, such as in the spectra of the radical intermediates in RNR catalytic cycle.
Prof. Dr. Marina Bennati
Robert Bittl, Alexander Schnegg: Sensitivity improvement in variable very high frequency EPR for applications in catalysis and protein research
High spin transition metal ions (TMIs) important in: a) oxygen transport proteins; or b) photochemical water oxidation catalysis (“water splitting”) will be characterized by novel variable very high frequency EPR methods. TMI complexes frequently exhibit paramagnetic metal cores with multiple unpaired electrons in high spin states. Due to instrumental constraints, investigations of high electron spin states exhibiting large zero field splittings (ZFS) have been limited until now. To lift this restriction, dedicated very high frequency (263 GHz) EPR and Frequency Domain Fourier Transform THz-EPR (FD-FT THz-EPR) methods with increased detection sensitivity and application range will be established. Instrumental upgrades of a synchrotron based FD-FT THz-EPR detection system and the development of novel FD-FT THz-EPR and 263 GHz EPR probe heads will allow single crystal EPR measurements on protein samples and in-situ characterization of high spin ions during water oxidation catalysis, which can involve application of electrical potential and light. Numerical EPR routines based on the public domain EPR simulation package EasySpin will be developed to simulate FD-EPR spectra. The intention is to apply the unique facilities at HZB, in particular FD-FT THz-EPR and 263 GHz EPR to provide information on high spin Mn complexes, currently being developed for photocatalytic “water splitting” as well as high spin Fe in respiratory proteins.
Dr. Alexander Schnegg
Prof. Dr. Robert Bittl
Prof. Dr. Enrica Bordignon
This project focuses on the implementation of an electrical detection scheme to investigate defect centers on single crystal surfaces under well defined under ultrahigh vacuum (UHV) conditions. The properties of surface and interfacial defects are intimately correlated with the performance of semiconductor devices. However, due to a lack of in-situ analytics with sufficient sensitivity the impact of these modifications on the surface defects become very difficult to assess. To this end, we will implement an in-situ EDMR experiment to an UHV-apparatus to investigate defects on semiconductor surfaces starting with defects low index silicon surfaces and their reactivity with gases such as hydrogen and oxygen. In a second step this strategy will be expanded to electrically active defects at the surface of SiC.
Prof. Dr. Thomas Risse
Methods based on the electron spin resonance (ESR) effect are amongst the most powerful analytical techniques in medicine as well as in the natural and material sciences because they enable to study the structure, dynamics and spatial distribution of paramagnetic species in a large variety of samples. In this context, the main goal of this project is to improve and/or to enhance the functionality of existing ESR-instruments for two specific applications from the fields of life science and material science by efficiently using the capabilities of modern integrated circuit (IC) technologies for the manufacturing of miniaturized, highly sensitive detectors for inductive and electrical measurements of the ESR effect. The first main subproject aims at developing a user-friendly, portable and yet highly sensitive Ku-band point-of-care (PoC) ESR spectrometer for the ESR based analysis of oxidative and nitrosative stress in whole blood samples using the spin trapping method. In cooperation with end users from the medical school and the psychology department in Ulm, we are developing an ESR spectrometer, which will make ESR spectroscopy as the gold standard for the assessment of oxidative and nitrosative stress accessible to a large community of clinical end users by featuring both a high user-friendliness and an excellent spin sensitivity. The second subproject deals with the development of integrated circuits for combined ESR-EDMR experiments. To both enhance the sensor sensitivity and render it more robust against external interferences, we will co-integrate the transimpedance amplifier required for the EDMR detection together with the inductive detector on a single integrated circuit, resulting in a highly miniaturized, highly sensitive system for the combined measurement of inductive ESR and EDMR signals. In summary, in this project, we will design two electron spin detection systems which optimally exploit the capabilities of modern nanometer scale integrated circuit technologies to create sensors with high degrees of miniaturization and improved user-friendliness combined with an excellent sensitivity.
Prof. Dr. Jens Anders
Fedor Jeletzko, Martin B. Plenio, Jörg Wrachtrup: Single spin EPR and NMR with diamond atomic spin sensors
Sensing small amounts of electron and nuclear spins has a very wide application in chemistry, physics and biology. In this work we plan to develop novel type of spin detection techniques, which will allow to reach ultimate sensitivity and detect a single electron and nuclear spins at ambient conditions as well at cryogenic temperatures. Moreover, since we use an atomic sized single optically active spin in diamond as detector, we will be able to image spins with nanometer resolution by using scanning probe techniques. In order to realize this goal, we will first theoretically design novel sensing methods that will not be disturbed by the presence of unwanted spin fluctuations. This will allow the detection of single spins of interest even in complicated biological environment, where the spin noise is unavoidable. These new techniques will be demonstrated experimentally by detecting spin labels on diamond in controlled conditions.
Prof. Dr. Fedor Jelezko
Prof. Dr. Martin B. Plenio
Prof. Dr. Jörg Wrachtrup
Klaus Lips, Alexander Schnegg: Spin-dependent transport in fully processed silicon solar cells studied by pulsed multifrequency electrically detected magneticresonance below 600 MHz / 20 mT and at 263 GHz / 9.4 T
In both, thin-film silicon and wafer-based crystalline silicon (c-Si) solar cells, a 10 nm thin hydrogenated amorphous silicon (a-Si:H) layer acts as charge selective hetero contact. a-Si:H layers effectively passivate interface defects, still a small sub-ensemble of paramagnetic defects remain and influence the solar conversion efficiency. Since such heterointerfaces are also present in absorberlayers of microcrystalline silicon (μc-Si:H) their detailed understanding is a prerequisite for optimization strategies leading to a-Si/c-Si and TFS solar cells with higher efficiency. To access interface defects in state of the art silicon solar cells, the sensitivity limit of EPR has to be lifted and spectral resolution hasto be improved. In the proposed work, we combine recent breakthroughs in the field of pulsed electrically detected magnetic resonance (pEDMR) with the availability of high-field EPR spectrometers to study spin-dependent transport processes by pEDMR at resonance conditionsranging from below below 600 MHz/20 mT up to 263 GHz/9.4 T on the same solar cell. This approach will yield the following benefits. 1.) Separation of overlapping resonance signals and the extraction of structural information via g- and hyperfine tensors and their interpretation by density functional theory(DFT) calculations; 2.) Insight into spin-dependent transport and recombination processes by field and temperature induced switching between different transport an recombination regimes.
Prof. Dr. Klaus Lips
Dr. Alexander Schnegg
Within the scope of this project we intend to develop a fundamental understanding of charge transfer and charge transport in organic solar cells. For this purpose we will employ suitable EPR-based techniques. In particular, we will develop the experimental method of transient electrically detected magnetic resonance (transient EDMR). Combining the sensitivity of EDMR and the time resolution of transient EPR will provide us with the possibility to systematically investigate the interplay between charge transfer complexes, which are generated upon photoexcitation and subsequent charge transfer between donor and acceptor, and the photocurrent in polymer: fullerene blends. In a first step we will study the magnetic properties of these intermediate complexes in material combinations incorporated in fully processed efficient organic solar cells. We will further investigate the crucial role of triplet excitons in organic solar cells and analyse whether they influence the photocurrent via loss mechanisms or whether the long lifetime of triplet excitons can be utilized to increase the yield of free charge carriers.
Prof. Dr. Jan Behrends
Typically only a fraction of spins is excited in electron paramagnetic resonance (EPR)spectroscopy, with this fraction being particularly small for metal centers. Compared to hypothetical experiments that excite all spins in an optimal manner with respect to their relaxation behavior, the established experimental schemes entail a large reduction in sensitivity. This reduction results from the problem that even with the shortest achievable rectangular pulses and the highest achievable microwave powers, excitation bandwidth is much smaller than typical spectral widths. With the advent of arbitrary waveform generators with time resolution in the microwave range during the past few years, this problem can potentially be solved by variation of the excitation frequency throughout the whole required range. However, such an approach requires redesign of existing experiments or even design of new experiments from scratch to obtain the same information as before with higher sensitivity. In addition, the excitation waveform needs to be optimized with respect to the technical parameters of the spectrometer and to the properties of the spin system. The proposed project aims to develop such new experimental schemes and to introduce general methodology for such developments.
Prof. Dr. Gunnar Jeschke
Goal of the project is widening the applicability of electron paramagnetic resonance spectroscopy (EPR) under in-cell conditions. We will work on determining optimum conditions regarding microwave resonator type, and frequency band on the EPR side as well as regarding expression systems and cell types on the biological side. The studies on resonator types will be performed in close collaboration within the priority program "New Frontiers in Sensitivity for EPR Spectroscopy: From Biological Cells to Nano Materials" with the core project of D. Suter. Based on our experience with in-cell EPR on naturally occurring paramagnetic states in proteins containing organic radicals and transition metal center cofactors, we have chosen two specific examples in order to show the relevance of in-cell EPR. The first is the class of blue-light photoreceptors and the second is the class of oxygen tolerant hydrogenases. As a long term goal, together with Suter's group having the necessary know-how in microresonator technologies an attempt will be made to push in-cell EPR to the limit of single-cell detection.
Prof. Dr. Robert Bittl
Dr. Christian Teutloff
In EPR spectroscopy, the microwave resonator has 2 functions: it (i) converts microwave power into microwave magnetic fields that excite the transitions between the electron spin states and it (ii) converts the oscillating magnetic flux, whose source is the precessing spin polarization, into a travelling electromagnetic wave that can be measured by the detection electronics. For this purpose, most conventional EPR spectrometers working at <50 GHz use standing wave resonators, whose volume must be ≈ λ3 mw, where λmw is the wavelength of the microwave. If the volume of the available sample is significantly smaller than this, the filling factor and the sensitivity of the spectrometer become small. One approach to boost the sensitivity consists in reducing the size of the resonator by designing it in the form of sub-wavelength resonant structures. The aim of this project is to develop optimized sub-wavelength resonators for specific applications in other projects of this priority program. Depending on the requirements of these individual projects, we will design suitable structures, simulate them using finite element software and manufacture them, using lithographic techniques. Testing will be done in collaboration with the projects that will use the resonators.
Dieter Suter: Broadband EPR in microresonators
EPR microresonators provide an enormous sensitivity boost for small samples. Another specific property of microresonators,compared to classical cavity resonators is their large bandwidth. This is an important asset for experiments that require short switching times of the exciting microwave field, fast modulation of amplitude, frequency or phase, or irradiation with multiple frequency components and / or short dead times. In the present project, we will develop the experimental techniques that utilize this potential for the detection of broad lines and rapidly decaying signals. In addition, we will also develop related broadband techniques that further increase the overall sensitivity of microresonators by using concurrent data acquisition for all spins in the resonator bandwidth. This additional sensitivity boost also exploits the large bandwidth of microresonators and compensates the sensitivity reduction due to the low quality factor. The techniques developed in this project will be useful for many other projects in this priority programme that use microresonators for sensitive detection of signals from small samples.
Prof. Dr. Dieter Suter
Prof. Dr. Heinz-Jürgen Steinhoff
Dr. Johann P. Klare
Gregor Hagelüken, Olav Schiemann: Trityl radicals: New spin labels for nanometer distance measurements with higher sensitivity at room temperature and within cells
Structural biology engages ever larger biomolecular systems in ever more complex environments. EPR spectroscopy offers for such studies methods that allow determining structures and structural changes on the nanometer scale by measuring the dipolar coupling between spin-centers. Promising are in this context Pulsed-Electron-Electron-Double-Resonance (PELDOR) or Double-Quantum- Coherence-(DQC) experiments. Many of the interesting systems are diamagnetic, which requires that they are spin-labelled with nitroxides. However, the chemical and EPR spectroscopic properties of these nitroxides lead to several limitations. In this project, we will functionalise trityl radicals in such a way that they can be used as spin labels for proteins. On model systems we will optimize the pulsesequences and measuring protocols for trityl-based distance measurements. We will use the one order of magnitude narrower spectral width to increase the signal intensity, to increase the PELDOR modulation depth to 100% and to enable routine DQC experiments. The TM-relaxation time on the microsecond time scale will enable us to perfome distance measurements under sample cooling with liquid nitrogen instead of liquid helium and in the long run allow for distance measurements in liquid solution at room temperature, meaning under truly biological conditions. The superior chemical stability of the trityl radicals, will be used for more sensitive in-cell experiments.
Dr. Gregor Hagelüken
Prof. Dr. Olav Schiemann
Adelheit Godt: Ln3+- and Ln3+/nitroxyl-labeled compounds for the development of EPR-based distance measurement techniques (DEER, relaxation enhancement)
Distance measurements by pulsed electron paramagnetic resonance techniques (DEER, PELDOR) have become very useful tools to elucidate the structure of (bio)macromolecules and their assemblies in a disordered, native state. For this purpose the molecules of interest are twofold site-selectively spin labeled with radicals. Usually nitroxyl radicals are used. A significant advancement of DEER in terms of sensitivity, measurement time, measurable distance range, precision, and data evaluation is expected from the recent development of high field EPR spectrometers. To gain full advantage of the high field other labels than nitroxyl radicals are needed. A very promising candidate is gadolinium in the oxidation state 3. To sound its potential tailor-made compounds are needed. The development and preparation of such model compounds of the type Gd3+-spacer-Gd3+ and Gd3+-spacer-(nitroxyl radical) is a primary goal of this project. The distance between the unpaired electron of the label and their geometrical arrangement will be well-defined and will be deliberately varied. With substituting Gd3+ by Dy3+ in Gd3+-spacer-(nitroxyl radical) the demand for model compounds to explore relaxation enhancement of nitroxyl radicals as a complementary EPR tool for distance measurement will be addressed. Moreover the syntheses of the model compounds will provide a construction kit and an instruction which will be of general use to assemble compounds which will support the further exploration and development of EPR techniques.
Prof. Dr. Adelheid Godt
Malte Drescher, Daniel Summerer: Distance measurements in the nanometer range by in-cell electron paramagnetic resonance spectroscopy
The direct observation of the function of proteins in their natural intracellular environment is a central goal of cell biology. In addition to the detection of intracellular localization processes the elucidation of protein structures and conformational dynamics is in the focus of current research. Intracellular electron paramagnetic resonance (In-cell EPR) spectroscopy of spin-labeled proteins offers unique features for these objectives, such as the measurement of absolute distance distributions between individual, strategically selected amino acids. This allows a comprehensive insight into the molecular architecture of proteins and protein complexes as the basis of their cellular function. On the other hand, EPR investigations of endogenous spin-labeled proteins directly in their natural intracellular environment have not been possible so far, because no cell-compatible approaches for this kind of protein spin labeling exists. To enable such studies for the first time, we will synthesize spin-labeled amino acids and develop aminoacyl-tRNA synthetases for their genetic encoding by directed evolution. We will examine the EPR spectroscopic properties of these amino acids and develop methods for their use in intracellular EPR spectroscopy in E. coli. We will use these insights to examine basic DNA recognition mechanisms of transcription activator-like effector (TALE) proteins in the non-crystalline state for the first time, both in vitro and directly in cells. TALE proteins have DNA-binding domains with a programmable sequence specificity and are key tools for the modification and analysis of genome functions.
Prof. Dr. Malte Drescher
Prof. Dr. Daniel Summerer