Archive of Research Highlights

Published as: “How Metallic Fe Controls the Composition of its Native Oxide”, Phys. Rev. Lett. 101 056101 (2008).

What happens to an ultrathin film of iron oxide when metallic iron is deposited on top of it? We have investigated this question by X-ray absorption spectroscopy performed in-situ during the deposition process. Typically, the native oxide is a non-stoichiometric mixture of different Fe oxide phases, containing even some Fe atoms in a metallic-like state. Deposition of metallic Fe on such an oxide leads to a reduction of the higher-valent oxides, leaving only iron atoms in a FeO-like phase. This approach allows one to prepare nanometre thin films of single-phase FeO. Further studies show that the Néel temperature of such FeO films is significantly enhanced compared to bulk FeO, and that its magnetic properties lead to remarkable coupling effects between two adjacent metallic Fe layers. [More]

Published as: “A Comparative Study on the Experimentally Derived Electron Densities of three Protease Inhibitor Model Compounds”, Org. Biomol. Chem. 6, 2295-2307 (2008).

In order to contribute to a rational drug design with ground-state electron densities, it is important to evaluate the deduced functions and properties that provide information on reactivity. Electrostatic potentials mapped on molecular surfaces and Zero-Laplacian iso-surfaces can be referred to as reactive surfaces because they show centres of nucleophilicity or electrophilicity. Derived from experimental densities, these surfaces are directly dependent on intermolecular interaction patterns within the crystal environment. Therefore, the information deduced from experimental electron densities are generally a better approximation for biological enzyme-ligand interactions than gas phase calculations or properties derived from the liquid phase. Within this study, we have determined ... [More]

Published as: "Structural basis of Dcp2 recognition and activation by Dcp1", Mol. Cell 29, 337-349 (2008)

Small-angle X-ray scattering (SAXS) is rapidly becoming a streamline tool in structural molecular biology providing unique information about overall structure and conformational changes of native macromolecules in solution. Of particular interest is the joint use of SAXS with the high resolution methods like crystallography to study functional complexes. For a Dcp1p-Dcp2n protein complex playing a major role in the process of messenger RNA degradation, crystallographic analysis combined with SAXS and functional studies revealed that Dcp2p exists in open and closed conformations, the latter being the catalytically more active form. This suggests a decapping mechanism controlled by a conformational change between the open and closed complexes. [More]

Published as: “Multistep Ionization of Argon Clusters in Intense Femtosecond Extreme Ultraviolet Pulses”, Physical Review Letters 100, 133401 (2008)

The soft X-ray free electron laser FLASH allows for the first time to probe the super intense light pulse – matter interaction at short wavelength, providing the basis for a large variety of studies with femtosecond X-ray pulses. We have performed a photoelectron spectroscopy study complemented with Monte Carlo simulations about the ionization dynamics of atomic clusters at FLASH at power densities exceeding 10¹³ W/cm². We find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. In contrast to earlier studies in the infrared and vacuum ultraviolet regime, there are no indications for efficient plasma heating processes. The current findings have large implications for the ionization and dissociation dynamics of ... [More]

Published as: “A chemically driven insulator–metal transition in non-stoichiometric and amorphous gallium oxide”, Nature Materials Vol. 7, 391-398 (2008)

Insulator–metal transitions are well known in transition metal oxides, but inducing an insulator–metal transition in the oxide of a main group element is a major challenge. We succeeded in demonstrating that highly nonstoichiometric, amorphous gallium oxide shows an insulator–metal transition, with a conductivity jump of seven orders of magnitude at a temperature around 670 K. Through experimental studies and density-functional theory calculations we confirmed that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga₂O₃. This novel mechanism – an insulator-metal transition driven by a heterogeneous solid state reaction within an amorphous matrix – opens up a new route to achieve metallic ... [More]

Published as: “Automated macromolecular model building for X-ray crystallography using ARP/wARP version 7”, Nature Protocols 3, 1171-1179. PMID: 18600222 (2008).

ARP/wARP is a software suite to build macromolecular models in X-ray crystallographic electron density maps. Structural genomics initiatives and the study of complex macromolecular assemblies and membrane proteins rely on advanced methods for 3D structure determination. ARP/wARP 7.0 meets these needs by providing the automated tools for: iterative protein model building including a high-level decision-making control module; fast construction of the secondary structure of a protein; building flexible loops in alternate conformations; placement of ligands, including a choice of the best-fitting ligand from a ‘cocktail’; and finding ordered water molecules. All protocols are easy to handle by a non-expert user and the time required is typically a few minutes, although iterative model ... [More]

Published as: “Mechanical Properties of Silk: Interplay of Deformation on Macroscopic and Molecular Length Scales”, Phys. Rev. Lett. 100, 048104, (2008)

Using an in situ combination of tensile tests and X-ray diffraction, we have determined the mechanical properties of both the crystalline and the disordered phase of the biological nanocomposite silk by adapting a model from linear viscoelastic theory to the semicrystalline morphology of silk. We observe a strong interplay between morphology and mechanical properties. Silk‘s high extensibility results principally from the disordered phase; however, the crystals are also elastically deformed. [More]

Published as: “Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies”, Nature Materials 7, 326 - 332 (2008)

Organic molecules provide fascinating possibilities for fabricating inexpensive novel electronic devices, organic light-emitting diodes, and flexible environmentally-friendly low-cost solar cells [1]. The efficiency of organic solar cells depends on the chemistry of the materials employed and critically on the electronic properties and nanomorphology of the molecules in the active layer [2]. We have used synchrotron light at HASYLAB to investigate how the orientation of the organic molecules in thin films affects the electronic structure [3]. [More]

Published as: “Massively parallel X-ray holography", Nature Photonics 2, 560–563 (2008)

The ultrafast pulses from short-wavelength free-electron lasers (FELs), such as FLASH, give us the possibility for imaging processes on the timescale of atomic motions. The upcoming X-ray FELs such as Linac Coherent Light Source and European XFEL will additionally allow us to probe dynamic processes approaching inter-atomic length scales. For many such experiments it is necessary to extract spatially-resolved information from a single pulse since sample damage may prevent stroboscopic methods. We have developed an extremely photonefficient method to form a real-space image of the object. Borrowing concepts from X-ray astronomy, our method of “Massively Parallel Holography” utilizes a special kind of coded pattern called a uniformly redundant array to form a hologram of the object. We showed that the pattern amplified the holographic signal of a bacterial ... [More]

Published as: "Visualization of a lost paintig from Vincent van Gogh using synchrotron-radiation based X-ray fluorescence elemental mapping"

Many paintings by Vincent van Gogh cover a previous composition, because the artist frequently re-used the canvas of abandoned paintings. Using synchrotron induced X-ray Fluorescence mapping we visualized a woman’s head hidden under the work Patch of Grass. The resulting elemental distributions and additional X-ray absorption spectroscopy on selected points on the painting enabled an approximate colour reconstruction of the hidden face. This proved to be the missing link in comparing the head with Van Gogh's known oeuvre. [More]

Published as: “Localization and loss of coherence in molecular double-slit experiments“ in: Nature Physics, VOL 4, 649-655 (2008)

Nitrogen molecules were irradiated with soft x-rays over a wide energy range. Because of this irradiation electrons are emitted from the molecule with a kinetic energy that depends on the choice of photon energy.  This means slow or fast, where the velocity determines the resolving power of the electron for its environment. The researchers succeeded in demonstrating that there is a transition between a cooperative and self-centred behaviour of the electron and that this behaviour depends on its kinetic energy during intramolecular scattering. Beyond a critical energy the scattered electron no longer superposes with its phase-coupled neighbouring electron, but with itself, thereby generating an image of its environment. This demonstrates that the electron’s cooperative and resolving powers are ... [More]

Published as: “Ultrafast single-shot diffraction imaging of nanoscale dynamics”, Nature Photonics 2, 415–419, (2008).

Combining an optical laser with FEL pulses from FLASH in a pump-probe set-up enables imaging with high temporal and spectral resolution. A nanostructure is ablated by an optical laser and this process is followed with lensless coherent diffractive imaging using FLASH pulses. A spatial resolution of better than 50 nm is achieved.  By taking pictures of a succession of exploding targets, a movie can be made following the dynamics of the solid material on a 10-ps timescale. With short-wavelength X-ray FEL sources this method will enable higher spatial resolution imaging and could be used to measure the dynamics of highly correlated systems at nanometer length scales.

Published as: U.E.A. Fittschen et al., Anal. Chem. (2008), 80, 1967-1977

Nowadays microscopic analysing techniques are easily capable of resolution of a few micrometers and beyond. The accuracy of the techniques rely on the comparison to standard reference material. However, reference materials which are homogeneous on the micrometer-scale are often not available or tedious to prepare. In contrast the preparation of dried residues derived from pico liter volumes using thermal ink jet printing technology discussed and characterized in this study is very simple and has the potential to become accessible to a wide variety of laboratories and may become wide spread in analytical science in the future.
In this paper we characterized the dried residues of pico liter droplets (ca. 130 pL) prepared by a slightly modified ink jet printer HP DeskJet 500C. The characterization was ... [More]

In recent years, continuous evolution of synchrotron radiation (SR) sources has resulted in a dramatic increase of brilliance with respect to older designs. Among the most exciting properties of third generation facilities of today is a high flux of coherent X-rays [1]. The availability of intense coherent X-ray beams has triggered the development of a number of new experimental techniques based on coherence properties of light such as scattering of coherent X-ray radiation, X-ray photon ... [More]

Published as: C. Gahl et al., Nature Photonics 2, 165-169 (2008)

For a fundamental understanding of ultra fast dynamics in chemistry, biology and materials science it has been a longstanding dream to record a molecular movie, where both the atomic trajectories and the chemical state of every atom in matter is followed in real time with time resolved pump/probe spectroscopy. X-ray free-electron lasers (FEL) provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range. To cross-correlate and synchronize the FEL with separate optical lasers we exploit the peak brilliance of the free-electron laser at Hamburg (FLASH) and establish X-ray pulse induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough en route to a molecular movie and – equally important – opens the novel field of femtosecond X-ray induced ... [More]

Published in Phys. Rev. B 75, 075111 (2007)

Research on new optical materials is fundamental for the development of various applications, such as laser systems, plasma TVs, or new detectors for medical diagnosis. Many of these materials make use of ions of rare earth elements (RE), which can absorb and emit light in the ultraviolet and visible wavelength range efficiently.

In our work, we have focused on the luminescence and excitation properties of 5d-4f transitions of gadolinium (Gd³⁺) and lutetium (Lu³⁺) ions doped into different fluoride crystals using high-resolution spectroscopy. We could analyze the excitation spectra using model calculations, and assign the fine structure observed in the spectra to different mechanisms. We regarded both pure electronic transitions within the RE ions and phonon-assisted transitions, in which the excited ions interact with the crystal lattice. While the 5d-4f transition ... [More]

Published as: J. Wenisch et al., Phys. Rev. Lett. 99, 077201 (2007).

Controlling the magnetic properties of semiconducting materials is certainly one of the key issues in the field of modern electronic device technology. An important material system in this context is (Ga,Mn)As, which has been the focus of many studies over the last years. As the understanding of its complex transport and magnetic properties increases, the focus of interest shifts from basic research towards its application in devices. For this, it is necessary to obtain a fundamental understanding of the nexus of magnetic properties and the structure of the materials at the device level. We present a systematic study of the role of strain relaxation in patterned (Ga,Mn)As nanostructures as the dominating factor contributing to the magnetic anisotropy. Before patterning, a (Ga,Mn)As layer shows biaxial easy axes along [100] and [010]. We then produce narrow stripes by lithographically ... [More]

Published as: S. Förster et al., Nature Materials 6, 888 (2007)

Shear forces are one of the most widely used methods to order and align soft crystals. Small-angle x-ray and neutron scattering experiments revealed for the first time the detailed structure of soft crystals. In particular, such crystals exhibit Bragg-peaks that were difficult to index assuming homogeneous crystal structures. Their origin had been attributed to changes in the crystal structure that are induced by the shearing process. We demonstrate that these Bragg-peaks are an inherent property of soft crystals related to the finite coherence of their crystalline lattice. Their consideration allows a detailed and quantitative analysis of the diffraction patterns of seemingly unrelated materials such as lyotropic liquid crystalline phases, mesoporous materials, colloidal dispersions, block copolymers, electro-rheological fluids, and photonic crystals. It further enables to develop a ... [More]

Published as: R. Terzano et al., Environ. Sci. Technol., 41, 6762-6769.

It is commonly recognized that soil is the major sink for heavy metal (HM) contaminants released into the environment by human activities and that the mobility, bioavailability and toxicity of metals strongly depend on their solubility and therefore on their geochemical forms. Soils contaminated with HM can cause serious risks to human health, e.g. if vegetables cultivated on contaminated soils are consumed. The correct identification of HM chemical forms in soil is therefore of paramount relevance for a proper risk assessment and for the formulation of effective remediation strategies.
The major geochemical forms of Cr, Ni, Cu, Zn, Pb, and V in a soil from an industrial polluted site in the South of Italy were determined by means of synchrotron X-ray microanalytical techniques. On the basis of the geochemical forms identified, among the others, two major former industrial ... [More]

Published as: A. Bóta et al., J. Phys. Chem. B, 111 (8), 1911 (2007).

The formation of the cadmium sulfide (CdS) particles in the gaps between the stacks of bilayers inside multilamellar vesicles is described illustrating a new pathway in the preparation of nanometer-scale particles. The in situ structural characterization of both the CdS particles and the vesicles as reaction medium was performed in the early and final states of the process by using anomalous small-angle X-ray scattering (ASAXS) and freeze-fracture methods. The ASAXS method provides the separation of the scattering of nanoparticles present in low amount whereby the monitoring of their formation and growth in the whole time range of manufacturing become possible. [More]

Published in : Phys. Rev. B 75, 212402 (2007), Phys. Rev. B 75, 020101(R) (2007), Phys. Rev. B 73, 020102(R) (2006)

Multiferroic compounds are materials that exhibit two or more switchable states, such as polarization, magnetization or strain which are again interdependent from each other. Magneto-electric multiferroics have recently attracted a lot of interest due to the exceptional properties which allow the control of spontaneous ferroelectric polarization by applied magnetic fields, ultimately leading to new types of magneto-electric devices. Unlike in conventional ferroelectrics, in the manganite perovskites, such as TbMnO3 and DyMnO3, ferroelectricity arises from a peculiar coupling of the lattice to a spiral ordering of Mn-spins. A strong anisotropic dependence of the spontaneous electric polarization on the direction of the magnetic field is found which can be related to the spiral ordering of the magnetic moments. The work presented here is a ... [More]

Published in: Physical Review A 74, 011401(R) (2006)

The Free electron LASer in Hamburg (FLASH) is a unique source for intense, ultra-short extreme-uv (xuv) light pulses. In view of these unprecedented characteristics, an associated time-synchronized optical laser facility opens up new and particularly exciting research opportunities. First characterization of this pump-probe set-up was obtained recently via time-dependent two-photon experiments on rare gases. The process of above threshold ionization (ATI), which is very sensitive to the spatial and temporal overlap of both pulses, was used to demonstrate the capability and the stability of the combined application of two independent femtosecond laser sources, namely the XUV free electron laser and the synchronized optical laser. [More]

Published in: Appl. Phys. Lett. 88, 083114 (2006)

Large scale arrays of polymer nanochannels are important tools for many applications, such as micro-fluidics and fabrication of bioanalytical assays. Polymer nanochannels are defined as the alignment of polymeric material in walls with negligible interconnection in perpendicular direction. We present here a new, unexplored route to create such polymeric arrays. Their large-scale order as well as their orientation with respect to the underlying substrate is probed using high-resolution grazing incidence small-angle x-ray scattering. [More]

Published in: Nature Physics 2, 839 (2006)

Using the FLASH facility we have demonstrated high-resolution coherent diffractive imaging with single soft-X-ray free-electron laser pulses [1]. The intense focused FEL pulse gives a high resolution low-noise coherent diffraction pattern of an object before that object turns into a plasma and explodes. Our experiments are an important milestone in the development of single-particle diffractive imaging with future X-ray free-electron lasers [2,3]. Our apparatus provides a new and unique tool at FLASH to perform imaging of biological specimens beyond conventional radiation damage resolution limits [2, 4] and to acquire images of ultrafast processes initiated by an FEL pulse or other laser pulse. [More]

Published as: G. Catalan et al., Phys. Rev. Lett. 96, 127602 (2006)

Ferro-piezoelectrics are the active elements in a very large number of applications from gas lighters to alarm buzzers. Their properties arise from the presence of a spontaneous polarization. Thin films of the classical ferroelectric PbTiO₃ subjected to a small tensile strain have been successfully grown unit-cell-by-unit-cell and investigated by using x-ray diffraction. This work demonstrates that i) controlling the growth of thin films at the atomic level allows creating novel phases not existing in bulk, and thus fine-tuning the functional performance through symmetry engineering at the unit-cell level, and ii) x-ray analysis of diffuse scattering around diffraction peaks is a powerful tool to establish the polar symmetry of ultra-thin ferroelectric films, a notoriously difficult problem in this field. [More]

Published as: J.-D. Grunwaldt et al., J. of Phys. Chem. B 110, 8674 (2006)

Structure-activity relationships gained by studying catalysts at work are considered the key to further development of homogeneous and heterogeneous catalysts. In situ X-ray absorption spectroscopy (XAS) is a well-established technique to study catalytic reactions, since it can monitor chemical states of both crystalline and X-ray amorphous structures even in gas phase, liquid phase or at high pressure. However, spatial variation of the structure within a catalytic reactor has hardly been investigated up to now. Here, we report on 2D-mapping of the structure of a heterogeneous catalyst using an X-ray camera for acquisition of X-ray absorption images. As title reaction, the partial oxidation of methane over Rh/Al₂O₃ was studied, which is a promising reaction for the production of hydrogen from natural gas. The X-ray camera was installed behind the ... [More]

Published as: P. Korecki et al., Physical Review Letters 96, 035502 (2006)

Conventional x-ray crystallographic methods, which are used for almost 100 years, detect x-rays diffracted by the crystal planes. In a recent paper it was demonstrated that the atomic structure can be imaged directly from real-space projections sensed by absorbing atoms inside a crystal. The use of white x-rays allowed neglecting the diffraction effects and treating the x-ray beam as a searchlight which directly produces x-ray projections of the main atomic planes in the crystal. The recorded x-ray patterns were processed in a way similar to the tomography technique which is used e.g. for 3D visualization of macroscopic objects in medical imaging. [More]

Published as: P. Zou et al., Nature 439, p. 229 (2006)

The Z-disk of striated and cardiac muscle sarcomeres provides the architectural framework to assemble and anchor the largest known muscle filament systems by an extensive network of protein-protein interactions. High resolution X-ray diffraction data unravel how the N-terminus of the longest filament component, the giant muscle protein titin, is assembled into an antiparallel (2:1) sandwich complex by the Z-disk ligand telethonin. The structure provides a molecular paradigm of how major sarcomeric filaments are cross-linked, anchored, and aligned within complex cytoskeletal networks. [More]

Published as: D. Rolles et al., Nature 437, p. 711-715 (2005)

One of the most counterintuitive aspects of quantum physics is that for some scenarios which are perceived as “either or” in everyday life, there is also the "as well as" possibility. A natural example of such behaviour, however, has now been seen in molecular nitrogen where each electron, even a highly localized core electron, is on both atoms of the molecule at the same time. When the molecule is ionised, this gives rise to coherent electron emission from both ends of the molecule, as in the single electron double-slit experiment. This coherent character of electron emission in such molecules has been demonstrated for the first time by scientists from the Fritz-Haber-Institute using ionizing radiation from synchrotrons at HASYLAB (DORIS III) and BESSY. They could also show that distortions of the mirror symmetry in these molecules by isotope substitution lead to a loss of coherence, ... [More]

Published as: M. Diaconu et al., Cell 121, 991-1004 (2005)

Genomes of rather divers species like humans, mice or worms have been found to be amazingly similar. In these and all other organisms, the genetic information is translated by the ribosome, in a way that the small but crucial differences are accurately maintained. To understand how the ribosome works, and how antibiotics block specifically bacterial ribosomes, molecular structures of the ribosome and the related co-factors are essential. In a collaborative study employing different physical and biochemical methods we were able to add another piece to the ribosomal puzzle, by a detailed characterization of one of the key elements of the ribosome, the so called L7/12 lateral stalk of the ribosome. [More]

Published in: Physical Review Letters 95, 113401 (2005)

Clusters and nanocrystals are of great scientific and technological interest due to their size dependent properties. Changes in their electronic structure have been successfully described with the quantum confinement model, which predicts increasing band gaps with decreasing particle sizes due to shifts of the band edges towards higher energies. Now, soft x-ray absorption measurements of diamond clusters show that the bulk-related unoccupied density of states do not exhibit any such shift. Moreover, spectral features from the cluster surface termination appear below the bulk absorption onset, resulting in an effective shift of the lowest unoccupied states towards lower energies. These findings contradict the commonly used quantum confinement model. [More]

Published in: Physical Review Letters 94 (2005) 04780

Water is the most important liquid on earth and it may be surprising therefore that water still keeps some of its secrets. The average static properties of a system of molecules, such as pressure or the average structure, should at a given temperature not depend on the total mass of the molecules or the mass distribution if the interaction is classical. Contrary to that expectation, isotopic forms of water, D₂O and H₂¹⁸O exhibit different static properties and a different static structure. These structural differences have been investigated. Simulations indicate that the observed changes are best interpreted as decrease in size of the hydrogen bonded tetrahedron with increase in mass. The distance of non-bonded water molecules relatively close to the central molecule remains in contrast ... [More]

Published in: Embo J. 24(2) 251-260, 2005

The ribosome deciphers and translates the genetic code in all living organisms. This process - termed protein biosynthesis – requires several co-factors accelerating or catalyzing various steps. Termination is the last step of protein biosynthesis and requires the combined action of the Ribosome Recycling Factor (RRF) and the Elongation Factor G (EF-G). We recently determined the crystal structure of RRF bound to Deinococcus radiodurans 50S subunit at 3.3 Å resolution. These results provide a high-resolution structural description of the specific interaction of RRF with the ribosome and the conformational changes induced by RRF. The data permit to derive a accurate model for the synergistic action of EF-G and RRF during termination of protein biosynthesis and ribosome recycling. [More]

Published in: Europhys. Lett., 66 (3), pp. 331-337 (2004)

Polyelectrolytes are highly charged macromolecules. A large variety of such systems, where the role of electric charge is essential for the structure and proper functioning has been discovered in nature like for instance nucleic acids, numerous enzymes or self assembling proteins. Due to the high charge density, concentrated on the chain, polyelectrolytes show a strong interaction between the segments of the same chain and other macromolecules as well as they exhibit a characteristic counterion condensation around their charges. This in turn influences the conformational properties of the chains. Determination of the counterion distribution may thus help to better understand folding properties of polyelectrolytes and their specific functioning. The counterion distribution was analyzed by Anomalous Small-Angle X-ray Scattering (ASAXS). [More]

Published in: Catalysis Letters 90, 221-229, 2003 and Catalysis Today 91-92, 1-5, 2004

Supercritical fluids have recently attracted considerable attention in catalysis, extraction, separation, crystallization, and polymer technologies due to their unique physical properties. Although they have led to better catalytic performance in a number of heterogeneously catalysed reactions, fundamental understanding is still lagging behind and necessitates in situ spectroscopic studies at high pressure. For the first time, a heterogeneous catalyst has been probed by X-ray absorption spectroscopy (XAS) in "supercritical" carbon dioxide (scCO₂) under reaction conditions using a continuous-flow fixed-bed reactor. The catalytic activity was simultaneously determined by gas chromatography (GC). [More]

Published in: J. Appl. Cryst. 36, part 4, 1075-1081 (2003)

The lattice parameters of sapphire (α-Al₂O₃) have been measured in a temperature range from 4.5K to 250K with a relative accuracy of better than 6 x 10^-6. Sapphire is a potential new material for X-ray crystal optics, especially attractive in applications as Bragg back-scattering mirrors since it allows unlike silicon exact Bragg back-scattering with high reflectivity for X-rays in the 10-50 keV spectral range. However, lattice parameters reported in the literature differ by up to 10^-4. This has imposed large uncertainties in the prediction of the back-reflections and the relevant crystal temperatures. [More]

Published in: Acta Cryst. A59, 437-451, 2003

Charge density investigations play an important role in order to determine the crystalline and electronic structures of materials. Theoretical models are usually tested by experimental data, but up to now this was difficult for high-T_c superconductors. Using high-energy synchrotron radiation, structure factors of YBa₂Cu₃O_6.98 were measured at lambda = 0.124 A. Experimental and theoretical results generally agree well. A topological analysis shows that the structure is mainly ionic, but depletion of charge density between Cu and O gives hints for covalency, and confirms the general consensus of hole localization in these bonds. [More]

Published in: Mol. Cell 10, 1007-1017, 2002

While the function of most small signalling domains is confined to binary ligand interactions, the peroxisomal Pex13p SH3 domain has the unique capacity of binding to two different ligands, Pex5p and Pex14p. We have used this domain as a model to decipher its structurally-independent ligand binding sites. By the combined use of X-ray crystallography, NMR spectroscopy and circular dichroism, we show that the two ligands bind in unrelated conformations to patches located at opposite surfaces of this SH3 domain. Mutations in the Pex13p SH3 domain that abolish interactions within the Pex13p-Pex5p interface specifically impair PTS1-dependent protein import into yeast peroxisome [More]

Published in Nature 420, 482 (2002)

Fourth-generation light sources based on free-electron lasers (FEL) will be a source of intense, short-wavelength radiation for a range of applications, including biological imaging. The first results reported on December 5, 2002 in Nature, from experiments with the FEL at the TESLA test facility at DESY shed light on the interaction of short-wavelength, short-pulse radiation with matter. Here, unexpectedly strong absorption of the laser radiation by Xenon clusters was observed, resulting in the explosion of the clusters and the ejection of high-energy, multiply charged ions. Such a highly nonlinear interaction between matter and soft X-rays below 100 nm has never been seen before. [More]

Published in: Phys. Rev. Lett. 88, 243001, June 17, 2002

Quadrupolar transitions can play an important role in X-ray absorption spectroscopy, especially when it is used for magnetic measurements, like in X-ray Magnetic Circular Dichroism or Resonant Magnetic Scattering. We show here that resonantly excited Ti KL_2,3L_2,3 Auger spectra of TiO2(110) carry a clear signature of quadrupolar transitions from the 1s to localized e_g and t_2g d-like states. The quadrupolar nature of the observed additional spectator lines are clearly demonstrated by their angular dependence, and their intensity is used to locate and quantify the quadrupolar transitions in the absorption spectrum. [More]

Published in: Science, Vol. 295, Issue 5558, pp. 1266-1269, February 15, 2002

Many organic molecules are chiral: Their S and R (left- and right-handed) enantiomers are non-superimposable mirror images of each other. Chemical reactions by default yield racemic mixtures consisting of 50:50 R and S enantiomer. However, in Nature proteins consist of only S-amino acids and RNA and DNA contain only R-sugars. This break of symmetry may have occurred through the amplification to 100% of a slight initial imbalance. In this study, an amphiphilic alpha-amino acid derivative was designed such that, organized in 2D crystalline self-assemblies at the air-water interface, its racemic mixtures could be converted into homochiral oligopeptides by a "lattice-controlled" polymerization reaction... [More]

Structural information on thin macomolecular and biomolecular films is hard to obtain. Scattering of hard x-rays offers the advantage of high resolution along all three dimensions. However, scattering intensities are often low due to a small scatting volume, small contrast and low degree of order. A new approach which may help to boost the signal to noise ratio is presented by Pfeiffer et al., by integrating a thin film composed of about ten lipid bilayers on silicon into a resonant x-ray structure or x-ray waveguide. In such a structure the sample itself makes up the waveguiding media with a a local field enhancement which can theoretically reach two orders of magnitude. This resonant field distribution is then used as a primary wave for grazing incidence diffraction, allowing the researches to observe lateral structural features of the lipid chain ordering, which would be hardly observable whithout the use of the x-ray resonances, when using comparable instrumental ... [More]

Fourth-generation light sources based on free-electron lasers (FEL) will be capable of producing X-rays of such extreme brilliance that new ways of focusing the radiation will have to be found. A device reported on November 8, 2001 in Nature, based on the simple concept of an array of pinholes, could be the answer. The 'photon sieve' exploits the monochromaticity and coherence of light from a free-electron laser to focus soft X-rays with unprecedented sharpness. The combination of an excellent focus with extreme flux will provide new opportunities for high-resolution X-ray microscopy and spectroscopy in both the physical and life sciences. [More]

TESLA stands for "TeV-Energy Superconducting Linear Accelerator - a 33-kilometer-long superconducting particle accelerator with built-in X-ray lasers, which is being developed in an international collaboration. The special advantage of TESLA: an accelerator enables particle collisions at the highest possible energy levels and at the same time serves as a source of "hard" X-ray light with laser properties. The Technical Design Report (TDR) describes the scientific aims and potentials as well as the technical design of TESLA and has just been published. [More]