H. Gorke

Femtosecond x-ray protein nanocrystallography

X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded. It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction ‘snapshots’ are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source. We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes. More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (∼200 nm to 2 μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes. This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage.

Single mimivirus particles intercepted and imaged with an X-ray laser

X-ray lasers offer new capabilities in understanding the structure of biological systems, complex materials and matter under extreme conditions. Very short and extremely bright, coherent X-ray pulses can be used to outrun key damage processes and obtain a single diffraction pattern from a large macromolecule, a virus or a cell before the sample explodes and turns into plasma. The continuous diffraction pattern of non-crystalline objects permits oversampling and direct phase retrieval. Here we show that high-quality diffraction data can be obtained with a single X-ray pulse from a non-crystalline biological sample, a single mimivirus particle, which was injected into the pulsed beam of a hard-X-ray free-electron laser, the Linac Coherent Light Source. Calculations indicate that the energy deposited into the virus by the pulse heated the particle to over 100,000 K after the pulse had left the sample. The reconstructed exit wavefront (image) yielded 32-nm full-period resolution in a single exposure and showed no measurable damage. The reconstruction indicates inhomogeneous arrangement of dense material inside the virion. We expect that significantly higher resolutions will be achieved in such experiments with shorter and brighter photon pulses focused to a smaller area. The resolution in such experiments can be further extended for samples available in multiple identical copies.

Balmer α transitions in antiprotonic hydrogen and deuterium

Abstract The strong-interaction shifts ϵ and broadenings Γ of the 2p levels in antiprotonic hydrogen and deuterium have been measured for the first time with a crystal spectrometer. In hydrogen, the 2 3 P 0 hyperfine state could be resolved from the three close-lying states 2 3 P 2 , 2 1 P 1 , and 2 3 P 1 . The hadronic shift was determined to be ϵ 2 3 P 0 =+139±28 meV (attractive). The value found for Γ 2 3 P 0 =120±25 meV is much larger than the spin-averaged 2p-level broadening Γ bal 2p as determined from earlier experiments measuring the intensity balance. The average shift of the three unresolved states is consistent with zero and a mean broadening of Γ 2( 3 P 2 , 1 P 1 , 3 P 1 ) =38±9 meV was measured. In deuterium, the spin-averaged hadronic shift and broadening were found to be ϵ 2p =−243±26 meV (repulsive) and Γ 2p =489±30 meV.

Measurement of the strong interaction parameters in antiprotonic hydrogen and probable evidence for an interference with inner bremsstrahlung

Abstract The spin-averaged values of the 1s strong interaction parameters in antiprotonic hydrogen were measured with good statistics. The results are: shift (ϵ 1s ) = −712.5 ± 20.3 eV and width (Γ 1s ) = 1054± 65 eV . These values are compatible and have approximately the same precision as the world average before this experiment. With some additional constraints, both the triplet and singlet components could be extracted from the same spectrum. The results are: 3 S 1 shift: −785 ± 35 eV , 3 S 1 width: 940 ± 80 eV , 1 S 0 width: 1200 ± 250 eV . In addition, precise values for the K α L α and Ltotal L α yields at low pressure (20 mbar) were obtained: 0.0176 ± 0.0016 and 1.45 ± 0.05 . Furthermore, the 2p mean absorption width of 30.8 ± 3.0 meV was deduced, also compatible with and of similar precision as the world average before this experiment. An indication of interference between the measured Lyman (Ki) x-ray series and the process of inner bremsstrahlung due to charged meson production after annihilation was seen because of the low background conditions of this experiment.

Measurement of the strong interaction parameters in antiprotonic deuterium

Abstract Antiprotonic deuterium K α X-rays were seen for the first time. Their analysis resulted in spin-averaged 1s strong interaction parameters, namely −1050±250 eV (repulsive) for the 1s shift and 1100±750 eV for the 1s width. The large errors are mainly due to low statistics. In addition, values for the K α /L α and L total /L α yields at low pressure (20 mbar) were obtained: 0.005±0.003 and 1.34±0.05. Furthermore, a 2p mean absorption width of 80 ≤ Γ 2p ≤ 350 meV was deduced. The results for the 1s ground state are somewhat surprising, especially the narrow 1s width which is similar to the spin-averaged antiprotonic hydrogen 1s width. However, our 1s width is in agreement with low-energy scattering data [A. Zenoni et al., Phys. Lett. B 461 (1999) 413] (see preceding paper).

Results of a fast pnCCD detector system

Fully depleted, backside illuminated pnCCDs with an integrated frame store area and an anti-reflective-coating for the optical and near infrared region have been fabricated. Measurement results of a 51 μm pixelsize device with an imaging area of 264 × 264 pixel will be presented. The devices, which feature a doublesided readout, allow to be operated at frame rates higher than 1000 frames per second. The electronic noise contribution of the entire detector system is slightly above two electrons at fastest readout modes. We will also present the concept of a data acquisition system being able to handle pixel rates of more than 70 megapixel per second. Decentral data reduction and analysis units allow for a centroid determination of sub--images with a very low latency time. The high speed, low noise and high quantum efficiency makes this camera system an ideal instrument for wavefront sensors in adaptive optics systems.

Optical test results of fast pnCCDs

Measurements with a high-speed pn-charge coupled device (pnCCD) system have been performed in the optical and near infrared (NIR) wavelength range. Some of the key parameters of the system that were determined include the overall quantum efficiency (QE), the point spread function (PSF) and the photon transfer curve (PTC). The results of these measurements will be presented below. The measurements have been carried out at the optical test bench of ESO in Garching, Germany. There we also demonstrated for the first time the feasibility of a fast readout scheme that allows the system to be operated at a speed of up to 1000 frames per second (fps) for use with optical light. Additionally astronomical test measurements have been performed at the Skinakas telescope on Crete, Greece.

Results of a pnCCD Detector System for High-Speed Optical Imaging

We present the design and optical imaging performance of a pnCCD detector system for highest frame rates and excellent sensitivity over a wide wavelength range from the UV to near IR region. To achieve frame rates higher than one thousand frames per second with an exceptionally low noise level, the devices are based on proven technology with column parallel readout and operated in a split-frame transfer mode. The CCDs are back illuminated and coated with an Anti-Reflective- Coating. The sensitivity over their full thickness of 450 &mgr;m allows for a quantum efficiency near 100% over a wide spectral range. At an optical test bench we determined the photon transfer curve, quantum efficiency and point-spread function within a wavelength region between 300 nm to 1100 nm for various detector parameter. To demonstrate the ability of a pnCCD to perform high-speed optical differential photometry, the crab nebula with the crab pulsar as central object were observed at the 1.3m SKINAKAS telescope on crete. For these observations the pnCCD was operated at a speed of 2000 frames per second. The high speed, low noise and high quantum efficiency makes this detector an ideal instrument to be used as a wavefront sensor in adaptive optics systems.

Large format imaging detectors for X-ray free-electron-lasers

New generation synchrotron light sources, the X-ray free electron lasers, require a two dimensional focal plane instrumentation to perform X-ray imaging from below 100eV up to 25keV. The instruments have to face the accelerator bunch structure and energy bandwidth which is different for existing (FLASH, Hamburg and LCLS, Menlo Park) and future photon sources (SACLA, Harima and XFEL, Hamburg). Within the frame of the Center for Free Electron Laser Science (CFEL), a joint effort of the Max-Planck Society, DESY and the University of Hamburg, the MPI semiconductor laboratory developed, produced and operated large area X-ray CCD detectors with a format of nearly 60cm2 image area. They show outstanding characteristics: a high readout speed due to a complete parallel signal processing, high and homogeneous quantum efficiency, low signal noise, radiation hardness and a high pixel charge handling capacitance. We will present measurement results which demonstrate the X-ray spectroscopic and imaging capabilities of the fabricated devices. We will also report on the concept and the anticipated properties of the full, large scale system. The implementation of the detector into an experimental chamber to perform measurements e.g. of macromolecules in order to determine their structure at atomic resolutions will be shown.

Conclusions from recent pionic--atom experiments

The most recent pionic—hydrogen experiment marks the completion of a whole series of measurements, the main goal of which was to provide conclusive data on pion—nucleon interaction at threshold for comparison with calculations from Chiral perturbation theory. The precision achieved for hadronic shift and broadening of 0.2% and 2%, respectively, became possible by comprehensive studies of cascade effects in hydrogen and other light exotic atoms including results from the last years of LEAR operation. In order to obtain optimum conditions for the Bragg crystal spectrometer, the cyclotron trap II has been used to provide a suitable X—ray source. To characterize the bent crystal spectrometer, the cyclotron trap has been modified to operate as an electron—cyclotron resonance source, which produces with high intensity narrow X‐ray transitions in the few keV range originating from highly charged ions.