SPEDEN is a computer program that reconstructs the electron density of single particles from their X‐ray diffraction patterns, using a single‐particle adaptation of the holographic method in ...crystallography Szöke, Szöke & Somoza (1997). Acta Cryst. A53, 291–313. The method, like its parent, is unique because it does not rely on `back' transformation from the diffraction pattern into real space and on interpolation within measured data. It is designed to deal successfully with sparse, irregular, incomplete and noisy data. It is also designed to use prior information for ensuring sensible results and for reliable convergence. This article describes the theoretical basis for the reconstruction algorithm, its implementation, and quantitative results of tests on synthetic and experimentally obtained data. The program could be used for determining the structures of radiation‐tolerant samples and, eventually, of large biological molecular structures without the need for crystallization.
The scanning tunneling microscope (STM) has revolutionized surface science since its invention in 1982 (Binnig and Rohrer, 1982) by providing a means to directly image atomic scale spatial and ...electronic structure. Using the combination of a coarse approach and piezoelectric transducers, a sharp, metallic tip is brought into close proximity with the sample. The distance between tip and sample is less than 1 nm, which means that the electron wave functions of tip and sample start to overlap. A bias voltage is applied between tip and sample that causes electrons to tunnel through the barrier. The tunneling current is a quantum mechanical effect: tunneling of electrons can occur between two electrodes separated by a thin insulator or a vacuum gap and the tunneling current decays on the length scale of one atomic radius. The tunneling current is in the range of picoamperes to nanoamperes and is measured with a preamplifier. In an STM, the tip is scanned over the surface and electrons tunnel from the very last atom of the tip apex to single atoms on the surface, providing atomic resolution. The exponential dependence of the tunneling current on the tip–sample distance can be exploited to control the tip–sample distance with high precision. There are four basic operation modes for any STM: constant current imaging, constant height imaging, spectroscopic imaging, and local spectroscopy. Their interpretation and realization will be briefly discussed below. For details about other modes and a comprehensive introduction to electron tunneling and STM see Wiesendanger (1994).
We demonstrate near-atomic-resolution Bragg diffraction from aerosolized single granulovirus crystals using an x-ray free-electron laser. The form of the aerosol injector is nearly identical to ...conventional liquid-microjet nozzles, but the x-ray-scattering background is reduced by several orders of magnitude by the use of helium carrier gas rather than liquid. This approach provides a route to study the weak diffuse or lattice-transform signal arising from small crystals. The high speed of the particles is particularly well suited to upcoming MHz-repetition-rate x-ray free-electron lasers.
Membrane proteins constitute >30% of the proteins in an average cell, and yet the number of currently known structures of unique membrane proteins is <300. To develop new concepts for membrane ...protein structure determination, we have explored the serial nanocrystallography method, in which fully hydrated protein nanocrystals are delivered to an x-ray beam within a liquid jet at room temperature. As a model system, we have collected x-ray powder diffraction data from the integral membrane protein Photosystem I, which consists of 36 subunits and 381 cofactors. Data were collected from crystals ranging in size from 100 nm to 2 μm. The results demonstrate that there are membrane protein crystals that contain <100 unit cells (200 total molecules) and that 3D crystals of membrane proteins, which contain <200 molecules, may be suitable for structural investigation. Serial nanocrystallography overcomes the problem of x-ray damage, which is currently one of the major limitations for x-ray structure determination of small crystals. By combining serial nanocrystallography with x-ray free-electron laser sources in the future, it may be possible to produce molecular-resolution electron-density maps using membrane protein crystals that contain only a few hundred or thousand unit cells.
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