Imaging of protein molecules--towards atomic resolution

Thin Layer Buckling in Perovskite CsPbBr3 Nanobelts

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have low impact on electronic properties, focus interest for future applications. Two-dimensional thin layer lead halide perovskites are hailed for their unconventional optoelectronic features. We report structural deformations via thin layer buckling in colloidal CsPbBr₃ nanobelts adsorbed on carbon substrates. The microstructure of buckled nanobelts is determined using transmission electron microscopy and atomic force microscopy. We measured significant decrease in emission from the buckled nanobelt using cathodoluminescence, marking the influence of such mechanical deformations on electronic properties. By employing plate buckling theory, we approximate adhesion forces between the buckled nanobelt and the substrate to be F_adhesion ∼ 0.12 μN, marking a limit to sustain such deformation. This work highlights detrimental effects of mechanical buckling on electronic properties in halide perovskite nanostructures and points toward the capillary action that should be minimized in fabrication of future devices and heterostructures based on nanoperovskites.

Electron Crystallography of Two-Dimensional Crystals of Membrane Proteins

Electron microscopy has become a powerful technique, along with X-ray crystallography and nuclear magnetic resonance spectroscopy, to study the three-dimensional structure of biological molecules. It has evolved into a number of methods dealing with a wide range of biological samples, with electron crystallography of two-dimensional crystals being so far the only method allowing data collection at near-atomic resolution. In this paper, we review the methodology of electron crystallography and its application to membrane proteins, starting with the pioneering work on bacteriorhodopsin, which led to the first visualization of the secondary structure of a membrane protein in 1975. Since then, improvements in instrumentation, sample preparation, and data analysis have led to atomic models for bacteriorhodopsin and light-harvesting complex II from higher plants. The structures of many more membrane proteins have been studied by electron crystallography and in this review examples are included where a resolution of better than 10 Å has been achieved. Indeed, in some of the given examples an atomic model can be expected in the near future. Finally, a brief outlook is given on current and future developments of electron crystallographic methods. Copyright 1998 Academic Press.

Computer-controlled spot-scan imaging of crotoxin complex crystals with 400 keV electrons at near-atomic resolution

400 keV electrons yield a better relative image contrast than 100 keV electrons for a beam-sensitive organic crystal when spot-scan imaging is used [J. Brink and W. Chiu, J. Microscopy 161 (1991) 279]. A FORTRAN 77 program has been written to operate the spot-scan imaging system on a computer workstation under the VMS operating system which is interfaced serially to the JEOL4000 electron microscope. We demonstrate the application of this implementation by imaging crotoxin complex crystals embedded in either vitreous ice or glucose to 2.5 A resolution. The intensity strength of the structure factors of this protein crystal are different at low (> 10 A) resolution but similar at high resolution (< 10 A) for the two embedding media as expected from their scattering contrast difference. Based on our experience as judged from the electron diffraction patterns of highly tilted crystals, flat crystals embedded in glucose can be readily obtained. Furthermore, our spot-scan imaging system also has the option of correcting the focus gradient that is present in images of tilted specimens.

Spot-scan imaging of microcrystals of an influenza neuraminidase-antibody fragment complex

Electron micrographs of two-dimensional microcrystals of a complex of an avian influenza virus neuraminidase and an antibody Fab fragment, termed 32/3, have been recorded using the spot-scan method of imaging. The crystals have a large unit cell (159.5 A x 159.5 A x 130.5 A) and a high solvent content (approximately 71% by volume) and are a challenging specimen for testing the spot-scan methodology. Crystalline order was preserved to beyond 4 A resolution as demonstrated by electron diffraction, using an embedding medium of a mixture of glucose and neutral potassium phosphotungstate. Using a Philips C400 computer control system interfaced to an EM420 electron microscope, and with the inclusion of additional software in the system, we have been able to record micrographs at low temperature with a relatively narrow (1500 A diameter) moving beam. There is evidence that the use of such a spot-scan beam reduces the effects of beam-induced specimen motion on the quality of micrographs. Conventional low-dose "flood-beam" images showed good isotropic optical diffraction in only 15% of cases whereas 30% of spot-scan images showed good diffraction. The best flood-beam images gave phases to only 15 A resolution after computer processing, whereas the best spot-scan images gave phases to 7 A resolution. Electron diffraction patterns were also recorded at low temperature, and the resulting diffraction amplitudes combined with phases from spot-scan images to yield a projection map of the structure. A 7 A resolution projection map of the complex is presented, and is compared with the projection map of the same avian influenza neuraminidase complexed with a different monoclonal Fab fragment, NC41, which has been solved to high resolution by X-ray diffraction.

Spot-scan imaging in transmission electron microscopy

The determination of the structure of proteins and other organic materials by transmission electron microscopy is a rapidly developing field. Obtaining high-resolution images of these radiation-sensitive specimens has, until recently, been problematic. The development of spot-scan imaging, in which the electron beam is focused to a spot with a diameter of about 1000 angstroms and moved over the specimen to record the image, has overcome some of the most severe problems, which result from beam-induced motion of the specimen and its image. Elimination of this motion greatly enhances the contrast of high-resolution features of the image and promises a significant increase in the speed with which future structural work can be accomplished.