The particle projections were not all identical, because small tilt variations on the support film led to different positions. The statistical analysis and classification showed that only a small number of projections had threefold rotational symmetry, indicative for a position parallel to the membrane (Fig. 2, lower row, left). The other two classes (middle and right) show the supercomplex this website in tilted positions. 3D reconstructions can be obtained from large sets of projections of objects under different angles. In favorable cases, the molecules show random orientation in the ice layer or on the support

film. If not, specimens can be tilted in the microscope in order to obtain 2D projection maps of the molecules viewed from different Lenvatinib angles. For the PSI–IsiA particle, such a 3D reconstruction was produced (Bibby et al. 2001), but it did not show much more details than that were

already visible in the 2D maps, because the complex is a rather flat object. However, in general, 3D information is much more valuable especially for spherical objects as ribosomes and virus molecules. In the 1980s and 1990s, single particle analysis was still a matter of hard labor, including the recording on photographic emulsion, scanning the images by densitometers and processing, which was less sophisticated (Fig. 3a). In recent years, single particle method has been developed much in a direction of automation Non-specific serine/threonine protein kinase of all steps, i.e., from automated particle CDK inhibitor collection to iterative improvements

of initial 3D reconstructions. The use of scanning slow-scan CCD cameras, which can be programmed to record hundreds of images in a semi-automated way, helped tremendously (Fig. 3b). In the near future, it is expected that direct electron counters with superior recording qualities will replace the CCD cameras (Faruqi and Henderson 2007) and that further automation will provide structures within hours after sample insertion in the microscope. In addition, much higher contrast of unstained specimens is possible by application of “novel” phase contrast electron microscopy such as the Zernike phase contrast microscopy (Yamaguchi et al. 2008). This is similar to the phase contrast light microscope, for which Frits Zernike was awarded the Nobel prize for physics in 1953. Implementation in commercial electron microscopes will be a logical next step in improving EM methods. Fig. 3 Example of single particle analysis on a large water-soluble protein, the 180-subunit hemoglobin of the earth worm Lumbricus terrestris. a (Boekema and van Heel 1989). b Sum of 1024 particles at 11 Å resolution in negative stain (R. Kouřil unpublished). c, d Two views of a 3D reconstruction at 13 Å resolution (W. Keegstra and G.T. Oostergetel, unpublished). e, f Model of the high-resolution (3.5 Å) X-ray structure (Royer et al.