Electron diffraction of helical particles.

The development of low-dose electron cryo-microscopy has provided the means to see structural details to better than 10 A resolution in helical structures. The application of techniques of image analysis to micrographs can yield accurate phases, but not amplitudes with which to generate three-dimensional maps of the structure. Electron diffraction can provide reliable amplitudes, which can be combined with the phases from the images. In order to collect amplitude data, two problems have to be overcome: the pattern should be obtained from a large well ordered sample of particles, and the inelastic background should be properly subtracted. In this paper, we present three simple methods to produce rafts of helical particles. Using these methods we have obtained electron diffraction patterns from TMV (with data out to 0.28 nm), TMV protein stacked disks (with data out to 0.3 nm) and bacterial flagellar filaments (with data out to 0.5 nm). In addition, we describe the algorithms used to extract the amplitudes from the diffraction patterns.