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General details on the particle picking method
Particles were picked using a program written in-house for selecting filaments. Lines are drawn over the filaments using the computer mouse, and the program selects coordinates for boxing the particles at a user-selected interval while keeping track of groups of particles coming from the same filament. This is useful where polarity can be determined per filament by two-dimensional averaging. The program also keeps track of the angle of the overlaid, user-drawn line, which is used for preliminary vertical orientation of the particles. 8440 particles of 80 x 80 pixel size were selected from the filaments.
A reference was established by selecting a well preserved particle and symmetrizing it 20-fold rotationally. Cross-correlation functions of this reference with images of digitized micrographs containing adsorbed particles of PM28 revealed correlation peaks at the particle positions, irrespective of their angular orientation. Using this particle picking method a gallery of 4096 particles was created.