From an analysis of electron micrographs of Alfalfa Mosaic Virus (AMV), evidence has been obtained which favors a cylindrical P6 lattice for the protein coat of the virus. For the analysis use was made of optical diffraction and computer processing of electron images of negatively stained virus particles.
The virus coat exhibits polymorphism. Two kinds of structure were found: a stacked and a helical type. In the stacked type of lattice the unit cells are arranged in staggered rings in such a way that two rings comprise a repeat distance of the structure. The selection rule for the optical diffraction patterns of the stacked form is 1 = n + 2m, in which n is an integer multiple of 3. The layerlines are equally spaced at a distance of approximately 1/80 Å−1.
In the helical type of lattice these rings of unit cells are transformed into turns of a double helix. The selection rule derived in this case is 1 = 6n − 17m, in which n is an integer multiple of 2. The repeat of the structure is approximately 440 Å.