Diffuse scattering resulting from macromolecular frustration

Distinctive diffuse scattering in the form of diffuse rings around Bragg positions has been observed in the diffraction patterns of a crystal of the N-terminal fragment of the Gag protein from Feline Foamy Virus. It is shown that these are caused by geometric frustration as molecules try to pack on the triangular b-c mesh of the space group P6₁22. In order to explain the strong diffuse scattering it is necessary for the crystal to contain occupational disorder such that each unit cell contains one or other of two different molecular arrangements, A and B. The frustration arises because the nearest-neighbour packing prefers neighbouring cells to be AB or BA, which cannot be achieved on all three sides of a triangle simultaneously. To explain the observation that reciprocal sections hk5n, where n = integer, contain only Bragg peaks it is necessary that A and B are identical molecular arrangements differing only by a translation of 0.2c. The implications of the disorder for solving the structure of the protein by conventional techniques as well as the possibility of using the diffuse scattering for this purpose are discussed.