The crystal structure of Tipula iridescent virus as determined by Bragg reflection of visible light.

Crystals of Tipula iridescent virus (TIV)3 diffract light just as ordinary crystals diffract X-rays, so that a crystallographic study is possible using visible light. The conditions of growth of TIV crystals have been investigated and a method developed for preparing large crystals from the microcrystalline centrifuged virus pellets. Optical obsei vations using monochromatic light have been made on crystals oriented parallel to the face of a perspex cell. The virus concentration in the normal wet crystals is calculated from the measured refractive index to be 17 4% w/v. The virus particles are packed in a face-centred cubic array, with an interparticle spacing of 2500 & 30 A, a distance nearly twice the diameter of the frozen-dried particle. The nature of the crystals is discussed and it is concluded that the (hydrated) virus particles in the crystal are not in contact but are separated by large distances of water (w 500 A). The crystals are thus probably held together by long-range forces operating at a distance comparable with the size of the particles themselves. The TIV crystals seem to represent the first clearly established instance of an ordering of iso-dimensional colloidal particles in three dimensions.