X-ray and Crystallographic Studies of Plant Virus

Almost the whole of our evidence for the internal structure of the particles is derived from x-ray observations, particularly those of the high angle or intramolecular photographs. In contradistinction to the low angle photographs, the most striking feature is that the general appearance of the pattern is independent of the state of hydration. Most of the photographs have been taken from oriented specimens: This is because on unoriented specimens the wealth of lines makes direct interpretation impossible, though with a knowledge of the oriented pattern they may all be identified. Table IV shows the close correspondence between the spacings of the spots on our oriented photographs (Plate 4, Fig. 25) 1 with the rings on the unoriented photographs of Wyckoff and Corey (1936). The oriented specimens gave typical fibre photographs. In the case of the large two dimensional crystals of the dry gel, which possess secondary orientation relative to the water surface, a careful search was made to see if the large-angle scattering also showed such orientation, but no trace of it was found (Plate 4, Fig. 23). It would therefore appear that the only mutual orientation which affects the inner portions of the particles is that of parallelism of their long axes. But this negative result may be due to the disorientation of the dry gel on shrinking. The large-angle scattering of the various virus preparations shows characteristic similarities and differences. Only four distinct patterns have been observed. The first, which has been most carefully studied, is common to all the strains of tobacco mosaic virus, a second barely distinguishable from it is given by the cucumber virus (Plate 3, Fig. 20). A third is given by both strains of the potato virus X (Plate 3, Fig. 21). The fourth pattern, which is difficult to compare with the others owing to the absence of orientation, is given by the isotropic bushy stunt virus of tomato (Bernal, Fankuchen, and Riley, 1938).