54 THE PHYSICS OF VIRUSES 



may modify conclusions regarding protein molecules. These 

 modifications may extend into virus figures, although, in view 

 of the excellent check given by the electron microscope, this is 

 not likely. 



X-Ray Diffraction Applied to Viruses 



X-ray difTraction is a powerful physical tool which is being 

 applied to supply evidence regarding crystalline protein struc- 

 ture. F'or diffraction studies to yield really valuable information, 

 relatively large, single crystals of material are needed. In any 

 event, some ordei- in the molecular arrangement is necessary. 

 Only for some preparations of plant viruses has it been possible 

 to apply this method. 



X-ray diffraction is concerned with three classes of measure- 

 ment. The first concerns the regular atomic arrangements inside 

 each virus i)article. The second concerns the size of each particle, 

 which appears as a repetitive diffracting distance. The third 

 concerns the space between viruses, which may be a very 

 regularly repeating value. All three types of diffraction have 

 been observed. The first two are of use in describing particle 

 size and shape and will be treated here. The inter virus spacing is 

 of more interest later on and will be deferred until then. 



Diffraction arises from ])hase-related scattering. It is simplest 

 to visualize this as due to regularly repeated scattering units, 

 which will cause diffraction maxima at certain definite angles 

 which are related to the repetition distance. The best illustra- 

 tion of this is the reflection from planes rich in whole virus 

 particles. A very beautiful picture of the nature of a small crystal 

 of tol)acco necrosis virus, taken with the electron microscope by 

 Wyckoff, is shown in lig. '2.9. It can be seen that there are 

 regular, closely p()])ulated planes, and indeed it can be seen how 

 the edges of the crystal are formed. Now any closely populated 

 plane is one which scatters X-rays richly. Bragg pointed out that 

 from such a plane equal angles of incidence and scattering give a 

 diffraction maximum. If, in addition, there are many parallel 

 planes, certain definite angles are selected, and these angles are 

 related to the X-ray wavelength, X, the separation of the richly 



