14 : 3/ Structure of Viruses 253 



large molecules and virus particles. Virus crystals have been studied 

 by the technique of X-ray diffraction. These studies have led to models 

 of the molecular arrangements within certain viruses. 



The density and uniformity of virus particle size can be determined 

 with an instrument known as the ultracentrifuge, in which the suspension 

 containing the particles is subjected to accelerations 10 4 or more times 

 gravity, by rapidly rotating it about an axis. The tube containing the 

 suspension is at an angle to the axis of rotation. Particles heavier than 

 the suspending medium will tend to migrate "down" the tube. The 

 analytical ultracentrifuge is equipped with optical systems to make it 

 possible to observe the migration of particles in the high gravity field 

 during rotation. 



By a series of calculations which will not be developed here, it is 

 possible to use ultracentrifuge data to determine the molecular weight of 

 small particles, as well as to determine' the uniformity of particle size and 

 shape. In addition to viruses, large, biologically important molecules 

 can be measured in this fashion. At one time, it was believed that 

 crystallization proved uniformity of particle size. Experiments with the 

 ultracentrifuge have shown more than one component in certain crystal- 

 line viruses. Only one of the ultracentrifuge components was active as 

 a virus. 



Another physical property of large molecules is the rate of migration 

 in an electrical field. This is called the electrophoretic mobility; it 

 depends on the pH of the solution. Viruses, just as living cells, have a 

 net negative charge at neutral pW and migrate to the anode. Electro- 

 phoretic studies have been used to demonstrate the uniformity of the 

 virus particles, as well as changes in their charge as a function of pH.. 

 These studies, combined with ultracentrifuge and crystallization studies, 

 have led to the picture of most viruses being uniform in particle weight, 

 size, shape, and net charge. 



A very different approach to virus studies consists in bombarding a 

 dried layer of virus particles with ionizing radiation. It is then possible 

 to apply target theory (see Chapter 16) to the virus and determine a 

 critical volume throughout which energy transfer may occur. Such 

 measurements show that on the average about 12 hits are necessary to 

 destroy the infective properties of some viruses, whereas others are in- 

 activated by single hit kinetics discussed in Chapter 16. These hits 

 occur in a critical volume almost as big as the entire volume of the 

 smaller viruses. For the larger viruses, the critical volume is much 

 smaller than the particle size. In every case, this critical volume is 

 about equal to the volume within the phage occupied by a class of sub- 

 stances called nucleic acids. Their properties are discussed further in the 

 following section. 



