INACTIVATION OF VIRUSES 373 



ionizing particle is liigh, and if one is assumed to suffice to inactivate, only 

 targets that are not hit by an ionizing particle will escape inactivation. Con- 

 sequently, the smaller the area of the target, not the volume, the greater the 

 chance of survival. 



Formally, data obtained by irradiating dried viruses with a-, X-, and 

 y-rays permit calculations to be made of area as well as volume. Tlie inter- 

 pretation of such calculations is simple if the data fit the assumption of a 

 single spherical target (which usually they do not). In the case of vaccinia 

 virus, the calculations suggested either a few hundred separate, spherical 

 targets, aU essential for reproduction, or a long, filamentous target (Lea and 

 Salaman, 1942). To identify the many targets with as many genes, as was 

 tentatively done, today appears highly speculative. 



The case of TMV is a great deal more significant because agreement has 

 been obtained between radiation analysis and independent chemical and 

 morphological evidence (see p. 371). This success is probably due to the 

 target being highly asymmetric and yet of simple cylindrical shape, and to 

 the fact that, apparently, one primary ionization is sufficient to inactivate. 



Structure analysis by means of ionizing radiation should be capable of 

 yielding more than estimates of target volume and area. The ionization 

 density along the track of a deuteron particle, for example, varies witli 

 its energy and, in principle, the thickness of the target, in the direction of 

 the beam, can be estimated from the formal values for the target area 

 obtained by varying the energy of the ionizing particles. For a detailed de- 

 scription of this experimental approach, see Pollard (1953). 



A number of viruses have been analyzed by Pollard and co-M^orkers, who 

 have combined data from a-particle or deuteron bombardment with X-ray 

 data (Pollard and Forro, 1951; Pollard and Whitmore, 1955; PoUard and 

 Kraft, 1955; Pollard and Setlow, 1956; TiU and PoHard, 1956). It is difficult 

 to evaluate the interpretations, some of which are illustrated by PoUard 

 (1953) in the form of models depicting the possible distribution of different 

 kinds of sensitive material within different virus particles. At the time of 

 writing, it would appear that more pertinent information about viruses is 

 being obtained from chemical and genetic studies, tracer experiments, and 

 autoradiographic work. In the future, some of the models may derive new 

 significance by being checked against independent evidence about the struc- 

 ture of these viruses; in this respect, the case of TMV, discussed above, is 

 encouraging. 



Very soft X-rays of Hmited penetration have been used m experiments 

 which suggest that the sensitive material of phage Tl (presumably its DNA) 

 is protected bya layer about 250 A thick (Davis, 1954). This value seems very 

 higli considering that the diameter of Tl is not much over 500 A. Guild (1952) 

 attempted to use soft X-rays for analytical work and demonstrated that 



