162 THE PHYSICS OF VIRUSES 



spectrum, but clearly enables the differentiation of various 

 virus properties in terms of their chemical constitution and, 

 possibly, their position in the virus. Thus, a virus having a 

 sensitive region consisting of nucleic acid imbedded in protein 

 will produce an absorption spectrum which is a composite of 

 both protein and nucleic acid. The action spectrum, as regards 

 the nucleic-acid part, will only show strong effect in the nucleic- 

 acid absorbing region. 



Work which could be extended to viruses has been carried 

 out by Morowitz (1953) on B. suhtilis spores. The lethal action 

 spectrum and the action spectrum for two deficiency mutants 

 were observed and compared with the absorption spectrum. 

 Definite differences were observed. 



Action Spectra for Different Viruses 



A summary of earlier work on the action spectra of viruses 

 was given by Hollaender and Oliphant (1944). Figure 6.10 

 shows a composite plot, adopted from their paper, for five 

 viruses. The remarkably low sensitivity of Rous sarcoma virus 

 at 2,470 A and the high sensitivity of TMV at 2,300 A are the 

 most interesting features of these studies. It is also interesting 

 that, although the absorption spectrum of TMV extends out to 

 2,900 A, the quantum yield at that wavelength is very small. 



This kind of experiment is difficult and laborious. However, it 

 is clear that if the character shown in the five curves plotted is 

 not due to experimental uncertainty, then virus action spectra 

 should be useful in characterizing viruses. 



Quantum Yield 



One important datum in ultraviolet studies is the quantum 

 yield for various kinds of inactivation processes. McLaren (1950) 

 has made a careful study of quantum yields (ratio of number of 

 molecules inactivated to number of photons absorbed) at 2,537 A 

 and has concluded that the larger the molecule (including 

 viruses) the lower the quantum yield. Inverting the figure and 

 talking of photons per inactivation, this number seems to be 



