156 THE PHYSICS OF VIRUSES 



The most striking result of this kind of study is the fact that 

 the characteristic small absorption peak of tryptophane at 

 2,900 A only shows for the case of light polarized perpendicular 

 to the axis of the capillary, and hence of the virus. It is thus con- 

 cluded that the aromatic rings of tryptophane are definitely 

 oriented in the TMV particle. In a less clear way it can also be 

 concluded that the nucleotides of ribosenucleic acid are also 

 oriented. Butenandt concludes that the planes of the purine and 

 pyrimidine rings are perpendicular to one another and per- 

 pendicular to the long axis of TMV. So also is the indole ring 

 of tryptophane. Seed and Wilkins (1950) consider that orienta- 

 tion is necessary but that, in fact, it is parallel to the long 

 axis of the virus. 



More work along these lines would be of the utmost value. 



Absorption Spectra of Other Viruses 



The absorption spectra of southern bean mosaic virus and 

 T-1 phage, as taken by Suprynowicz (1953), are shown in Fig. 

 6.7. These are both less full of character than is TMV and, in 

 addition, show no sharpening of absorption at low temperatures. 

 This last may partly be explained by the inability to orient the 

 viruses in any way. The same kind of broad analysis into protein 

 components and nucleic-acid components is possible. The 

 rather definite increase in absorption below 2,300 A is probably 

 due to peptide bond absorption, which has been shown by 

 Saidel and Goldfarb (1951) and by Setlow and Guild (1951) to 

 be responsible for the rise in absorption of dipeptides below 

 2,300 A. [Further interesting work on this has been reported by 

 Goldfarb, Saidel, and Mosovich (1951) and Hamm and Piatt 

 (1952).] 



Action Spectra 



Biological action spectra are, in principle, very powerful 

 methods of study of the internal structure and mode of operation 

 of viruses. The technique consists of irradiating the virus, either 

 dry or in solution, with monochromatic ultraviolet light and 

 measuring the survival ratio for a known amount of incident 



