14S THE PHYSICS or VIRUSES 



by a large molecule cannot be anything like as selective a matter 

 as absorption by an atom in free space. The third feature applies 

 in solution. Since the presence of free ions in a solution can 

 profoundly modify the stable electronic configuration of a com- 

 ])ound (particularly in the organic acid-base, or "zwitterion" 

 com])ounds), the absorption spectrum in solution may well 

 depend on the number and kind of ions present. 



In many cases the act of absorption can cause a bond to 

 break. In a small molecule this causes dissociation. In a large 

 molecule, however, the existing structure may be so strong that 

 no actual atomic motion takes place and ultimately the energy 

 is lost as radiation of some form, with consequent restoration of 

 the bond. This process, known as the "cage-effect," or the 

 Franck-Rabinowitsch effect, operates in the interior of a large 

 molecule. The subsequent fate of the energy caught by the mole- 

 cule in the act of absorption is complicated. In this respect the 

 large-molecule constituents of viruses and the behavior of small 

 crystals have much in common, so that virology has to look to 

 developments and discoveries in solid state physics for an inter- 

 ])retation of some of the experimental findings in ultraviolet 

 effects. 



One process of energy transfer has been discussed for photo- 

 synthesis by Oppenheimer and Arnold (1950). They point out 

 that the induction field of an excited bond may interact with 

 another bond at a rather large distance and cause a transfer of 

 energy without actually any radiation and absorption having 

 taken place. This energy transfer process is similar to that of the 

 "diffusion" of color centers in an alkali halide crystal, which has 

 been experimentally studied by Apker and Taft (1950) and 

 theoretically discussed by Heller (1951). Transfer of energy over 

 distances as great as a micron are possible. This means that it is 

 l)ossible for energy to be transferred rapidly from any part of a 

 virus to any other part. 



At the surface of a large molecule, de-excitation can occur as a 

 result of collisions with the solvent. Because of this, and because 

 of the cage effect just mentioned, the denaturation of proteins by 

 ultraviolet light is relatively inefficient. 



