88 



percentage of electrons in aromatic rings among the electrons of a biological 

 material is small. 



The whole biological process is designed to make use of the low grade exci- 

 tation of a prosthetic group, whereas ionizing radiations produce high grade ex- 

 citations of all groups indiscriminately. 



LINSCHITZ: Let's come back to that objection for discussion later, because 

 it's obviously important. For the time being, assume that we have an electron- 

 ically excited residue on a DNA chain. Then, if one has anywhere along the 

 chain a molecule with absorption bands which lie somewhat to the red of those 

 for the majority of molecules in the system, then one can expect by the trapping 

 mechanism shown previously that the energy will funnel into this site where it 

 will either give rise to fluorescence, intense local heating, or any of the radical 

 formation processes we spoke of earlier. I think that any of the arguments which 

 deal with the biological effects of radiation will have to take account of these 

 simple, and I think fairly reasonable physical, processes. 



KAMEN: May I ask one question that has bothered me for about two or three 

 years. That is, the significance of cavitation in supersonic radiation. Accord- 

 ing to all your notions you have to have enough energy to get at least a few vibra- 

 tional levels above the ground state. The amount of energy involved in super- 

 sonic radiation is not enough for that; yet you get enormous biological effects 

 out of these things. 



POLLARD: They seem to be frictional effects. In studying a bunch of vi- 

 ruses, the effect certainly goes with size. The bigger the virus, the more the 

 chance for damage. In other words, the more the structure, the greater the 

 chance for damage. 



PATT: Did Dr. Linschitz say a single activating event could involve 3, 000 

 DNA molecules? 



LINSCHITZ: No, 3,000 purine or pyrimidine rings. The same thing might 

 occur in proteins, of course, in which energy transfers can take place between 

 aromatic centers within the protein molecule. Photochemistry of the proteins 

 indicating that this does occur in some cases. For example, Franck and Liv- 

 ingston (16) have discussed the case of the photochemical splitting of the CO- 

 myoglobin complex, with light absorbed by the protein part of the molecule. 

 Presumably, this energy must be transferred to the porphyrin to cause dissocia- 

 tion. Using Forster's theory and the known concentration of porphyrin in the 

 system, one can estimate crudely the time required for energy transfer from 

 protein to porphyrin. This turned out to be of the order of 10" 12 seconds, which 

 is within the possible lifetime of the electronic energy in that molecule. In this 

 case it appears that such energy migration is quite likely. 



POLLARD: Should there be any thermal influence on this? If you cool a 

 molecule down should you get more or less transfer? 



LINSCHITZ: I imagine that you probably get less effect because as you cool 

 down you sharpen the fluorescence and absorption bands and thus diminish the 

 overlap. On the other hand, if you warm the thing up you can enhance the prob- 

 ability of vibrational quenching, in which case less energy is available for trans- 

 fer. What you would get would be a balance of these two factors. 



POLLARD: So you should have a maximum? 



