82 



Which of these various processes can occur depends ultimately of course on 

 the redox potentials of the various excited molecules and substrates, the rates 

 and just what happens to be in the neighborhood. 



Most of the available data on the various bimolecular processes represented 

 here come from experiments on very complex molecules, mainly dyes with low- 

 lying energy levels and with relatively stable semiquinones. There is certainly 

 a need for photochemical studies of molecules of intermediate complexity and 

 bio-chemical relevance, to ascertain which of the various processes may occur 

 (7). 



The main point to make is that these bimolecular processes afford a mech- 

 anism whereby one can obtain active radicals without the expenditure of the en- 

 ergy necessary to ionize water and either allow the ions to recombine or to 

 split water into radicals by the various processes that have been discussed by 

 Dr. Platzman and Dr. Magee. This is an economical way in general of reach- 

 ing such activated intermediates, and in particular H0 2 . 



KAMEN: Isn't there some difficulty about distinguishing between a direct 

 hydrogen transfer this way and the intermediate ionization of water? If water is 

 in the vicinity how do you tell if you have, for instance, a transfer of electrons 

 through a chain of intermediates like this or one where you simply ionize water 

 first and then produce the water radicals? There is no way of distinguishing it, 

 is there? Give us an example. How do you tell which reaction is going on in 

 the system? 



LINSCHITZ: If a model reaction can be shown to go with visible or ultra- 

 violet radiation, at least the possibility of its taking place directly under high- 

 energy radiation is demonstrated. To be entirely sure however, we would have 

 to know the actual yield of electronic excitation relative to water ionization. 

 This is a problem which I don't want to speak on at all. I had hoped that by this 

 time we could have some idea as to what the fraction would be. 



FANO: Aren't these the mechanisms that are quite effective when you have 

 photochemistry with ultraviolet light because the ultraviolet light can be made 

 use of only by these systems, but when you throw in ionizing radiation, the 

 fraction of ionizing radiation that would go into that would be very small because 

 there are few groups that can do this. 



LINSCHITZ: This problem will appear again and again in this whole session. 

 It seems to me that what you have to contend with here is first of all the total 

 amount of excitation that will be picked up by these molecules and, second, how 

 strategic these molecules may be in the subsequent radiobiological effect. 

 Granted, most of the cell is water but if energy is transferred specifically into 

 a single site, say on a nucleic acid chain, it may possibly, erg for erg, be more 

 effective there. 



BURTON: Do you mean that when you use ionizing radiation you can produce 

 a great diversity of excitations only one of which, say, can contribute to the 

 phenomenon that you are talking about? Is that what you are saying? 



FANO: No. What I was saying was that the number of groups that can do 

 this is comparatively small percentwise; therefore, only a small fraction of the 

 radiation energy goes into them. 



BURTON: However, most of the internal conversion processes may lead to 

 a single state which results in the radiobiological effect. I don't mean to say 



