100 



possible. You can go up to iodine, for example. This causes a tremendous ef- 

 fect. 10" ^ is the order of magnitude of the enhancement of the probability for 

 going from singlets to triplets, but the astonishing thing is you can remove the 

 bond and it still happens. In other words, you merely have to lay benzene on a 

 heavy atom surface or near a paramagnetic atom or molecule. The most com- 

 mon is the O2 molecule. You can show enhancement of the singlet- triplet trans- 

 ition in many different ways and with catalytic effects that are remarkable. 

 And it is possible that in some biological systems the presence of iodine or sul- 

 fur in the molecule or molecular oxygen near a molecule will cause a catalytic 

 effect which will make the triplet state more effective, if there are any restric- 

 tions against producing it. 



KAMEN: We have a good notion of some ways of explaining the effect of ra- 

 diation as such. How would you explain the effect of radiomimetic substances? 

 I am not asking for an explanation. I am just remarking that there are things 

 you can do with agents like nitrogen mustard which simulate remarkably the ef- 

 fects of radiation, -- ionizing radiation in this case. What would be the mech- 

 anism using such radiomimetic molecule, if you assume that the radiation acts 

 at an aromatic site like this? I must confess that I am stumped by it. You can 

 show that some of the oxidizing enzymes which are affected by radiation are 

 similarly affected by mustard. 



BURTON: What happens in the cases you mention is that you are getting a 

 nitrogen mustard effect similar to the effect produced by radiation, whereas 

 Dr. Kasha has been talking about the removal of the effect which one might ex- 

 pect as a result of irradiation because of the presence of these heavy atoms. 



KASHA: No, what I said is that this enhances the production of the triplet 

 states. 



MAGEE: This retains electronic energy in the system for long periods of 

 time. 



BURTON: What you are looking for is a connection with a theory for chem- 

 ical action. 



KAMEN: You have to get to where these two kinds of agents are about to 

 converge. Now what does a nitrogen mustard do in making an effect like radia- 

 tion? Any change in the cell enviroment very often can simulate a radiation 

 effect. I don't understand it. 



POLLARD: It is tough. I agree with you. 



BURTON: Of course, one of the things Dr. Kasha was talking about all the 

 time was going from the high singlet state to the lower triplet state, but it is al- 

 so possible, if there is a reaction which is a producer of energy, to go from a 

 high vibrational level of a low singlet state to a low vibrational level of the trip- 

 let state. This is particularly true if there is something present that favors or 

 induces the transition. If there is some kind of exothermal chemical action so 

 that the system can proceed from a low singlet state to a high triplet state, it is 

 also required that there be something present which promotes transition from 

 the singlet state to the triplet state. A mechanism must be provided for the 

 transition from a high triplet to a low singlet state and from a low singlet level 

 to a high triplet level. 



Suppose we set up a situation (I am deliberately not specific) in which some 

 kind of a chemical reaction occurs, so that in some way the molecule of interest 



