115 



This brings up the question of what kind of damage could you do to a thing 

 like a nucleotide which would not affect the binding. This recalls Sinsheimer's 

 (16) experiments on irradiation of purines and pyrimidines which we discussed 

 at our meeting two years ago. Sinsheimer mentioned isomerization and reac- 

 tions at double bonds all of which were away from the presumed bonding site be- 

 tween the purine and the protein. So you can see in this instance an actual dem- 

 onstration of the specific effect of radiation; that is, of the localization of the 

 damage which we heard about in model reactions all day yesterday. This again 

 is an observation which bears directly on the model reactions considered and 

 there are numerous others one can bring up. 



It has been requested that we elaborate yesterday's discussion about the 

 "triplet state". Well, to go ahead. In spectroscopy one characterizes energy 

 states by quantum numbers and one of them is the so-called "spin" quantum 

 number. 



It was originally introduced in order to explain spectral-line splitting in 

 externally applied magnetic fields, and it is usually thought of in terms of a 

 mechanical spin of an electric charge. This makes it possible for an energy 

 state to consist in fact of a number of states which are slightly different in ener- 

 gy because of the coupling of this spin with other electronic forces. The spin is 

 quantized for each electron, and the magnetic moment associated with it, in 

 units of the so-called Bohr magneton, has a value of 7. The number of states 

 that are possible depends on the total spin. If the total spin of the system is s, 

 then the number of states, or "multiplicity" is 2s + 1 . If the spin is 1, then the 

 multiplicity is 3 and the state a triplet state. Two unpaired electrons give a 

 spin of 1 . 



Radiative transitions are most likely to occur between states of like multi- 

 plicity. To get into a triplet state from a singlet state, one usually has to have 

 some collision process. Usually an excited singlet state can have associated 

 with it a triplet state. The question is how often do we get molecules in this 

 triplet state, and of what interest is this to radiobiology ? According to the 

 discussion which you heard yesterday the chances are that after all the energy 

 is distributed and you degrade it all down thermally, there may be left some en- 

 ergy which will be effective for chemical reaction. The energy which is avail- 

 able for chemical reaction must be that which is still in the molecule. It is at 

 this point you expect something like the triplet state to be important because that 

 may be the only thing left which will hold the energy in the molecule for a long 

 enough time so that there is reaction of some sort. The only questions are 

 whether there are triplet states, how many of them there are, what is the popu- 

 lation in the triplet state as a consequence of irradiation and what is the time 

 they live? 



KASHA: All the things you mention are measurable, and that is what people 

 are trying to do today. 



PLATZMAN: That is to say -- for any simple molecule, but not for example 

 for an enzyme. 



KAMEN: With the more complicated molecules the chances of long-lived 

 states increase. The life of the metastable state can be really quite long. 



KASHA: I think there is a limit on the order of seconds. 



LINSCHITZ: I don't think that one can refer to the "triplet state of a pro- 

 tein". Rather if you have one amino acid, let us say, which is coupled to the 



