116 



protein chain only by peptide bonds, then you can with fairly good approxima- 

 tion talk about the excitation energy of just the aromatic part of that amino acid. 

 So that what you would be doing would be holding the energy in one of the amino 

 acids. 



May I comment on the actual yield of the lowest triplet state? The claim has 

 been made that the quantum yield might be as high as 100 per cent starting from 

 any excited state and coming down. I wonder if that point could be discussed. 

 The very fact that you increase the yield of phosphorescence as you enhance the 

 spin-orbit coupling in the molecule by introducing heavy or paramagnetic groups 

 indicated that in the unsubstituted molecule you cannot get yields of the lowest 

 triplet states of 100 per cent. 



KASHA: This is a matter of the different types of electron systems which 

 can be studied. If you study aromatic hydro- carbons, i.e. it -electron systems, 

 you may get relatively small yields of triplet excitation which, in fact, go from 

 zero to some value like 0.6. Then if you make heavy atom substitutions in it - 

 electron systems you can enhance the triplet state yield by how much is left. If 

 there was 0.4 left you come up to 1. 



LINSCHITZ: Take the type of structures that lie on the range from zero to 

 0.6. A molecule like tyrosine, of course, would give you a good yield of triplet 

 state. But then take something like tryptophan or histidine. What kind of a 

 yield could you expect there? 



KASHA: No one has studied those. Somebody should study the amino acids 

 in this respect. But I know that the people who have done the yield measure- 

 ments (and this includes myself) have studied the aromatic hydrocarbons as the 

 chief group of molecules which allows that yield enhancement to be studied, and 

 in those cases they have found, for example, in the case of naphthalene the yield 

 is 0.01, but in iodonaphthalene it is about 1.0, so that there is a tremendous en- 

 hancement in that case. However, there are other electron systems like the n- 

 electron system which means chemically, e.g. , those containing a carbonyl 

 group, and in those cases yield of the triplet state is about unity and you can do 

 nothing to enhance it because there is no other energy available. That is all 

 there is to it, as far as I know. 



KAMEN: There has been considerable discussion and I think it is about time 

 to ask what suggestions for experiments arise from all this. In this connection, 

 I think everybody is equally eligible for suggestions. I should like to start the 

 discussion merely by pointing out some things which appear quite obvious. 



First of all, we have already had four or five specific suggestions. But in 

 addition, I think it is important that there be studies made on the effect of par- 

 tial and total unfolding of protein on surfaces using various kinds of radiation. 

 The effect of low energy electrons on such aggregates would be especially inter- 

 esting. Not alpha particles so much as low energy electrons. 



BURTON: By low energy what do you mean? 



KAMEN: Between 1 and 10 electron volts. 



MAZIA: We have made some studies of the relation between molecular 

 "folding" in surface films of proteins and their sensitivity to X rays. The con- 

 cept of "folding" of proteins is now so vague that I had better describe the actual 

 case. If a pepsin- albumin film is formed under a certain minimum pressure, 

 its activity as an enzyme-substrate complex is retained. If the film is permitted 



