446 LIGHT AND LIFE 



the early 1900's that there was carbon, oxygen, nitrogen and sulfur, and 

 in 1935 that there might be fat, proteins, and carbohydrates, and now that 

 we have cytochromes, flavins, quinones, and some metals and a few enzymes, 

 there are still many gaps. We cannot make a molecular balance sheet for 

 these particles. I feel that it will be very hard to explain what these particles 

 do without a molecular balance sheet. The same is true for other biological 

 structures of course. In this connection one asks what is the minimum 

 numljer of things that you need to make a phosphorylation system. Here a 

 biological approach is required. The problem is, how do you construct a 

 structure biologically which will begin to do something at a certain point 

 in its evolution? Nobody has done this yet. What has been done is take 

 organisms which are already established, chop them to pieces, and then 

 dig out the pieces and find the smallest piece and say this is the minimal 

 structure. The biochemical criterion is that if you get to a point where 

 nothing happens with some of these structures and you can add something 

 back to get back to activity, you have the minimal structure. This really 

 does not mean that you have the minimal amount. The point is how few 

 chlorophylls do you need, how few proteins, how few flavins, etc. Until this 

 question is settled, it will be very difficult indeed to decide about the metal 

 requirement, be it iron or copper or manganese. Copper and manganese 

 are certainly some of the possibilities, among many. 



Dr. Rabinowitch: I think that it makes the game less interesting to think 

 that everything is either pyridine nucleotide or cytochrome. 



Dr. Kamen: Well, at least we are making progress; up to now we were 

 restricted to x and )'. 



Dr. Linschitz: In connection with Dr. Kamen's remarks on possible inter- 

 actions between excited chlorophyll and cytochromes, I'd like to mention 

 some studies we have made on the quenching of triplet states by heavy 

 metal ions. The mechanism we have been forced to assume for this quench- 

 ing process on the basis of evidence which I do not want to take time to go 

 ijuo, but which is in a paper in press at the moment, involves a charge-transfer 

 interaction between the excited states, call it the triplet state, of anthracene 

 or porphyrin, and the metal ion. This is followed l)y a rearrangement of 

 charge inside the complex leading back to the ground state. One gets the 

 same kind of mechanism for quenching of anthracene or ])orphyrins. It is 

 now of interest to look at the eHect of complexing of the metal ions on the 

 rate of this process. One finds indeed that the quenching is very sensitive to 

 the presence of complexing groups which block oft the metal ions, but 

 if you use porphyrin complexes the rate continues at a high value. In fact, 

 we are now extending this work to the quenching of chlorophyll triplets 

 by other porphyrins. The only data I can give now is on the quenching of 

 anthracene triplet by copper, copper chloride uncomplexed, and copper por- 

 phyrin complex. The rates in each case are very high. That is to say, the 

 blocking off of the metal by this very large group in no way impedes the 

 formation of the charge-transfer complex. 



