SUMMARY 453 



as the pink intermediate known as the Krasnovsky product, which is 

 reduced chlorophyll. 



Not much is known about the photochemistry of chlorophyll itself. 

 It is an extremely reactive molecule even when not activated by light, 

 and I think it would not profit us to attempt a review of the chemistry 

 of chlorophyll now, but it would profit all of us to take a look at what 

 is known about it when we go back home. 



There is a question about the potential that is assigned to bacterio- 

 chlorophyll in the experiments of Goedheer, Kok, and others. The 

 potential given is 0.46 volts. Now if the chlorophyll which is receiving 

 this energy transfers an electron by a process of this type and we're 

 going down from chlorophyll to quinone (which everybody by popular 

 acclaim wants to have as a first resting place for the first electron) 

 at about 0.05 volts, then 0.4 volts is all we get out of this first quantum, 

 which has 1.3 volts in the infra-red band. So we throw away 0.9 volts! 

 This seems very wasteful. It seems more logical to assume that the 

 final resting place of the first electron is in a system which is at a 

 potential much more reducing then quinone. This is what worries me 

 about assuming an oxidation of chlorophyll, followed by a reduction of 

 quinone, as the sole result of the first photochemical act. And perhaps, 

 in private conversation, after this survey is over, I will hear why this 

 is no trouble. 



The observations of Goedheer appear to bear out that chlorophyll 

 is oxidized in the course of the photochemical act; but note, there are 

 many ways of getting oxidized chlorophyll besides charge transfer to 

 quinone. Just to give you one example, you could have two chlorophylls 

 in a complex which on absorption of light gives a practically complete 

 conversion of all the energy in a quantum, with production of oxidized 

 and reduced chlorophyll simultaneously. Then you have a double-sided 

 sandwich, a"Dagwood." with quinone on one side, and cytochrome on the 

 other. This seems to me to give a better looking picture, but I don't 

 hold it up as an example of anything beautiful, even though it looks 

 prettier than some others I've seen. 



Another thing I should like to emphasize again, as I have done be- 

 fore, is that there is another aspect to this reaction producing a re- 

 duced chlorophyll with a potential of minus one volt, or close to it. 

 Nobody has ever measured this reaction in chlorophyll, but they have 

 measured it in comparable chlorins and porphyrins. Let me remind you 

 again, the tetrapyrrole structure is symmetrical, around half- reduced 

 and half-oxidized semiquinone structures. You can get either one in 

 photochemistry, because photochemistry supplies the energy for both; 

 and what happens after the excitation depends entirely on the environ- 

 ment in which the tetrapyrrole then finds itself. If, as in the case of 

 the Krasnovsky reaction, for instance, the solvent is a basic solvent, 

 an electron donor, then there will be a reduction. If the solvent is an 

 acidic solvent, there'll be an electron acceptor there, and there will be 



