28 J- FRANCK 



Bassham: We are not working; with one atom or one molecule at a time. If we 

 accumulate charge at a certain potential, which is necessary, say, for the DPN 

 electrode, that is all we have to ac^complish. There can be a large population of 

 electrons. 



I might point out that in something like a solar battery, for instance, you can 

 remove an electron from one position. It will by conduction fall into a hole some- 

 where. This creates a charge. The charge can be used for the chemical current or 

 wherever you want to use it. It may be the same in photosynthesis. 



Strehler : There are some energetic considerations here that Dr. Bassham is not 

 taking into consideration — at least not here. (1) Energy will be dissipated (and a 

 fair amount of it) in the process of oxygen liberation (I assume we're speaking 

 about green plants). (2) In all likelihood that energy has to be available in addition 

 to the energy dissipated to form a triplet state or to store the usable energy in 

 some stabilized pool or intermediate reductant. If this were not the case the re- 

 ductant could react back at high rate. (3) In addition, not all of the energy in the 

 one electron carrier or free radical may be available to do chemistry since radicals 

 generally dismutate to nonradicals with a loss of energy — and DPNH is a 

 radical. 



I don't believe it is possible at present to formulate binding arguments of a 

 quantitative sort in support of either position. But the arguments raised on 

 purely physical chemical bases, such as given by Franck in his review paper in 

 the Archives of Biochemistry and Biophysics, or as presented in the Phosphorus 

 Symposium, where I developed essentially similar conclusions on entirely inde- 

 pendent basis, cannot simply be dismissed. We must be seriously concerned 

 about the question: Can one quantum of 40 kcal. energy content, considering prob- 

 able losses, transfer an electron from the potential of water to the potential of an 

 acceptor which reacts reversibly with fixed carbon dioxide? 



Bassham : Take 41 kcal. and throw away 7 or 8 kcal. in the first photochemical 

 step to make a triplet state. Then you still have enough left, as I said before, to 

 create hydrogen peroxide and to reduce TPN+ to TPNH. Then the hydrogen 

 peroxide proceeds spontaneously to water and oxygen. 



Strehler : How do you make the hydrogen peroxide? By the recombination of 

 OH radicals? 



Bassham : We don't necessarily go through the OH radical. Perhaps we have 

 a hydrated surface from which we have enough electrons all at one time to allow 

 instantaneous formation of peroxide. I am simply saying that we don't know 

 enough about it to rule out any possibility as long as there is enough energy to 

 carry out the net reaction which is there. 



Rabinowitch: The free energy of photosynthesis is approximately 120 kcal., or 

 about 30 kcal. per hydrogen atom transferred. 



If you want to transfer these four hydrogens by four protons from water to 

 something of the same reduction potential as your ultimate acceptor, which is 

 what you want to do in the case of TPN, you have practically nothing to spare. 



Bassham : I think you misunderstand me. I am not proposing that four quanta 

 will accomplish photosynthesis. I say that we can transfer four electrons. Other 

 electrons have to be used up to make high-energy phosphate to help us out later 

 on. To do what you are suggesting would require a better reducing agent than 



