270 C. p. WIIITTINGHAM 



Strehler: Did you say the quantum yield was the same in the two cases in the 

 beginning? 



Whittingham : Yes, as you extrapolate to zero light intensity we find that you 

 always get the same rate. This is measuring the oxygen production under identical 

 physical conditions. 



Gaffron : I would like to put this question to Dr. Livingston and to Dr. Rabino- 

 witch: Would it not be possible for the quinone reaction to occur in the singlet 

 state of chlorophyll? After all, quinone does quench fluorescence! In this case, its 

 reduction could occur with a better quantum yield than has been found so far. 

 The temperature effect also indicates it to be nearer to the primary photochemical 

 process and not so much subject to enzymatic limitations as is photosynthesis. 



Livingston : If the quinone reaction involves the primary excited or fluorescent 

 state, it probably could not be a diffusional reaction. The quinone would have 

 to be associated with the chlorophyll or aggregated chlorophyll in the dark, as a 

 thermally stable complex, or otherwise there would not be time enough for an effi- 

 cient reaction to occur at these low concentrations of quinone. 



Lumry : As far as higher plant chloroplast fragments are concerned, there are 

 two distinct regions of the Hill reaction with apparently quite different mecha- 

 nisms. These are controlled by oxidant concentration. At low concentrations 

 there is a region which is completely independent of oxidant, and there is no 

 quenching of fluorescence. But at the higher region, where a transition from one 

 Hill reaction to another occurs, you begin to get quenching. The quenching goes 

 along very smoothly parallel with the change from one Hill reaction to the other. 

 So it may very well be that what Kamen suggested happens at the higher region 

 though not at the lower. 



Amon : I would first like to make a point of some interest in connection with the 

 role of chloroplasts in photosynthesis. Although I cannot recall at the moment the 

 details of Hill's 1939 paper, in the 1950 paper, presented at Sheffield and published 

 in the Symposium of the Society for Experimental Biology, Hill definitely envisaged 

 chloroplasts as participating in the generation of some energy-rich compound, 

 subsequently oxidized by molecular oxygen. Hill's idea was that in order to bridge 

 the energy gap Kamen was talking about — that is, to compensate for the fact that 

 no Hill reagent is a sufficiently strong reductant to accomplish the reduction of 

 CO2— a reoxidation of part of the primary reductant with molecular oxygen is 

 necessary. His scheme was essentially analogous to that proposed by Warburg. 



I wonder if ^\^littingham has a comment on that. 



Whittingham: That is correct; but I think there is no evidence which would 

 compel Hill to say that you could not replace oxygen in the reoxidation process, 

 say, by a ferric complex or some other intermediate oxidant. As I see it, he used the 

 simplest possibility— that of molecular oxygen as oxidant; but there was no com- 

 pelling need for this assumption. 



Amon : My recollection is that he thought of this reoxidation as a respiratory 



process. 



Whittingham : What he had in mind was some kind of oxidative reaction, not 



necessarily autooxidation. 



Amon : I wanted to find out whether Hill would object to the idea of the absence 

 of respiration in chloroplasts— which we will suggest later— or whether respiration 



