MELVIN CALVIN 



323 



with para- and one with ortho-quinones; oxygen behaves like an 

 ortho-quinone in the transformation, and there is a very nice relation 

 between the potential and the photochemical yield. 



Unfortiuiately, none ot these transfers of hydrogen from chlorin 

 to these oxidation agents (hydrogen acceptors) involved the storage 

 of chemical energy. In every case, the thermodynamics is such as to 

 favor the system porphyrin -|- hydroquinone (over chlorin -{■ 

 qiiinone), and the light is simply overcoming the activation energy. 

 The kinetics of these reactions were studied in some detail (Borough 

 and Calvin, 21), and it was easy to demonstrate that a long-lived 



15 



^ K 10^ 



10 



PAHA 

 OUINONES 



1 2 3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 



E" (VOLTS) 



Fig. 4. Relation of quantum yield in photooxidation of zinc tetiaphenylchlorin to 

 the oxidation potential of hydrogen acceptor. 



excited state of the chlorin was involved, since the rate of the reac- 

 tion did not depend upon the concentration of the hydrogen acceptor 

 at all, down to very low concentrations. This led to the suggestion 

 that the excited state was the triplet state of the chlorin which has been 

 found in a whole variety of chlorins, including chlorophyll a (7, 21) . 

 Fig. 5 shows the kinetic analysis of this experiment. It is evident 

 that the quantum yield is dependent ujDon the ratio of ^3 to ^3 -f- ^5> 

 and what we suggest is that the ratio depends upon the quinone. You 

 will notice that the rate law does not contain a factor for the con- 

 centration of quinone, but the quantum yield does contain a factor 

 which is dependent upon luhich quinone you use. In this way we 



