PRIMARY QUANTUM CONVERSION: ELECTRON SPIN 

 RESONANCE EVIDENCE ^ 



R, H. RUBY and M. CALVIN 



Departments of Physics and Chemistry and Laivrence Radiation Laboratory, 



University of California, Berkeley 4, California 



In this paper we will discuss evidence provided by EPR on the na- 

 ture of the primary quantum conversion process in bacterial photo- 

 synthesis. We shall not discuss, except by reference, the peripheral 

 evidence offered by investigations of model systems, such as triplet 

 state studies or charge-transfer studies, but will try to demonstrate 

 evidence which may be obtained to identify the species producing the 

 observed light-induced EPR signal in living photosynthetic systems. 



There exist excellent discussions of EPR technique in general (1, 

 2,3), with reference to application in biological systems (4,5,6,7,8), 

 and with particular application to photosynthetic systems (9,10). A dis- 

 cussion of the technique and its early application to the problems of 

 photosynthesis is not warranted here. 



A MODEL 



One desires to choose a model with which as wide a range of ob- 

 served phenomena as possible may be explained. We shall discuss a 

 model suggested previously (2) which encompasses the photosynthetic 

 acts of both bacteria and green plants. We define the act of primary 

 quantum conversion as the series of events occurring between the 

 absorption of electromagnetic radiation and the appearance of the 

 primary transient oxidant and reductant which participate in the chem- 

 ical reactions of photosynthesis. Fig. 1 shows the assigned redox 

 relationships of species in the proposed electron transport pathway 

 adjoining the primary quantum conversion act. Only the step involving 

 \\V\ is assumed to apply to photosynthetic bacterial systems, in which 

 oxygen is not a product. The high efficiency of the photosynthetic 

 process requires that the oxidized species returns to its initial state 

 by mechanisms which allow only for small losses of energy to heat or 

 electromagnetic radiation. This requirement led to a physical depic- 



1 The work described in this paper was sponsored, in part by the U. S. 

 Energy Commission. 



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