J. A. BASSHAM AND M. CALVIN 



Hill reaction studied in the light (9) it was found that the initial 

 rate of oxygen evolution is increased by as much as 50% over 

 the rate observed in the control in which no thioctic acid was 

 added. This result could be interpreted as evidence for the 

 participation of thioctic acid in the primary conversion step 

 or as an acceleration of a dark reaction transfer of reducing 

 power to quinone. Thus the biological evidence for the role 

 of thioctic acid as a quantum converter is suggestive but 

 not unequivocal. However, since the oxygen evolution in the 

 Hill reaction is insensitive to inhibition by parachloromercuri- 

 benzoate, a more limited role of thioctic acid in which it acts 

 only as an acceptor of an electron from chlorophyll seems the 

 more likely than the above role, in which it removes this electron 

 from oxygen after receiving energy from excited chlorophyll. 



Of the various possible chemical reactions of chlorophyll 

 under the influence of light we shall consider only the transfer 

 of an electron. Since there is evidence that the light absorption 

 process functions with chlorophyll in an aggregated system, it is 

 interesting to consider, instead of the reaction of a single chloro- 

 phyll molecule, the possibilities that exist with some sort of 

 orderly array of chlorophyll molecules. This array is probably 

 not actually crystalline chlorophyll but may well be an orderly 

 arrangement of chlorophyll molecules associated with other 

 molecules and protein or lipoprotein. We may think of the 

 electronic system of such an aggregate as a single unit in which 

 the TT electrons of the chlorophyll molecules interact. The 

 absorption of an electromagnetic quantum will raise one 

 electron of this system from the ground state in which it is con- 

 fined to a single molecule into a state in which it may migrate 

 throughout the array, i.e., into a conduction level. If there is 

 built into this structure a permanent polar character such as 

 exists at a "p-n" junction, for example, these photoconduction 

 electrons will diff'use toward the positive end of the permanent 

 dipole, leaving a positive hole to diffuse in the opposite direction. 



Thus a separation of charge will be induced by the light 

 which may be neutralized by a suitable electron acceptor at one 



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