104 



James Franck and J. L. Rosenberg 



use of metastable excitation of Chi between A and other oxidants, 

 some as harmful as moleciolar oxygen. Although in vitro experi- 

 ments show that photo-oxidation of chlorophyll is much less 

 likely thaa simple oxygen quenching of excitation, the former 

 process does occur with a small yield. In any event, mere 

 impacts of oxygen with CM.* may divert the use of excitation 

 from photosynthesis . Since it is known that oxygen has a negli- 

 gible influence on photosynthetic rates in regions of low 

 irradiation intensity, we condLude that A must be present in at 

 least a high enough concentration (lO~^ molar or more) to win 

 out over Oo for the excitation. Among substances which do not 

 meet this criterion for A are most of the materials present in 

 catalytic amoiints, including TPN. Phosphoglyceric acid, PGA, 

 does meet the requirement and has been tentatively identified by 

 us with substance A. In support of this view is the fact that 

 other respiratory intermediates, normally present in smaller 

 concentrations than PGA, become the substitute oxidants for 

 Reaction (l ) during periods of unavailability of PGA, such as 

 cyanide inhibition, induction periods, and periods of greening 

 of etiolated leaves' ^^5 J. 



At intensities approaching saturation the photosynthetic 

 oxidant becomes limiting, and oxygen becomes a more effective 

 competitor. This is the explanation for the strong depressing 

 influence of oxygen on the saturation rate of photosynthesis. 

 Although photo-oxidation of the exposed chlorophyll still 

 proceeds with only a small quantum yield under these conditions, 

 in the absence of a recovery process there would be a progres- 

 sive accumulation of units whose exposed chlorophylls, oxida- 

 tively bleached, are unable to allow excitation to move to the 

 reaction centers. Another competing process occurring with a 

 probability rising with irradiation intensities is the collection 

 by Chl-^jj^g^ of a second quantiom of excitation before Reaction (l ) 

 has been sensitized. This double excitation, leading to photo- 

 ionization and the associated afterglow, will not be discussed 

 further in this paper, except for the remark that the connected 

 bleaching would res\ilt in a disruption of the flow and utiliza- 

 tion of excitation energy collected by the protected chlorophylls. 



The disruption of energy flow may be avoided by a built-in 



recovery process, (5), analogous to the reaction described by 



Kok(3) and by Wittv°) for the special case of the 7OO m|j. 



absorbing form of chlorophyll. This recovery of normal exposed 



Chi would be rapid if the Cyt in its reaction center complex is 



Chi + Cyt ^ ^ Chi + Cyt (3) 



red ox ^ -^ ' 



