CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1523 



reduction equilibrium (say, between two intermediate redox catalysts, 

 A/AH2 and B/BH2). What can be said about the secondary oxidations 

 and reductions which can follow this displacement? In the first place, 

 one has to remember that "reducing power" (say an accumulation of 

 BH2) arises simultaneously with "oxidizing power" (say, an accumulation 

 of A), so that to speak of only one of them (one often hears of the "reducing 

 power" of illuminated chloroplasts) means postulating that the systems 

 A/AH2 and B/BH2 are either separated spatially immediately after the 

 photochemical transfer of H from A to B; or else that, because of enzymatic 

 specificity, the next system in the reduction sequence (say, C/CH2) "sees," 

 chemically speaking, only the reduced system B/BH2 and not the simul- 

 taneously present oxidized system A/AH2 (an analogous assumption must 

 be made on the "oxidation side" of the primary photoprocess). 



The specificity of enzymes and the heterogeneous structure of the photo- 

 synthetic apparatus make one — or both — of these important postulates 

 not implausible. 



The next question (selecting arbitrarily the "reducing power" rather 

 than the "oxidizing power" for further consideration) is: what secondary 

 reductions can be achieved by the system B/BH2, in which the ratio [BHo]/ 

 [B] has been increased by light? Let us assume that the normal potential 

 of the pair B/BH2 is E^; if the logarithm of the concentration ratio [BH2]/ 

 [B] has been shifted to 5 {~> 1), the redox potential of the pair becomes 

 E = Eq -\- 0.03 5. On paper, by increasing 5, E^ can be increased in- 

 definitely; e. g., with b = 4^, E^ = E^ + 0.12 volt, and so on. In reality, 

 however, the possible contribution of this concentration term to the reduc- 

 ing capacity of BH2 in the steady state is very limited, because an extreme 

 value of the concentration ratio (such as 10*: 1) cannot be maintained when 

 a reaction is in progress. 



This is particularly true of a photochemical reaction with a high quan- 

 tum efficiency. Such a reaction is only possible if no (or relatively few) 

 quanta are wasted in the primary process ; and if this process includes the 

 transfer of hydrogen (or electrons) from AH2 to B, there must be enough B 

 present at all times to accept the proffered H atoms (or electrons). In 

 other words, the steady-state concentration of B cannot drop low in light 

 if this light is to be effectively used, and this means that no extremely high 

 photostationary ratios [BH2]/[B] are permitted, and the oxidation-reduc- 

 tion potential of this system in light cannot be much different from ^o- 

 To this consideration one must add a second one : to prevent exhaustion of 

 B in light, the absolute rate of reaction of BH2 with the next hydrogen (or 

 electron) acceptor, C, must be sufficiently high. To use a concrete ex- 

 ample, if B is called upon to accept one H atom every second, and if one- 

 half of the total B -\r BH2 must be available in the oxidized form to ensure 



