1522 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



to reach a stationary value and decaying in about 1 min. in darkness. Reductants de- 

 creased the positive effect or even changed its sign. 



If a rise of potential to a level 0.35 volt above the normal potential of 

 ascorbic acid (about —0.04 volt at pH 7) could be maintained in the steady 

 state of a photochemical reaction, chlorophyll-sensitized reduction by as- 

 corbic acid of compounds with normal potentials of the order of +0.3 

 volt (as described by Krasnovsky and Brin, cj. above) would be plausible. 

 However, this low photogalvanic potential corresponds to a wide shift of 

 the concentration ratio of a very dilute oxidation-reduction system, Chi/ 

 rChl; and as soon as the reductant in this system finds an oxidant with 



mm. 



Fig. 35. 7B. Changes in redox potential of chlorophyll in pyridine: (A) 

 10-* mole/1. Chi (a + 6), 0.6 X 10-^ mole/1, ascorbic acid; (B) same + 

 10-* mole/1, safranin T; (C) same + 4 X 10 -* mole/1, riboflavin (after 

 Evstigneev and Gavrilova 1953). 



which it can react, the concentration of the reductant must decline and the 

 potential go up ("photogalvanic depolarization"); the reaction will be 

 slowed down or stopped altogether. This effect of hydrogen acceptors on 

 the photogalvanic potential actually was noted by Evstigneev and Gavri- 

 lova. Figure 35.7B shows that the presence of 4 X lO""* mole/liter ribo- 

 flavin reduces the potential drop to less than one-half of its original value. 

 In other words, the photogalvanic potential cannot be taken as a measure 

 of the effective reducing power of an illuminated redox system. What 

 oxidants can be reduced in light, and at what rate, is a problem of kinetics, 

 and not, or only partly, of equilibrium (or pseudoequilibrium) potentials. 

 This may be a proper place for a general remark concerning the use of 

 oxidation-reduction potentials in the discussion of photochemical mecha- 

 nisms. Let us assume that the action of light is to displace an oxidation- 



