CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1521 



Evstigneev and Gavrilova (1953^) measured the photogalvanic effect 

 in an illuminated solution of chlorophyll and ascorbic acid in pyridine. In 

 the dark the oxidation-reduction potential of the solution is determined by 

 the ascorbic acid-dehydroascorbic acid system. In light the potential 

 rises, first rapidly, then more slowly, as shown in figure 35. 7A; in the dark 

 it returns almost to its initial value. (Here again we note that the reaction 

 can be repeated several times with the same sample!) The widest poten- 

 tial change observed was about 0.29 volt with chlorophyll a (or a + 6), 0.25 

 volt with chlorophyll b, 0.35 volt with pheophytin (a + 6), 0.33 volt with 

 Zn-pheophytin and 0.29 volt with Mg-phthalocyanine. Since the dilute 

 (lO"* M) chlorophyll could not have oxidized more than about 1% of the 

 0.01 M ascorbic acid, and any change in potential caused by this oxidation 

 must have been toward more negative values, the rise of potential actually 

 observed in the illuminated solution indicates a high sensitivity of the elec- 

 trode to the presence of a small amount of reduced chlorophyll. (The situa- 

 tion is similar to that in thionine-ferrous ion photogalvanic cells, described 

 by Rabinowitch in 1940; there, too, a shift of tlie oxidation-reduction 

 equilibrium in light leads to the electrode potential becoming more nega- 

 tive, i. €., the electrode responds to the reduction of the dye-leucodye sys- 

 tem more sensitively than to the oxidation of the Fe+VFe+' system. 



The main potential-determining process in the illuminated chlorophyll- 

 ascorbic acid solution must be the transfer of electrons from reduced chloro- 

 phyll to the positive electrode, and their return to dehydroascorbic acid via 

 the negative electrode; i. e., an electrode-catalyzed back reaction in the 

 photochemically displaced chlorophyll-ascorbate equilibrium. 



Evstigneev and Gavrilova pointed out that the value -fO.35 volt (the 

 maximum photogalvanic potential obtained in these experiments) is the 

 same as was derived by Krasnovsky and Brin (1950) from the capacity of 

 photoreduced chlorophyll solutions to reduce various oxidants (cf. above). 



Evstigneev and Gavrilova noticed that after light has been switched off, 

 the photogalvanic potential disappeared within a minute, while the red 

 absorption band of chlorophyll was not fully restored until 30-40 minutes 

 later; they concluded that the electrochemical effect is caused by a par- 

 ticularly unstable, intermediate form of reduced chlorophyll. This seems 

 to call for a parallel experiment on the rate of disappearance in the dark of 

 the bands at 518 m/x and 585 mn, ascribed by the same authors (1953^) to 

 the ionic and the neutral form of a semiquinone, respectively (cf. chapter 

 37C, section 2). 



Evstigneev and Terenin (1951) made experiments on the photovoltaic effect (Bec- 

 querel effect) with electrodes (Pt, graphite) coated with chlorophyll, phthalocyanine or 

 pheophytin. With chlorophyll in the presence of air they found a positive photoeffect 

 (coated electrode became more positive in light) requiring about 1 min. of illumination 



