DIFFERENT OXIDANTS 1575 



Table 35.IXA 



Normal Potentials of Five Dyes and the Potentials Measured in Chloroplast 

 Suspensions Containing These Dyes before and after Illumination (after 



Macdowall 1952) 



2,6-Dichloro- Indigo 



phenol-indo- Toluylene Cresyl Methylene disulfo- 



phenol blue blue blue nate 



^o' (pH 6.5, 15° C.) -0.283 -0.169 -0.116 -0.060 -0.067 



i; (before illumination) -0.435 -0.448 -0,496 -0.471 -0.470 

 E (after illumination) -0.071 -0.239 -0.046 -0.011 -0.009 



with the fifth. The meaning of the potentials measured before the illu- 

 mination is doubtful. The highest positive potential was reached with 

 toluylene blue. 



A similar but more extensive study was made by Wessels and Havinga 

 (1952, 1953; c^. Wessels 1954), who observed the decolorization in light 

 and the photogalvanic effect (change of redox potential in light) of fifteen 

 quinonoid dyes with normal potentials (pH 6.5, 30° C.) from —0.255 volt 

 (2,6-dichlorophenol-indophenol) through —0.137 volt (toluylene blue), 

 -0.028 volt (methylene blue), +0.098 volt (indigo disulfonate) to +0.269 

 volt (safranin T). They found, in air, photogalvanic effects for all 9 dyes 

 with Ei^' values up to —0.028 volt, and none for all 6 dyes with more posi- 

 tive redox potentials. Decolorization w^as observed for 7 dyes wdth poten- 

 tials up to —0.137 volt, and not for the 8 dyes with more positive poten- 

 tials. Under anaerobic conditions 10 dyes were studied, and photogalvanic 

 effects were observed with all but 3 of them, with a dividing line between 

 + 0.098 volt (indigo disulfonate) and +0.130 volt (indigo monosulfonate) . 

 Decolorization could be noted, however, only for dyes with potentials up to 

 —0.137 volt (as under aerobic conditions). (The potentiometric test is 

 more sensitive to small shifts in oxidation-reduction equilibrium than 

 visual observation of color changes.) 



The photostationary redox potentials, measured in illuminated solutions 

 in which a photogalvanic effect could be observed, ranged from 0.00 volt 

 to —0.10 volt, without obvious relationship to the E^ of the dye (or qui- 

 none). (About the limited significance of such photostationary potentials, 

 see p. 1522.) 



A study was also made by Wessels of Hill reaction with a mixture of two 

 oxidants (quinones or quinonoid dyes). If both components were "good" 

 Hill oxidants, both were reduced; if one was a "borderline case" (such as 

 thionine, E^' = —0.0777 volt), it was hardly reduced at all. The presence 

 in the mixture of an oxidant too positive for its own reduction did not afTect 

 the reduction of a "good" Hill reductant, except in the case of phthiocol 

 (which is known to inhibit the Hill reaction). 



