1572 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



Aronoff (1946) repeated Warburg's experiments with a variety of naph- 

 thaquinones and anthraquinones and found that they reacted at a rate 

 roughly parallel to their oxidation-reduction potentials — in other words, 

 stronger oxidants were reduced faster than the weaker ones. (This com- 

 parison refers to the saturation rate in strong light, not to quantum yield 

 in weak light!) These results are in agreement with Holt and French's ob- 

 servations on dyestufts (cf. Table 35.VIII). 



Wessels and Havinga (1952, 1953; cf. also Wessels 1954) prepared and 

 investigated 28 quinones with a range of normal potentials (measured at 

 pH 6.5) from —0.444 volt (tetrachloro-o-benzoquinone) , through -0.332 

 volt (p-benzociuinone), -0.181 volt (1,2 naphthaquinone), -0.086 volt 

 (1,4 naphthoquinone), +0.020 volt (2-methyl-l,4-naphthoquinone), 

 +0.135 volt (phthiocol), to +0.21 volt (chloranilic acid). They measured 

 the effect of illumination on the redox potential of the chloroplast suspen- 

 sions containing these quinones, and found, under aerobic conditions, a 

 "photogalvanic effect" (change of redox potential in light) with all (17) 

 quinones having normal potentials from -0.444 up to 0.058 volt (phen- 

 anthrene quinone) ; and no photogalvanic effect for all (11) quinones with 

 potentials above 0.058 volt. Under anaerobic conditions, 7 quinones, with 

 normal potentials from -0.444 to +0.180 volt were studied, and a photo- 

 galvanic effect was observed in six of them, with normal potentials up to 

 +0.090 volt (2-hydroxy-l,4-naphthoquinone); no effect was observed 

 only with sodium anthraquinone-2-sulfonate (Eo' = +0.180 volt). Similar 

 results were obtained with quinonoid dyes (see below). 



(e) Organic dyestuffs 



Closely related to quinones are many organic dyestuffs that contain 

 closed, conjugated double bond systems, and can be converted to "leuco- 

 dyes" by the addition of two hydrogen atoms. In the leuco- 

 dyes the conjugated system is either destroyed or, at least, reduced in 

 length, wdth consequent weakening of absorption bands and their shift 

 into the ultraviolet region. Many of these dyes form strong electrolytes, 

 and their oxidation-reduction potentials can be easily determined. 



Holt and French (1948) first observed that a number of reversibly 

 reducible organic dyes can serve as photochemical oxidants of water in the 

 presence of chloroplast suspensions. Attention must be given to photo- 

 chemical effects produced by light absorption in the dye, and to the sepa- 

 ration of this absorption and its chemical effects from those of chlorophyll 

 (or other chloroplast pigments). 



Quantitative experiments were made by Holt and French (1948) with 

 the red dye phenol indophenol. No reaction could be observed in the dark, 

 or with chloroplasts preheated to 50° C. Tests of the purity of the dye 



