1574 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



reduction intermediate (such as a semiquinone) . The thermodynamic 

 equiUbrium hes practically always on the side of the dye and water, not on 

 that of the leuco dye and oxygen. Therefore a back reaction is always 

 possible — although it may be so slow as to be practically insignificant. 

 The redox potential of the dye-leuco dye system is more likely to affect the 

 rate of the dark back reaction than the quantum yield of the photochemical 

 forward reaction. When the back reaction is fast, the Hill reaction becomes 

 unobservable by ordinary methods. It may still be detectable by isotopic 

 tracers, or by "trapping" the product chemically or physically, or by sensi- 

 tive methods revealing a small shift of the oxidation-reduction equilibrium 

 in light. We recall that in Hill's original experiments the rather fast re- 

 oxidation of ferrous oxalate by oxygen had necessitated the addition of ferri- 

 cyanide as "stabihzer." Leucothionine or leucomethylene blue, if they are 

 formed in the Hill reaction together with oxygen, ^\dll be reoxidized even 

 more rapidly. Leucophenol indophenol, on the other hand, is reoxidized 

 much more slowly, thus permitting practically complete decoloration in 

 light. Only when the back reaction is negligible does the rate of decolora- 

 tion provide a true measure of the photochemical reactivity of the suspen- 

 sion (i. e., of the quantum yield of the photochemical reaction). 



Additional quinonoid dyes were tried out by Holt, Smith and French 

 (1951). Table 35.IX shows the results. 



Table 35.IX 



Indophenol Dyes Reduced by Illuminated Chlokoplasts 

 (after Holt, Smith and French, 1951) 



Color at 



Dye (sodium salt) 



pH 6.5 



pH 8.0 



2,6-Dibromobenzenone-indo-3'-carboxyphenol 

 2,6-Dibromobenzenone-indo-2'-bromophenol 

 2,6-Dibromobenzenone-indo-3'-methoxyphenoI 

 2,6-Dibromo-2 '-methyl-5 '-isopropylindophenol 

 Ben zenone-mdo-2 '-methy 1-5 '-isopropylphenol 

 2,6-Dibromobenzenone-indophenol 

 Benzenone-3 '-methyl-6 '-isopropylphenol 



Red-violet" Blue" 



Blue" 

 Blue" 

 Blue" 

 Pink* 

 Blue" 

 Red*- 



Blue" 

 Blue" 

 Blue-^ 

 Blue" 

 Blue" 

 Olive green" 



" Means strong decolorization. 

 '' Means little decolorization. 

 " Means no decolorization. 



Macdowall (1952) illuminated a suspension of washed Swiss chard 

 chloroplasts in distilled w^ater for 2 V4 hours wdth light of about 2 klux in the 

 presence of nine dyes, and determined the degree of reduction at the end of 

 exposure either photometrically or potentiometrically. The potentials 

 reached with five oxidants are Hsted in table 35.IXA. 



The figures in table 35. IX A show that reduction was practically com- 

 plete with the first three dyes, far-reaching with the fourth, and neghgible 



