1600 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



the degree of reduction becomes doubtful. The potential may now be af- 

 fected by some minor component or an impurity; and even if it does reflect 

 correctly the state of the main redox system, this state may be maintained 

 not by the (photochemical) forward and (thermal) back reaction, but by 

 some side reactions (such as the reoxidation of the reduced form by a 

 minor component) which are difficult to avoid even in pure solutions, not 

 to speak of complex biological systems. For this reason, the stationary 

 "photogalvanic" potentials measured by Wessels in various quinones and 

 quinononoid dye systems can be used for kinetic speculations only when the 



-£ inmV 



350 



200 



150 



WO 



ss. 



add, 

 o-pht 



N^. 



10 15 



Fig. 35.22D. Back reaction induced by darkening or o-phenanthroline 

 poisoning in chloroplast suspension in the presence of tokiylene blue (E'a = 

 -0.137 volt) (Wessels 1954). 



concentrations of the two forms are not too different in order of magnitude. 

 This is the case in the presence of oxygen, or in weak light, as illustrated by 

 figure 35.22C. The normal potential of DCIP is -255 mvolt (on the 

 "physicochemical" scale used in this book!); photostationary potentials 

 of —285 mvolt and —225 mvolt thus correspond to 90% oxidation and 

 90% reduction, respectively. 



That the photostationary potential is in fact determined by a back reac- 

 tion is illustrated by figure 35.22D, which shows how the reduction of 

 toluylene blue {Eo = —137 mvolt) yields to reoxidation in darkness, or 

 when o-phenanthroline is added, inhibiting the photochemical reaction. 



pH Effect. The optimum pH for the Hill reaction depends on the 

 oxidant (and, according to Punnett, 1954, on whether one works with whole 

 or broken chloroplasts) . 



With HilVs mixture (ferric oxalate + ferricyanide) Holt and French 

 (1946) found a maximum initial rate at pH 7.6 at 3° C, and at pH 7.0 at 

 10° C. Clendenning and Gorham (1950^ also found for this oxidant an 



