KINETICS 1605 



chapters 27 and 28 applicable to the Hill reactions are those on pages 1020- 

 1047, covering the effect of back reactions in the photochemical apparatus 

 and the influence of "finishing" reactions of limited capacity, such as the 

 enzymatic liberation of oxygen. 



An additional factor, which can play an important role in the Hill reac- 

 tion, is the back reaction between the two final products, molecular oxygen 

 and the reduced oxidant (ferric salt, or h;ydroquinone, or leucodye). In 

 photosynthesis this kind of back reaction is the cellular combustion of car- 

 bohydrates, an enzymatic reaction which is slow compared to photosynthe- 

 sis, at least in strong light. In the Hill reaction many fully reduced oxi- 

 dants are rapidly autoxidizable, so that a photostationary state is estab- 

 lished in light (as described earlier in this section). The position of this 

 state depends on light intensity, oxygen concentration, and the rate con- 

 stants of the several forward and back reactions. 



One of the few attempts to date at kinetic analysis of the photostation- 

 ary state of the Hill reaction, is that of Wessels (1954). He postulated the 

 simple sequence of three reactions : 



light 



(35.35Aa,a') H2O + X . XH2 + HO2 



dark 



(35.35Ab,b') XH2 + Q v QH2 + X 



(35.35AC) QH2 + 3^02 > Q + H2O 



where X is an intermediate redox catalyst common to all Hill systems (and 

 probably to photosynthesis as well) and Q (for quinone or quinonoid dye) is 

 a specific Hill oxidant. Comparison with reaction schemes such as 28. IB 

 (page 1026) shows one important (and probably unjustified) omission — 

 both back reactions, (35.35Ab') and (35.35Ac), are supposed to involve 

 molecular oxygen, and no provision is made for reoxidation by oxygen 

 precursors (systems Z/ZH2 or A'H0/A'0H2 in our earlier discussions, cf. 

 for example reaction 28.21d in scheme 28. IB). 



The reaction sequence (35.35Aa,b,c) leads to expressions for the photo- 

 stationary state which the reader can easily derive. These are of the form : 



[oxidant ] a [X ] [O2 ] '/2 + ?; [O, ] + c [oxidant ] [O. ] V2 



(35.35B) R = 



[reductant] d/[Chl] 



Depending on whether the back reaction (35.35Ab') or (35.35Ac) pre- 

 dominates, either the first, or the second and the third term in the numera- 

 tor can be neglected. In the second case (if one assumes the concentration 

 of the intermediate X to be proportional to that of chlorophyll), R becomes 

 independent of both chlorophyll concentration and the concentration of 

 the oxidant, in agreement with Wessels' experimental results (cf. e. g., 

 figure 35.23A). In the second case, R depends on both [Chi] and [oxi- 



