CHEMICAL KINETICS OF THE HILL REACTION' 370 



reduced. The rapidity of this effect, which appears to ])e catalyzed 

 by fragments, varies widely with different oxidants. Within experi- 

 mental error, rates as measured with ferricyanide, for example, are 

 independent of oxygen partial pressure up to 1 atm. except for lengthy 

 runs at low light intensities. Some substituted naphthocjuinones, on 

 the other hand, are so sensitive to back oxidation that they can be 

 used only under highly anaerobic conditions. This effect is probably 

 responsible for the various reports on the relation between the half- 

 cell potentials of different oxidants and Hill reaction velocities rather 

 than any fundamental relationship between reducibility of oxidants 

 and the reducing power of illuminated chloroplasts. The literature 

 shows clearl}^ that under proper conditions illuminated chloroplasts 

 can reduce very ''poor" oxidants such as TPN about as rapidly as 

 "good" oxidants like ferricyanide and benzoquinone (9). It can be sug- 

 gested, then, that most Hill oxidants act in about the same way 

 and permit about the same maximum rate of reaction when artifacts 

 are eliminated. As a consequence the energy requirements for photo- 

 synthesis and the Hill reaction must be about the same. In this lab- 

 oratory quantum requirements for the Hill reaction with light of 

 6750 A (as extrapolated to zero light intensity) of 8 have been ob- 

 tained — a result which is in good agreement with the values for photo- 

 synthesis as obtained by Emerson, Daniels, and many others. 



The Hill reaction rate of chloroplast fragments is linearly de- 

 pendent on chloroplast concentration (as measured by chlorophyll 

 content) over a rather wide range. When studied as a function of 

 oxidant concentration, however, it appears that there are three dis- 

 tinct regions of behavior as the oxidant concentration varies from 

 10"^ M to 10~^ M. The Hill reaction is very difficult to study at ex- 

 tremely low^ oxidant concentrations (10~^ M to 5 X 10 ~^ M) be- 

 cause of back-oxidations due to oxygen and because even highly 

 washed chloroplast fragments retain some reducing power which 

 cannot be conveniently oxidized away without altering the chloro- 

 plast properties. In this low oxidant region the rate of the Hill reaction 

 drops off rapidly with decreasing oxidant concentration. It is prob- 

 able that in this range the reaction rate is limited by oxidant concen- 

 tration according to a first-order diffusion process. 



A middle region with respect to the relation between oxidant con- 

 centration and Hill reaction rate is found in the range 5 X 10~^ il/ 

 to lO-^' M. As shown with ferricyanide as oxidant in Fig. 2, the reac- 



