CHEMICAL KINETICS OF THE HILL REACTION' 375 



obtained at satiiratiiis li^lit intensities. The hijj;h velocities obtained 

 with such material may result from the removal of natural inhibitors 

 or from the prevention of inhibitor formation during the activating 

 process. In this sense these high rates may represent the true in- 

 trinsic chloroplast reaction. On the other hand, since activation is in 

 general obtained only with chloroplasts prepared under nitrogen, 

 it may be that some oxygen-sensiti\T, high-energy compound is 

 present which can contribute a part of the energy requirement of the 

 Hill reaction. Treatment with oxygen or other oxidants during 

 illumination removes the activation effect. 



The most serious problem encountered in attempting to make 

 precision rate measurements is the rapid second-order inactivation 

 reaction previously described (3). Two additional observations by 

 Bishop on this spontaneous reaction are worth noting: (1) Both low- 

 light and high-light slow steps are decreased in exactly the same 

 way; (2) the reaction requires oxygen, as judged from the observa- 

 tion that of a variety of attempts made to slow down the loss rate, 

 only a very considerable reduction of the oxygen partial pressure 

 had a large effect. 



THE RELATION BETWEEN LIGHT INTENSITY AND HILL REACTION 



RATE 



It is necessary in this chemical-kinetic approach first to establish 

 the relation between reaction rate and light intensity. Then any pro- 

 posed mechanism for the process, to be correct, must first of all satisfy 

 the requirements of the observed rate vs. light intensity kinetic re- 

 lationship. A tremendous number of light curves, especially for photo- 

 synthesis, have appeared in the literature. Early work in this labora- 

 tory resulted in light curves which closely approximated rectangular 

 hyperbolas. However, the variability was such that it was not possible 

 to say that the relationship was actually that simple. With improved 

 techniques Rieske (4) in this laboratory has shown that the relation 

 is actually that of a rectangular hyperbola to a high level of signifi- 

 cance. He has been able to establish by statistical methods that the 

 following steady-state rate law (which is in the form of a rectangular 

 hyperbola) provides an excellent fit to rate data secured with finite 

 layers of reaction mixtures (up to several millimeters) with optical 

 densities up to O.GO: 



K + I A 



