382 R. LUMRY AND J. D. SPIKES 



studies of the overall photosynthetie process (for example, see the 

 remarks which follow the paper of A. H. Brown in this volume.) 

 However, they represent a serious problem in kinetic studies of the 

 Hill reaction, since they occur in the range of oxidant concentration 

 customarily used in manometric measurements of Hill reaction ac- 

 tivity. Bradley and Calvin (7) in studies on the participation of 

 thioctic acid found effects of this compound which appear to be 

 closely related to those described above. The quenching of chloro- 

 plast fluorescence with increasing oxidant concentration, as studied 

 in this laboratory by Mayne, follows a relationship rather similar to 

 that demonstrated by A;^ as described above. It is apparent that oxi- 

 dants exert complicated effects on chloroplast properties in this high 

 concentration range. Thus, all studies of the Hill reaction made 

 under such conditions must be examined with great care in order to 

 distinguish between the fundamental processes involved and sec- 

 ondary reactions which may be artifacts. In order to avoid these 

 problems, most of the work discussed in this paper was carried out in 

 the middle oxidant range, where complications seem to be at a 

 minimum. If this could not be done, at least some studies w^ere made 

 to establish that effects were the same in both the middle and the upper 

 concentration ranges. 



Slightly soluble oxidants cannot be studied with the usual man- 

 ometric methods, since the acceptor must be at least several thou- 

 sandths molar in order that enough oxygen will be evolved to be 

 measured conveniently. A new technique has been developed in this 

 laboratory in which an excess of an inert, insoluble terminal electron 

 acceptor is added to the chloroplast system together with a low con- 

 centration of the electron acceptor being studied. As the experimental 

 electron acceptor is reduced by the illuminated chloroplasts, it will 

 in turn be converted back to the oxidized form by the terminal ac- 

 ceptor. This technique was used by Hochster and Quastel (8) in 

 studies on a number of respiratory systems. Such an approach also 

 permits measurements to be made at a constant redox potential 

 and in the very low concentration ranges of soluble oxidants. 



It was found that colloidal manganese dioxide would reoxidize at a 

 useful rate a lumiber of compounds which were of interest as possible 

 electron acceptors for the Hill reaction and which could not be 

 studied by the usual manometric techniques. Slightly soluble com- 

 pounds such as tohiquinone, anthraquinones, ribofla^'in, o-benzo- 



