388 H. LUMRY AND J. D. SPIKES 



lack of (luantitative data which would allow the determination 

 of the method of inhibition, the thermodynamic binding quantities, 

 and the particular elementary steps of the whole process which are 

 influenced. A full account of our results in this area of investigation 

 would be out of place here but several generalizations may be men- 

 tioned. 



We have investigated a variety of urethanes and related compounds, 

 metal chelating agents, sulfhydryl inhibitors, Hill oxidants, and a 

 scattering of other powerful inhil)itors, as well as a large number of 

 less specific agents effective only at high concentration. All strong 

 inhibitors, including the so-called narcotics which have been pre- 

 sumed to be nonspecific denaturing agents for proteins, give mass-law 

 behavior with small orders of the inhibition reaction in inhibitor con- 

 centration. Failure to find exact mass-law behavior in some cases ap- 

 pears to be due to lack of control in the experiments and to com- 

 plications introduced by the presence of Hill oxidants. For example, 

 at high oxidant concentrations phenylurethane inhibits only ko 

 but experiments in the middle range of oxidant concentration show a 

 pronounced A-^ inhibition. Inhibitors differ in their effect even in the 

 middle region depending on the oxidant employed. 



Inhibitor studies are complicated by inactivation of chloroplasts 

 during long experiments as well as by local heating under intense il- 

 lumination. Most of our inhibitor results must thus be considered to be 

 of a preliminary nature. However, some observations of a qualita- 

 tive nature are valid. In general, strong inhibitors reduce both A'd 

 and Ul though with different orders and at different concentrations. 

 In terms of a simple geometric "photosynthetic unit," an inhibitor in- 

 fluencing any slow step along the chain would appear to have an 

 identical inhibiting effect on any detectable preceding slow steps, 

 since mass-action kinetics would not apply in a localized array of 

 molecules. Such does not appear to be the case, and we must seek a 

 more complicated mechanism — a conclusion which can be seen to 

 be also a consequence of the rate law. No purely Ax inhibitors have 

 been found except Hill oxidants, perhaps suggesting that Al and ko 

 slow steps are closely related. Detailed investigation of the inhibition 

 reactions should thus provide a means for relating these slow steps 

 and for determining the characteristics of the molecules involved. In 

 the few cases thus far examined, A^ inhibition is paralleled by quench- 

 ing of fluorescence so that we may tentatively conclude that some in- 



