1592 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



tiire of thionine and ferrous sulfate, responds to the reduction of the dye 

 more than to the oxidation of ferrio salt (cf. part A, section 5 above). In 

 following the Hill reaction with a redox electrode, one is actually measuring 

 the photogalvanic effect in a system in w^hich only one of the two compo- 

 nents is definitely known, and assumes that the other has no effect on the 

 potential at all (e. g., because it remains confined in the chloroplasts and 

 only escapes after the conversion of the oxidized form to molecular oxygen, 

 which makes it "electrode inactive"). This is not necessarily true — some 

 intermediates in oxygen liberation from water may diffuse into the medium, 

 and may be electrode-active there. Another possible complicating factor 

 is that in the presence of several Hill oxidants (e. g., of the "natural" hy- 

 drogen acceptor together wdth the added one), the electrode may respond 

 preferentially to one of them — and not necessarily to the one present in 

 the highest concentration. 



An elegant method for the study of the Hill reaction is the measurement 

 of isotopic oxygen exchange by means of a mass spectrograph. This method 

 alone permits measuring, simultaneously, oxygen-liberating and oxygen- 

 consuming processes. Its application by Brown and co-workers to photo- 

 synthesis and respiration is described in chapter 37 D (section 3) ; it was 

 applied to the Hill reaction by Mehler and Brown (1952) and Brown 

 (1953). 



(6) Rate and Yield under Different Conditions 



We include in this section a summary of measurements of the rate of the 

 Hill reaction under different conditions even though the reproducibility of 

 these measurements is not too well assured. In particular, it has not yet 

 been possible to maintain a constant rate of reaction in chloroplast sus- 

 pensions for more than a short period of time (of the order of an hour at 

 best, and often much less). The comparison of the efficiencies of the Hill 

 reaction with different preparations and different oxidants is therefore best 

 based on measurements of the initial rate, made in the first few minutes 

 of illumination of a fresh preparation (or a preparation preserved without 

 loss of activity as described in section 3(6) above). 



The most important kinetic constants of a reaction such as photosyn- 

 thesis, or Hill reaction, are the maximum quantum yield as observed in 

 weak light and the saturation rate in strong light. In studying the Hill re- 

 action, attention has also been paid to the total amount of the added oxidant 

 that can be utilized before the reaction comes to a standstill; however, 

 this proportion has more to do with the chemical stability of the oxidant 

 and its reactions with various components of the chloroplastic matter 

 than with the efficiency of the photochemical apparatus. 



