KINETICS 1589 



donor). He based this hypothesis on the finding of an "induction period" in the poten- 

 tionietric observations of photochemical processes in crude suspensions of Avena chloro- 

 plasts. It could be ascertained that during this period the ascorbic acid content of the 

 chloroplasts decreased. (Gerretsen suggested that photoxidation of ascorbic acid may 

 be the cause of induction phenomena also in photosynthesis.) In agreement with this 

 hypothesis, the "induction period" could be prolonged by tlie aildition of extra ascorbic 

 acid. This hypothesis is equivalent to the assumption that chloroplasts complete, in 

 light, a "Krasnovsky reaction" before they begin to carry out the "Hill reaction" and 

 liberate oxygen. However, it is also possible that the photoxidation of ascorbic acid 

 occurs by a mechanism of the type suggested by Mehler, i. e., via the formation of hydro- 

 gen peroxide by Hill reaction with molecular oxygen as oxidant, and secondary' reaction 

 of the peroxide with the substrate of photoxidation. 



(g) Miscellaneous Organic Compounds 



Some oxygen evolution was observed by Aronoff (194G) with salicylic 

 aldehyde and benzaldehyde, but considerable chlorophyll bleaching oc- 

 curred in this case. Some oxygen was also evolved with benzoyl peroxide; 

 none with salicylic acid, fructose, methanol or butadiene monoxide. 



Gurevich (1947) found that illuminated chloroplast suspensions (from 

 Primula, Stellaria or Atriplex) can reduce o-dinitrobenzene, NO2C6H4NO2, 

 first to NO2C6H4NHOH, and then to NO2C6H4NH2. Since this reagent is 

 a strong enzymatic poison, it was used in the form of a deposit on a filter 

 paper strip suspended in the illuminated chloroplast suspension; in other 

 words, it must have been reduced secondarily by products diffusing from 

 the chloroplasts into the aqueous medium. 



5. Kinetics 



(a) Methods for Measuring Reduction 



The common method for measuring the rate of the Hill reaction with 

 any oxidants is the manometric (or chemical) determination of liberated 

 oxygen. An advantage compared to photosynthesis is that only one gas 

 is exchanged as a result of the photochemical reaction. The dark reactions 

 also are less significant than respiration in living cells — at least with some 

 chloroplast preparations and some oxidants. However, a correction for 

 dark reactions, either consuming oxygen and liberating carbon dioxide 

 ("respiration") or liberating CO2 without consuming O2 ("fermentation") 

 often is needed {cf. section c) . 



With certain oxidants the determination of oxygen can be replaced or 

 supplemented by convenient determinations of the oxidant. Physico- 

 chemical methods are more valuable than chemical assaj^s because they 

 permit the progress of the reaction to be followed without taking samples for 

 titrimetric or gravimetric analysis. 



