'different oxidants 1567 



(35.3lAa) 2 H.O — ^^ 2 {H} + 2 {0H| 

 (35.31 Ab) 2 {OH} > H.O + V2 O2 



(35.31AC) 2 {H} + O2 > H2O2 



catalase 



(35.3lAd) H2O2 + CaHsOH > C2H4O + H.O 



(35.31Ae) V2O2 + C2H5OH ^ C2H4O + H2O 



catalase 



The normal redox potential of the system O2/H2O2 is 0.27 volt (cf. table 

 11. 1), well within the region of other efficient Hill oxidants. Aerated 

 chloroplast suspensions contain about 10~^ mole/1, oxygen; quinone and 

 other common Hill oxidants need to be present in concentrations of 2 X 

 10~^ mole/1, or higher (cf. section 5(6) below) to ensure high efficiency of 

 oxygen liberation. This indicates that oxygen is a highly effective com- 

 petitor of the other Hill oxidants, and emphasizes how remarkable is the 

 fact that it is not an effective competitor of carbon dioxide in live, aerobi- 

 cally photosynthesizing cells. 



Mehler suggested that photoxidations in vivo (described in chap. 19) 

 may be based on a Hill reaction with oxygen as oxidant (as suggested in 

 Vol. I, p. 544), in which some of the hydrogen peroxide, formed as reduc- 

 tion intermediate, is intercepted by peroxidases, causing it to react with the 

 photoxidation substrates (in competition with its dismutation by catalase) . 



Mehler and Brown (1952) confirmed the above-suggested mechanism of 

 the ''Mehler reaction" by showing mass spectrographically that the net 

 consurnption of oxygen, according to equation (35. 31 A), is in fact the result 

 of superposition of an oxygen evolution (eqs. 35.31Aa,b) upon a 

 (twice as large) oxygen consumption (eq. 35. 31 Ac). For the purpose 

 of this demonstration, the reaction was carried out in O^^ and O^^ enriched 

 oxygen (in a helium atmosphere) . The experimental system also contained 

 quinone, in addition to catalase and alcohol. In light, quinone was reduced 

 first, causing 62^^ to be evolved, while the 02^** concentration remained con- 

 stant. When practically all quinone was exhausted, the "Mehler reaction" 

 got under way; both 62^- and 62^* were now consumed, but the second one 

 much faster than the first. The curves showing the consumption of the two 

 isotopic species were in quantitative agreement with predictions based on 

 mechanism (35. 31 A). 



Mehler (195P) studied more closely the competition between quinone 

 and oxygen as hydrogen acceptors in the Hill reaction. He found that in a 

 chloroplast suspension containing both quinone and ethanol -f catalase, 

 quinone was hydrogenated at the usual rate until the reaction was 90% 

 complete, at which time the oxygen consumption began; remarkably 



