HYDROGEN PEROXIDE HYPOTHESIS 



283 



Table 11. 1 

 Thermodynamic Data on Hydrogen Peroxide and Water 



Reaction 



(11.1') 02(g.)+H2 ^HjOaCaq.) 



(11.2') H202(aq.) + H2 > 2 H20(l.) 



(11.1',2') 02(g.) +2H2 



->• 2 H20(l.) 



(11.3) H202(aq.) 



-> H.Od) + i 02(g.) 



AH 



45.7 

 91.1 



-136.7 



- 22.7 



AF 

 (298° C.) 



31.5 

 81.7 



-113.1 



- 25.1 



Eo 

 (pHO) 



-0.69 

 -1.77 



-1.23 



Eo' 

 (PH7) 



-0.27 

 -1.35 



-0.81 



Because of the instability of hydrogen peroxide, oxygen acts in 

 reaction (11.1') as a comparatively weak oxidant (£^0' = — 0.27 volt), 

 and the larger part of the energy of oxidation is released in the second 

 oxidation step (11.2'), which corresponds to Eo = - 1.35 volt, or in the 

 dismutation of the peroxide (Eq. 11.3). For example, when a standard 



— C — C — bond is oxidized by oxygen to a 



H H 

 oxidation step 



-C=C — bond, the first 



(11.7) 



— C— C— + O2 



I I 

 H H 



-C=C— + H2O2 



liberates only 5 kcal, while 23 kcal is released by the subsequent dismu- 

 tation of hydrogen peroxide, and 50 kcal by the oxidation of a second 



— C — C — group by H2O2. If, in the oxidation of a triose, all hydrogen 



H H 

 would pass through the stage of hydrogen peroxide, the energy liberation 

 would be divided as follows : 



(11.8a) 

 (11.8b) 



(11.8) 



-> 3 CO2 + 3 H2O2 + 267 kcal 



CaHeOs + 4^02 — 



3 H2O2 > 3 H2O + 1^ O2 + 69 kcal 



CaHeO, + 3 O2 



-» 3 H2O + 3 CO2 + 336 kcal 



The main channels of animal respiration by-pass hydrogen peroxide 

 (although it can be formed by certain side reactions, as in the direct 

 autoxidation of yellow respiration enzymes). One reason for the avoid- 

 ance of hydrogen peroxide may be that the energy of peroxide dismutation 

 cannot be utilized for muscular work as easily as is possible with the 

 energy of various oxidation-reductions (by coupling them with trans- 

 phosphorylations, cf. page 224). Another reason may be that the 

 formation within the cells of oxidants with a potential as negative as 



