98 A SYMPOSIUM ON RESPIRATORY ENZYMES 



tration and a consequent decrease of the negative charges around 

 the iron atom. 



However, Smythe's views are not shared by Weiss (78). This 

 author beheves, in direct contradiction to the hypothesis that only 

 coordinatively bound ferrous iron is autoxidizable, that only "free" 

 ferrous ions react, in general, with molecular oxygen. By "free" ions 

 Weiss means iron with incompletely filled electron orbits. The 

 autoxidation of ferrous sulfate is formulated in accordance with 

 Weiss's theory of simple electron transfers as follows: 



O2- + H* ?^ HOT 



HO2- + H^ = H2O2 

 Equation 15 explains the formation of hydrogen peroxide as an 

 end product of autoxidation processes. If, however, it is not caught 

 in statu nascendi, e.g., by cerium hydroxide, the hydrogen peroxide 

 is relatively rapidly decomposed by reaction with ferrous iron- 

 ferric ions and hydroxyl ions or water being formed as final prod- 

 ucts. During the entire process the oxygen molecule is stepwise 

 reduced by four electrons with the formation of four hydroxyl ions. 



The sequence given above, according to Weiss, provides a logical 

 explanation for the slow rate at which an acidified ferrous sulfate 

 solution is autoxidized. The latter does not indicate that the inter- 

 action between the ferrous ions and oxygen (reaction 13) is slow 

 but, on the contrary, that the rate of the back reaction 14, which 

 involves a reduction of ferric ions, is quantitatively significant. 

 Under stationary conditions and for a given partial tension of oxy- 

 gen, the rate of reaction 15 is defined by the ratio of the velocities 

 of the partial reactions 13 and 14; in other words, by the ratio 

 [Fe^*] to [Fe^^*]. The greater this ratio the more positive will be the 

 oxidation-reduction potential and the higher will be the rate of 

 ferrous salt oxidation. The essential feature in reactions of this type, 

 according to Weiss, is not the formation of stable ferrous iron com- 

 plexes but the fact that the ferric ions are protected against reduc- 

 tion by the formation of still more stable ferric iron complexes. Thus, 

 in instances where the ferrous complex is more stable than the corre- 

 sponding ferric complex, as in the case of ferrous tridipyridyl sulfate, 

 no autoxidation takes place. 



A decidedly more mechanistic explanation is offered by Theorell 

 (73) for the failure of cytochrome c to react with molecular oxygen 

 in the physiological pH range: "The heme of the cytochrome is 

 . . . built into the protein component in a manifold way: by means 



