OXIDASES, PEROXIDASES, AND CATALASE 97 



(electron) from a nucleus which already carries two positive charges. 

 Non-ionized ferrous hydroxide, on the other hand, should be more 

 easily oxidized, since this process represents only the separation of 

 the electron from an electroneutral substance. If this reasoning is 

 sound, one would expect that ferrous iron, if built into a non-ionized 

 compound, should be readily autoxidizable regardless of the acidity 

 of the medium. The rate of autoxidation of ferrous complexes would 

 then be expected to be a function of the pH only in so far as the 

 stability of the complex is affected by changes in hydrion concen- 

 tration. In other words, under any given condition the velocity 

 of the reaction with oxygen should be proportional to the concen- 

 tration of the ferrous complex, which in turn is a function of the 

 concentrations of the ferrous ions and the complex-forming anions. 

 Since the latter, within a given pH range, depends on the hydrion 

 concentration, one would expect that within this range the rate of 

 oxidation would vary with the pH. Smythe proceeded to test this 

 working hypothesis on a series of inorganic and organic iron com- 

 plexes of relatively simple configuration. His manometric study of 

 the rate of autoxidation of ferrous pyrophosphate and ferrous meta- 

 phosphate, which are stable over a wide pH range, bore out the 

 prediction. The reaction rate varied strongly with the hydrion con- 

 centration, decreasing markedly in both instances with an increase 

 in acidity. The case of ferrocyanide is of special interest because its 

 structure bears a certain resemblance to the core of iron porphyrin 

 complexes. In both instances the iron atom is surrounded by four 

 groups, containing nitrogen linked to carbon atoms. In the case 

 of the ferrocyanide these groups carry negative charges. To escape 

 from the metal atom, an electron must pierce this "negative atmos- 

 phere," which is obviously difficult. We are not surprised, therefore, 

 to find that the rate of autoxidation of ferrocyanide is extremely small 

 in spite of the fact that the iron is present in covalent rather than in 

 ionic linkage. If it were possible to reduce the negative charge on 

 the groups encircling the iron atom, its reaction with oxygen should 

 be facilitated. Indeed we find that if the charge is reduced by re- 

 placing one of the CN~ groups by ammonia, a compound, penta- 

 cyanoammine-ferroate, is formed which is attacked by oxygen at 

 an appreciable rate. The striking observation of Baudisch and 

 Davidsohn (7) that the oxygen uptake is more rapid at pH 2 than 

 at 7 or 12, in contrast to the findings obtained with iron pyro- or 

 metaphosphate, is explained by Smythe in terms of a suppression of 

 the ionization under the influence of the increased hydrion concen- 



