PHILIP GEORGE 



There is evidence for hydrogen atom transfer (or H+ + 

 e-) in the reactions of both FcMb and Fcper (11,23). Measure- 

 ments of the /?H changes that occur in unbuffered solutions show 

 that for each equivalent of acidic ferrimyoglobin Fey"b(H20) 

 oxidized to Fe^b by IrClg", two equivalents of H ^- are formed; 

 and correspondingly on reduction by Fe(CN)6~, two equivalents 

 of H+ are removed from solution; i.e., 



Feii+b(H20) 



FcIJk + 2H+ + .- 



'Mb 



(7) 



From a study of the pH. variation of the equilibrium constant for 

 the reaction of ferrimyoglobin with chloriridate, it has been 

 found that these 2H+ arise in the oxidation process itself, and not 



Fig. 5. Full curve: Variation of 

 E'a with pH, at 20.4 °C. and ionic 

 strength 0.04 for the couple involving 

 the higher oxidation state of ferrimyo- 

 globin (24), Fei^X + 2H+ + ^- -^ 

 Fe^(H20). a, point of inflection due 

 to heme-linked ionizations (compare 

 curve C, Fig. 2) ; b, line changes slope 

 due to the ionization of the iron-bound 

 water molecule in ferrimyoglobin, 

 Fei+(H20) - FeMbOH2+ + H+ p^ 

 = 9.00 (15). Dotted curve: Hypo- 

 thetical variation for the Compound 

 I/Compound II couple of peroxidase, which, if the variation for the Com- 

 pound Il/Peroxidase couple is similar to that for myoglobin, would account 

 for the stability relationships in the peroxide system. 



through the operation of the heme-linked ionization, the effect 

 of which can readily be recognized by analyzing the data using 

 the equation given in Section II (24). The pK. of the group in 

 Fe^b is estimated as about 7.5, compared to 6.1 in ferrimyo- 

 globin, at 20.4 °C. and an ionic strength of 0.04. Using these 

 figures the p¥L variation of Eq for the FeMb/Fey5(H20) couple 

 can be calculated ; the results are plotted in Figure 5 and show 

 that in acid solution Fe^b is quite a strong oxidizing agent with 

 E^' > 1.0 V. 



372 



