OXIDATIOX-REDl'CTIOX POTENTIALS 193 



metal poipliyiins could not \>v iiieasuit'd. TaMe \ III shows tliat tlu- j)otcn- 

 tials of the base cobalt and manganese i)()iphyrins are lower than those of 

 the corresponding iron compounds. 



TABLE VIII 



Oxidation-Reduction Potentials of Cobalt and Manganese 

 Mesoporpliyrin Compounds" 



"According to Clark and co-workers (Jto2). ^30° C. 



7. OXIDATION-REDUCTION POTENTIALS 

 OF HEMATIN COMPOUNDS 



The change of iron v^alency plays a fundamental role in the function 

 of cytochromes (c/. Chapter VIII); the study of the oxidation- 

 reduction potentials of the simpler hemochrome and hematin systems 

 is therefore of great interest. In addition, it has been found possible 

 to derive from oxidation-reduction potential data a good deal of 

 information concerning the equilibria involved in hemochrome for- 

 mation. 



7.1. Oxidation-Reduction Potentials at Constant pH 

 7.1.1. The Henie-Hematin System. It has proved difficult to 

 establish reliable data on the heme-hematin system, stable potentials 

 being obtained only over a narrow range of hematin concentration 

 (180). The system is relatively inactive electrochemically (c/. Chap- 

 ter II, Section 7.2.6.). The first studies were carried out by Conant 

 and co-workers {Jt.72,Jf.81), who titrated hematin in borate buffer with 

 titanous chloride as reducing agent and ferricyanide as oxidizing 

 agent. They established that one equivalent per iron atom was 

 involved and that the ])otential decreased with increasing pH. 

 Conant's curves fitted the assumption of monomeric heme and 

 dimeric hematin, the potential becoming more negative with increas- 



