194 V. HEMATIN COMPOUNDS 



ing hematin concentration. Barron (180) found, however, a positive 

 shift under the same conditions, individual curves fitting the assump- 

 tion that both heme and hematin were monomeric. Barron's study 

 still provides the best values of the protoheme protohematin system.* 

 When hemes derived from different porphyrins are used, the same 

 difficulties obtain, and are in some cases enhanced, e.g., with meso- 

 hematin, the reductant in this case being so insoluble as to make 

 redox potential measurements impossible (Davies, 536). 



7.1.2. Systems Involving Coordinating Substances. When a 

 coordinating substance is added to a solution containing heme and 

 hematin, the potential usually becomes quite stable, so that precise 

 measurement is possible. Conant and Tongberg made the first 



TABLE IX 



Oxidation-Reduction Potentials of Hemochrome-Hemichrome Systems" 



"According to Barron (180,1<S2,1S3) and Clark and co-workers H52). 

 '> ;;H 9.63, 30° C. 



measurements on pyridine hemochrome, and other hemochrome sys- 

 tems were studied by Barron (180,184^). The most extensive investi- 

 gation so far recorded is that of Clark and collaborators {536,51^2,51^3, 

 2872,27^9), of which the theoretical treatment has been given in 

 Chapter II. 



By combination with bases or cyanide the characteristic potential 

 is shifted in a positive direction in increasing order by cyanide, 



* Although n = 1, Shack and Clark conclude that both heme and hematin are 

 dimeric {2538a). 



