196 V. HEMATIN COMPOUNDS 



It will be seen in Chapter VIII that the potentials of the cyto- 

 chromes (except cytochrome b) are much higher than those of any 

 of the hemochromes listed above. The denatured globin-hemochrome 

 system has a potential of — 0.158 v. at pll 9.17, and — 0.098 v. at 

 pH 7.06, values rather close to those of the pilocarpine and histidine 

 hemochrome systems (180). The potential of the hemoglobin- 

 hemtglobin system is much more positive (cf. Chapter \T, 5.1.7.). 



7.1.3. Determination of Constitution of Hemochromes. It 



was shown in Chapter II that oxidation-reduction potential data at 

 constant />II might be expected to yield values for the number of 

 molecules of base combining with oxidant (hematin) and reductant 

 (heme), the state of aggregation of all four components of the system 

 (i.e., heme, hematin, hemochrome, hem/chrome), and the number of 

 electrons involved in the reduction process. 



As has been noted in previous sections of this chapter, this has 

 been possible only to a limited extent, diie to the conditions imposed 

 by the particular systems. Thus, for example, the mesoporphyrin 

 system is restricted by the insolubility of mesoheme compounds. 



The equations given in Chapter II involved certain assumptions 

 as to properties of the system. If a system is found to conform to 

 these equations, the correctness of the assumptions is demonstrated. 

 If it does not, alternative formulations must be employed which fit 

 the experimental facts. Assumptions of this nature are: (a) that 

 there i? no change of aggregation on coordination with base; (6) that 

 equal numbers of base molecules, namely two, combine with both 

 oxidant and reductant; and (c) that there is no stepwise association 

 of heme or hematin with base.* 



There are, however, other assumptions which require further con- 

 sideration. These are, firstly, that there is no interaction of buffer 

 anions with oxidant and reductant — this is discussed in Section 7.3.; 

 and secondly, that the carboxylic acid groups in the side chains may 

 be considered fully ionized, and as having no effect on the potential. 

 The latter assumption is probably correct at the rather alkaline pH 

 used in Clark's studies, although the existence of the monoimidazole 

 monopotassium salt of Hamsik (cf. Section 4.2.3.) renders it a little 

 doubtful. However, in those i)orphyrins where the carboxyl groups 

 are separated from the porphyrin nucleus by a saturated chain, they 



* Cf. further di-soussioii in the paj)er of Shack and Clark (25JSa). 



