RED, RED-GREEN, AND GREEN HEMINS 201 



than that of the ferric form is shifted further to the alkaUne side, 

 away from the biological pH range. This does not necessarily hold, 

 however, for all hematin cyanide compounds, particularly not if the 

 hematin compound combines with undissociated hydrocyanic acid 

 (c/. Chapter VIII). 



7.3. Influence of Buffer Anions 



Barron claimed that buffer anions combined with hematin and 

 thus exerted an influence on the oxidation-reduction potential, but 

 his data show this only for borate ion; the points for phosphate and 

 veronal buffers lie on the Ek/pH curve of a solution of hematin in 

 alkali without buffers. 



Clark et al. were unable to find evidence for any such effect, 

 although admitting that it had not been entirely excluded (c/. 2835a). 

 There is no doubt, however, that under certain circumstances, the 

 presence of buffer anions may bring about a change in the activity 

 coefficients of oxidized and reduced metalloporphyrins and of their 

 compounds with bases, so that allowance must be made for this in 

 the equations developed in Chapter II. 



Evidence has been adduced for the combination of anions with 

 other hematin compounds, in the case of peroxidase and catalase 

 (c/. Chapter IX), 



8. HEMATIN COMPOUNDS OF AZA- AND 

 OXYPORPHYRINS AND OTHER HEMATIN COMPOUNDS 



8.1. "Red, Red-Green, and Green Hemins" 



At a time when our knowledge of hematin compounds was still 

 little developed, Warburg {2928) suggested that three different types 

 of hematin compounds existed, red (porphyrin) hemins, green (chlo- 

 rophyll) hemins, and green-red pheohemins (respiratory ferment and 

 spirographis hemin). This classification has now been shown to be 

 much too primitive, and should no longer be used. Azahematins 

 and oxyporphyrin hematins have little structural relationship to 

 formylporphyrins of spirographis or of pheoporphyrin b tj'pe, but 

 still would have to be classed as red-green hemins. The shift of 

 ^absorption bands required to transform a "red" into a "green-red" 

 hemin can be brought about in several very different ways, by 

 structural alterations both in the side chains (the substances in this 



