CHAPTER V 



HEMATIN COMPOUNDS 



1. BASIS OF METAL COMPLEX FORMATION 



In the chapters on porphyrins and bile pigments, the ability of 

 these substances to combine with metals has repeatedly been men- 

 tioned. In the present chapter the main stress will be on the nature 

 and properties of the metal compounds as complexes, while the 

 structure of the organic part of the molecule will be treated as a 

 secondary matter. Several different metals can form such complexes, 

 and certain of these can combine with additional inorganic and organic 

 molecules. The latter combination not only greatly influences their 

 properties, but also is of profound importance for understanding 

 their biological function. 



Willstatter recognized that the metal compounds of porphyrins and 

 similar substances {e.g., chlorophyll) were not salts, but that, since 

 the metal compounds were soluble in organic solvents, the metal was 

 held in complex combination. Combination occurred with both 

 porphyrin esters and with etioporphyrin; free carboxyl groups were 

 therefore not necessary. It was then postulated, in the terminology 

 of Werner's theory, that the metal replaces the two hydrogen atoms 

 of two pyrrole rings while it is bound simultaneously by coordinate 

 linkage to the two tertiary nitrogen atoms of two pyrrolene rings. 



1.1. Stereochemistry of Complex Formation 



Haurowitz {1167) pointed out that complex salt formation left the 

 molar volume of porphyrin almost unaltered, and concluded that the 

 metal atom replaces the two central hydrogen atoms without straining 



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