COMPOUNDS OF PORPHYRINS AND METALLOPORPHYRINS 241 



with base to form hemochromes; in the compounds which these metallo- 

 porphyrins form with globin, therefore, the metal atom may take part in 

 the Hnkage (r/. Chapter V, and also peroxidase, Chapter IX). In the copper 

 and zinc metalloporphyrins, however, it is unlikely that the valency shell of 

 the metal atom would permit the formation of a stable metal protein bond. 

 The insertion of the metal atom has also saturated the valencies of the 

 pyrrole nitrogens. 



In the copper or zinc mesoporphyrin globin compounds neither the metal 

 atom nor the pyrrole nitrogens are able to take part in the linkage; this must 

 therefore be either an electrostatic linkage between carboxyl groups in the 

 porphyrin and basic groups in the protein or must be based on van der Waals 

 forces between the flat aromatic porphyrin plate and the protein. The 

 similarity between the spectral changes observed when the porphyrins com- 

 bine with globin and when they combine with simpler substances (Section 

 3.3.4.) favor the former of these two possibilities. In this connection reference 

 may be made to the observations of Granick and Gilder {1035), who found 

 that porphyrins other than protoporphyrin, but not their esters, competi- 

 tively inhibit the incorporation of protoporphyrin into the molecule of 

 respiratory enzymes of hemoflagellates. 



3.3.4. Compounds of Porphyrins and Metalloporphyrins with Simple 

 Substances. Globin is not the only substance which causes sharpening of 

 the bands in the visible region of the spectrum and the appearance of a well- 

 marked Soret band on combination with porphyrins and metalloporphyrins. 

 The rather indistinct spectra which are observed in aqueous solutions of 

 porphyrins, nonferrous metalloporphyrins, and hematins are probably due 

 to interference with the resonance of the porphyrin ring by formation of 

 polymers (Chapters III and V). 



The mechanism by which polymerization is prevented is not clear in all 

 cases. In the porphyrins it is possible that polymerization takes place by 

 the formation of hydrogen bonds between the carboxyl on one nucleus with 

 one of the central hydrogens on another. Rupture of these intermolecular 

 links by combination with solvent or with other substances may again 

 establish the integrity of the resonance system within the single molecule. 



The state of porphyrins and metalloporphyrins in organic solvents such as 

 ethanol is irrelevant to the question of the linkage to globin. In aqueous 

 solution, however, J. Keilin has shown that the spectrum is sharpened by the 

 addition of surface-active substances such as bile salts or sodium dodecyl 

 sulfate, or by bases such as caffeine, 1-methylimidazole, or pilocarpine (1504.). 

 While the former may exert their effect by van der Waals' interactions or by 

 bond formation to tlie pyrrole nitrogens, the latter class of substances almost 

 certainly combine with the carboxyl groups. The evidence for the change 

 in the state of the porphyrin induced by caffeine does not rest solely on 

 spectroscopic grounds, since the porphyrin was no longer precipitated by 

 calcium carbonate. 



Keilin's observations on turacin (copper uroporphyrin, cf. Chapter III) 

 make it unlikelv that van der Waals interactions are involved in these 



