68 III. PORPHYRIN CHEMISTRY 



doubt that acid causes a large increase of the porphyrin concentration in 

 the shed urine. The porphyrin formed in this way can hardly be uniform 

 uroporphyrin III, if Waldenstrom's assumption that it is formed from por- 

 phobilinogen is correct. Two dipyrrolic precursors would be needed to yield 

 uroporphyrin III, and their autocondensation would lead to the formation 

 of other isomeric uroporphyrins. 



It is also remarkable that Fischer found turacin to be the copper complex 

 salt of uroporphyrin I, while Rimington (2263) obtained coproporphyrin III 

 by the decarboxylation of the uroporphyrin from turacin. Before more 

 reliable methods of identifying the isomers of copro- and uroporphyrin are 

 worked out, it would appear wise to draw conclusions from these identifica- 

 tions only with caution {cf. Chapter XII). 



3.4.3. Porphyrins with Five to Seven CarI>oxylic Acid Groups. The evi- 

 dence for a heptacarboxylic acid in "uroporphyrin III" has just been dis- 

 cussed. The conchoporphyrin of the Pteria shell was found to be a penta- 

 carboxylic acid (.S'J.9), yielding coproporphyrin I on decarboxylation; melt- 

 ing point of the ester is '■271-'-27;}°. It is possible that some other porphyrins 

 encountered in human excretions {-'/JO^'SUIf) belong to this class of porphyrins 

 between coproporphyrins and uroporphyrins. Pentacarboxylic acids have 

 also been observed as decarboxylation products of uroporphyrins (lOoG). 



4. ASPECTS OF THE PHYSICAL CHEMISTRY 

 4.1. Solubility and Acid-Base Character 



Porphyrins are amphoteric compounds with isoelectric points or 

 zones at about /;II 8 to 4.5; their acid character depends on the car- 

 boxyl groups in their side chains and is therefore lacking in the 

 etioporphyrins; their character as weak bases depends on the presence 

 of two tertiary nitrogens in the pyrrolene nuclei. "Zwitterions play 

 no role. Porphyrins are therefore easily flocculated and practically 

 insoluble in dilute acetic acid. 



The partition of jjorphyrins between water and organic solvents, 

 particularly ether, depends on the nature of the side chains and on 

 the pH. The large plate of the porphin ring confers upon the mole- 

 cule a hydrophol)ic character which is counteracted by the hydro- 

 philic carboxylic acid groups in the side chains, particularly under 

 conditions under which they carry electric charges (as in alkaline 

 solution). Porphyrins without acidic side chains can be extracted 

 from ether only by strong minora] acids, which form dihydrochlorides. 

 Porphyrins with two to four carboxylic acid side chains pass from the 

 aqueous phase into ether at a pll of .3-4, at which their ionizable 

 groups carry no charges; they are extracted from ether by mineral 



