PORPHYRINS WITH TWO CARBOXYLIC ACID GROUPS 59 



latter synthesis if carried out at 90-95° C. yielded a porphin with an 

 absorption spectrum rather similar to that of Fischer's preparation, 

 but not quite identical (c/. below). At 150° C. another "porphin" 

 with different absorption spectrum and lower HCl number (c/. 

 Section 6.3.) was obtained, which Rothemund considers to be an iso- 

 meride. The existence of two isomeric porphins would be of consid- 

 erable theoretical significance (cf. Section 6.3.) if it could be confirmed; 

 the present evidence is unconvincing and the porphin of Rothemund 

 is probably a mixture of porphin with other substances (cf. Pruckner, 

 S188; see also 125a). 



3.2. Etioporphyrins 



Willstatter believed that etioporphyrin was the noncarboxylated 

 porphyrin of both hemin and chlorophyll. Fischer showed, however, 

 that the chlorophyll-etioporphyrin was a mixture of two porphyrins, 

 phylloetioporphj^rin and pyrroetioporphyrin. The etioporphyrin from 

 hemin has the constitution C2oH6N4(CH3)4(C2H5)4, Pyrroetiopor- 

 phyrin, C2oH6N4(CH3)4(C2H5)3H, has one ethyl group less, and phyllo- 

 etioporphyrin has the same side chain arrangement as this, but has 

 one additional methyl group on one of the methene groups of the 

 central ring (cf. Table III). Etioporphyrin should be called more 

 correctly mesoetioporphyrin, since it arises by decarboxylation of 

 mesoporphyrin, while protoporphyrin can be decarboxylated to a 

 similar porphyrin with vinyl side chains, C2oH6N4(CH3)4(C2H5)2 

 (€2113)2. The properties of etioporphyrin III and its preparation by 

 synthesis or by decarboxylation of mesoporphyrin IX and copro- 

 porphyrin III are described in Fischer's book (861, p. 199). Etiopor- 

 phyrin, having no carboxyl groups, is an extremely weak acid with a 

 pK of 16 (1808). 



3.3. Porphyrins with Two Carboxylic Acid Groups 



3.3.1. Mesoporphyrin IX. C2oH6N4(CH3)4(C2H5)2(CH2CH2C02H)2. 



This substance was first prepared by Nencki (2036) by mild treat- 

 ment of hemin with hydriodic acid and phosphonium iodide. The 

 iron is removed and the vinyl groups of protoporphyrin are hydro- 

 genated to ethyl groups. Instead of phosphonium iodide, sodium 

 sulfite can be used for the removal of excess iodine (2652). Good 

 yields (60%) are obtained if protoporphyrin is reduced by hydriodic 

 acid in acetic acid in the presence of ascorbic acid (2^78,1058). 



