DIPYRROLIC AND RELATED PIGMENTS IN NATURE 151 



compound had been described as early as 1870 by Stokvis, who called it 

 "reduzierbares Nebenprodukt" {cf. 1366). Fischer and co-workers (807,856) 

 had assumed propentdyopent to be an Q!,a'-dihydroxydipyrrylcarbinol, with 

 a hydroxymethylene group in the center, but the investigation of Siedel 



_/\ ApA A__, _ XT r^/\ ApA A„-, 



O N ^ N OH2O NaO N ^ N ONa 

 H H H H, H 



A B 



Fig. 29. Propentdyopent (A) and pentdyopent (B), according to Siedel 



and Moller {2558) suggests that it is rather an a.cc'-dihydroxydipyrrylketone 

 with the carbonyl group in the center. Pentdyopent itself was considered 

 by Siedel in 19-10 as the alkali salt of the dipyrrylcarbinol but from its prop- 

 erties a ketyl (radical) structure (Fig. iS) appears more likely. In a later 

 review {2553), Siedel suggested the formula shown in Figure 29A for pro- 

 pentdyopent and the formula in Figure 29B for the red alkali salt of pent- 

 dyopent, but the formulas in Figure 28 appear to be more likely. 



8.3. Porphobilinogen 



By chromatographic analysis, Waldenstrom {2906,2911; cf. also Prunty, 

 2192) isolated a chromogen from the urine of patients with acute porphyria. 

 He called it porphobilinogen, since, on treatment with acids, it was trans- 

 formed into uroporphyrin and the urobilene-(b) porphobilin {cf. Section 6.5.). 

 Diffusion experiments indicated a molecular weight corresponding to only 

 two pyrrole rings. 



While condensation of p-dimethylaminobenzaldehyde with mesobilane 

 gives a chloroform-extractable red pigment with only one absorption band, 

 the condensation of it with porphobilinogen gives a chloroform-insoluble red 

 pigment with a two-banded absorption spectrum {570,2192,2837,2906). 

 Similarly neither porphobilinogen nor porphobilin can be extracted from the 

 urine by organic solvents. 



By the action of alkali, porphobilinogen may be changed into a tetra- 

 pyrrolic chromogen giving an Ehrlich dye with only one absorption band. 

 This is probably the true leuco compound of porphobilin (urobilane); on 

 autoxidation it yields porphobilin. 



The insolubility of these compounds in organic solvents as well as the 

 ease with which uroporphyrin is formed from the dipyrrolic chromogen are 

 good evidence that this contains acetic and propionic acid side chains like 

 uroporphyrin. It is probably a dipyrrylmethane substituted with these side 

 chains, while the tetrapyrrolic chromogen into which it is converted by alkali 

 is a urobilane, and porphobilin the corresponding urobilene-(b). 



Ehrlich diazoreaction in urines. Some urines give a positive Ehrlich diazo 

 reaction (coupling with diazobenzenesulfonic acid to a red dye). Heilmeyer 



