66 III. PORPHYRIN CHEMISTRY 



3.4.2. Uroporphyrins, C2oH6N4(CH2C02H)4(CH2CH2C02H)4. In 



contradistinction to the porphyrins described so far, uroporphyrins 

 are ether-insoluble substances. Uroporphyrin I was discovered by 

 Fischer in the urine of a patient with chronic porphyria (see 779). It 

 also occurs in the shells of the Pteria mussel (see 819,835, and 264-1) • 

 Its structure has so far been determined by exclusion of other possi- 

 bilities rather than by direct proof or synthesis. On decarboxylation 

 (heating in oil bath or in 1% hydrochloric acid under pressure at 180° 

 C.) coproporphyrin I is obtained, though in rather small yield. This 

 proved uroporphj'rin to be a tetracarboxylated coproporphyrin. 



At first it was assumed that uroporphyrin contained methyl and 

 methylmalonic acid, CH2CH(C02H)2, side chains. Later it was 

 found, however, that natural uroporphyrin I was neither identical 

 with the synthetic isouroporphyrin I containing these side chains, 

 nor with the corresponding synthetic porphyrin with methyl and 

 succinic acid, CH(C02H)CH2C02H, side chains. Both gave oxidation 

 products (hematinic acids) and octamethyl esters differing from those 

 obtained from natural uroporphyrin I, in contrast to earlier findings 

 (c/. 825); the absorption spectrum of isouroporphyrin was also not 

 identical with that of uroporphyrin (681, p. 508 ff.). The porphyrin 

 with four succinic acid side chains has four active carbon atoms, and 

 a porphyrin of this structure would be expected to be found in nature 

 in optically active form; natural uroporphyrin is, however, optically 

 inactive. The only alternative is the placing of the four extra car- 

 boxylic acid groups on the four methyl groups of coproporphyrin. 

 Pyrroleacetic acids are in fact remarkably easily decarboxylated {835), 

 and it was shown that a synthetic porphyrin with four CH2CO2H 

 groups and four methyl groups was decarboxylated under the con- 

 ditions under which uroporphyrin is transformed to coproporphyrin 

 (855). Hence uroporphyrin I is very probably formula I of Figure 11. 

 The melting point of its octamethyl ester was found by various 

 authors as 284-291° C. Fischer gives 302° (cor. 311° C), while 

 Watson and collaborators {1056) find the melting point of the ester 

 purified by chromatography to be 284°; these authors ascribe higher 

 melting points to admixture of complex salts or of products of partial 

 saponification. 



"Uroporphyrin III (?)" was isolated by Waldenstrom (2905,2906, 

 2908,2910) from the urine of patients with acute porphyria (c/. 

 Chapter XII. Section 4.3.4.). It is not extractable with ether, but 

 can be extracted with ethvl acetate from solutions in dilute acetic 



