A Crystalline Iron Chloride Molecular Compound of Urobilin and Stercobilin. ∗

Evidence was presented previously 1 for the identity of crystalline urobilin and stercobilin as isolated from human urine 2 and feces, 3 respectively. These crystalline substances unfortunately do not have sharp melting points, nor is the “urobilin” absorption spectrum specific, 4 hence it was desirable to provide further means of positive identification, particularly for the study of substances which may be isolated subsequently. In this regard, Heilmeyer and Krebs 5 have recently mentioned certain differences in a stercobilin which they isolated by means of the procedure described by the writer. This question will be discussed in a later publication.

It has long been known that urobilin is capable of forming a number of metallic compounds. Heretofore, none of these have been isolated. A crystalline iron chloride molecular compound of mesobilirubin was described by H. Fischer, Baumgartner and Hess. 6 The corresponding compound of mesobilirubinogen is amorphous. 7

In the present investigation it has been found that both urobilin and stercobilin yield a beautifully crystalline molecular compound with iron chloride. This crystallizes out of hot 25% hydrochloric acid in a yield of approximately 75%. Crystallization is best brought about by adding a hot, concentrated solution of FeCl₃ in 25% HCl, drop by drop to a tube containing the crystalline urobilin or stercobilin dissolved in hot 25% HCl, the reaction being carried out in a hot water bath with constant stirring. Immediately upon the addition of the FeCl₃ solution, the molecular compound comes out in an amorphous condition, but dissolves on stirring and further heating. Crystallization often commences while the solution is still hot, the crystals becoming more plentiful on cooling. They possess a glittering red-brown color.