Gray and Nicholson. 3085 627. The Chemistry of the Bile Pigments. The Structures of Stercobilin and d-Urobilin. By C . H. GRAY and D. C. NICHOLSON.Stercobilin and d-urobilin have been degraded with the formation of fragments from the end ring and middle ring of each. The structure of stercobilin as a hydrogenated i-urobilin, i.e. , mesobilene-b, has been confirmed and the P-positions of the end rings have been established as the sites of attachment of the extra hydrogen atoms. The production of ethylmethylmaleimide and haematinimide by the oxidation of d-urobilin and the quantitative hydrogenation of this pigment suggest that it is related t o i-urobilin in that one ethyl group is replaced by a vinyl group. The isolation of two new urobilinoid pigments, namely, d-urobilin-IXa and racemic dehydro-iurobilin, is described.MOST bile pigments are tetrapyrrolic compounds derived from biliverdin [4 : 5 4 -2 ' -carboxyethyl-1 : 3 : 6 : 7-tetramethyl-2 : 8-divinylbilatriene-a,b,c (I)] by different degrees of reduction. They are classified according to the number and position of the double bonds joining the bridge carbon atoms (a,b,c, in I) to the pyrrole nuc1ei.l Biliverdin is formally derived from protoporphyrin-IX (11) by oxidative removal of the cr-carbon linkage, and all naturally occurring bile pigments are therefore assumed to be IXa-compounds in that the arrangement of the P-substituents corresponds to that in biliverdin. Biliverdin and a number of related compounds have been synthesised and their IXa-structures established beyond doubt.Pigments in which the vinyl groups have been reduced to ethyl groups are given the prefix " meso " because of their formal relation to mesoporphyrin (111). The structure usually assigned to biliverdin is the lactim form of the bislactam (I), but the lactam structure is more consistent with recent knowledge of the " hydroxypyrroles " and of the bile pigments: and is therefore used in this paper.An important group of bile pigments includes the urobilins which are thought to be mesobilenes-b. These include stercobilin ([a],, -4000" in CHCl,), d-urobilin 51 ([a]D +5000" in CHClJ, and the optically inactive i-urobilin. i-Urobilin receives the trivial name, urobilin-IXa, although all the naturally occurring bile pigments have the IXaarrangement of B-side chains. Of these compounds only the structure of i-urobilin has been established by degradation and synthesis 7 as 4 : 5-di-2-carboxyethyl-2 : S-diethyl-1 : 3 : 6 : 7-tetramethylbilene-b (XIV or XVI).Elementary analyses suggest that stercobilin and d-urobilin are tetrahydro-and dehydro-derivatives, respectively, of i-urobilin. The two pigments cannot, therefore, be structural isomers or enantiomorphs as suggested by With although the racemisation of d-urobilin described in this paper implies the existence of a true enantiomorph of that pigment. Both pigments presumably contain a dipyrromethene structure since their
Spectrophotometric titration curves of four main types of bile pigments and related dipyrryl compounds reveal a predominantly basic character in urobilins, violins, and verdins, the basic strengths decreasing in that order, and a predominantly acidic character in rubins. Implication of these results on the structure of the pigments is discussed.ALL naturally occurring bile pigments contain carboxyl groups, separated from the main chromophores by two methylene groups, so that their ionisation is not detectable spectrophotometrically. Thus, the spectra of esters of verdins and of carboxyethylporphyrins are identical with those of the corresponding free pigment^.^.^ Changes in the spectra of bile pigments accompanying change of pH, therefore, arise from addition of protons to, or their removal from, the main chromophores of these compounds.We have studied rubins, urobilins, verdins, and violins as well as simple dipyrrylmethenes representative of the chromophores of the first two classes of these pigments. We first used McIlwain's buffer s o l ~t i o n s , ~ but these are not of constant ionic strength, as shown by an approximately six-fold change of conductivity over their entire range. Their pH range was unsatisfactory for the spectrophotometric study of the urobilins and, less so, of the rubins, which were, therefore, also examined in veronal-acetate buffers which are of constant ionic strength (p = 0.1).5 These buffers were, however, unsuitable for the investigation of the verdins, but were employed for mesobiliviolin. Results with the two buffer systems were nevertheless almost identical.* For an alternative formula see ref. 6.
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