THE biological breakdown of haem proceeds through a series of intermediates of which the orange-yellow bile pigment, bilirubin, the chromophore responsible for coloration in the various forms of jaundice, is a key member. In the adult human, for example, erythrocytes have a lifetime of about 3 months and their destruction produces about 300 mg bilirubin each day1,2. Although there were some early doubts (arising from the possibility of a cyclisation between the CIS vinyl group and the oxygen function at C19)3, the gross chemical structure (Fig. 1) for bilirubin has been beyond reasonable dispute for many years4. There remain, however, considerable structural uncertainties with respect to (1) stereochemistry, the geometry around bridges 5 and 15 being unknown, and represented variously as E or Z in the current literature. (The two configurations, E and Z, have been recognised in a model pyrromethanone system, however, (see ref. 5 and Fig. 2)); (2) tautomerism, although spectroscopic evidence favours the lactam formula for rings A and D (shown in Fig. 1) over the lactim6-9; and (3) conformation, although several speculations, supported by spectroscopic evidence and model making, are available10-17. We report the X-ray analysis of bilirubin—the first such analysis of a naturally occurring linear tetrapyrrole—which shows that bilirubin has the Z configuration at the C4-C5 and C15-C16 bonds, and possesses in the crystal a ridge tile conformation with considerable intramolecular hydrogen bonding.
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