The structure of bilirubin illustrates many of the recurring themes of this column dealing with the use of three-dimensional representations as pedagogic tools. In 2003 Ritter pointed out that many two-dimensional representations of bilirubin display incorrect stereochemistry (2). The molecule is often shown as the E,E isomer rather than as the naturally occurring Z,Z isomer. In 2004, Dinan took further the issue of how bilirubin should be drawn in an interesting “case study” (3). Dinan points out that in addition to the confusion over the E–Z stereochemistry, there are also problems with the way that the terminal pyrrole rings in bilirubin are represented, with some depictions showing them as the lactam tautomer and some as the lactim.

Here are two-dimensional line models of the E,E and Z,Z forms with the pyrrole rings shown in the lactam tautomeric form.

Comparison of these structural models with the images of three-dimensional models

offers students several additional insights into structure, particularly into non-bonded interactions. As shown in the JCE Online molecule collection, the four nitrogen atoms in the Z,Z isomer of bilirubin and the Z,Z,Z isomer of biliverdin are no longer coplanar, and beginning students should be encouraged to think of explanations for this lack of coplanarity, given that the molecules are derived from a system, the heme system, where the nitrogen atoms do lie in the same plane.

Fully manipulable (Chime and Jmol) versions of the E,E and Z,Z isomers of bilirubin, the Z,Z,Z isomer of biliverdin, and lactam and lactim tautomers of a model pyrrole system are available at the JCE Digital Library Web site.

Literature Cited

1. Pillay, A. E.; Salih, F. M. J. Chem. Educ. 2006, 83, 1327–1329.
2. Ritter, S. K. Chem. Eng. News 2003, 81 (26), 29.
3. Dinan, Frank J. J. College Sci. Teaching 2004, 33, 18–22.

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