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Photochemical Oxidation of Bilirubin to Biliverdin
The Featured Molecules for this month are related to the paper by Pillay and Salih on the photochemical oxidation of bilirubin to biliverdin (1). Biliverdin is a breakdown product of hemoglobin which is reduced by biliverdin reductase to bilirubin, the molecule that is responsible for neonatal jaundice.
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.
Below 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 isomers of both bilirubin and 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.
Literature Cited
- Pillay, A. E.; Salih, F. M. J. Chem. Educ. 2006, 83, 1327–1329.
- Ritter, S. K. Chem. Eng. News 2003, 81 (26), 29.
- Dinan, Frank J. J. College Sci. Teaching 2004, 33, 18–22.
Viewing Requirements
In addition to the static images, two fully manipulable versions
(Jmol, MDLChime)
of these molecules appear below.
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Chime (registration required)
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