The structure of bilirubin

Crystals of bilirubin are triclinic, space group P1̄ with cell dimensions a = 19.439, b = 11.707, c = 15.500 Å, †, α = 97.19, β =100.22, γ = 118.20°, Z = 4. The structure was solved by direct methods incorporating information derived from reflexion quartets and by using a set of diffractometer data containing 1408 reflexions with F obs ≥ 3σ( F obs ). The structure was refined with bond-length constraints to a final R of 0.125 by using a set of microdensitometer-measured photographic data containing 1323 unique observed reflexions. Both data sets were collected from a matured crystal whose diffraction pattern had improved considerably during a period of several weeks. Bilirubin is shown to have the Z configuration at both the C5 and C15 bridges. The two independent molecules of bilirubin in the crystal structure have very similar ‘ridge tile’ conformations: rings (A + B) and rings (C + D) form two good planes with an interplanar angle of approximately 97° and the molecule is stabilized by six intramolecular hydrogen bonds. There is no evidence for any intermolecular hydrogen bonding. Observed bond lengths suggest that bilirubin can be regarded as a 2,2' - dipyrrolylmethane with conjugating substituents at the α positions. When freshly prepared, most crystals of bilirubin have reflexions hkl ( h odd) diffuse: this observation is discussed in terms of the molecular conformation of bilirubin and the particular packing arrangement which these molecules adopt in the crystal structure.