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Semiempirical methods were utilized in the computation of a fully optimized structure of bilirubin. Bond lengths and bond angles obtained using either AM^ or P M~ calculations showed excellent agreement with those obtained by X-ray diffraction. This indicated that molecular orbital methods satisfactory reproduced the complex conjugation found in bilirubin. Dihedral angles of the crucial "hinge" and the dihedral angles of the propionic acid side chains agreed well with those found by X-ray diffraction. Calculated hydrogen-bond parameters (distance and angles) showed substantial differences from experimental values, probably due to inherent weakness in the parameterization of the molecular orbital techniques. Conformational studies were carried out using AM^ by rotating the C9-C10 bond in 5" increments showed that the most stable structure exhibited a minimum at about 125" and exhibited a structure similar to those postulated from X-ray and NMR experiments. The hydrogen bonds showed remarkable tenacity during rotation of the C9-C10 bond and resisted breaking until the molecule was under extreme strain.

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