Syntheses, Crystal Structures, and Conformations of 10,10-Spiro-Bilirubins |
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Authors: | Brahmananda Ghosh Vincent J Catalano David A Lightner |
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Institution: | (1) Department of Chemistry, University of Nevada, 89557-0020 Reno, Nevada, USA |
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Abstract: | Summary. Crystal structure determinations of two novel bilirubin analogs with spirocyclohexyl and spirofluorenyl groups at C(10) are reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those from molecular dynamics calculations, and to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like bilirubin, the component dipyrrinones of spirocyclohexyl and spirofluorenyl rubins are present in the bis-lactam form with (Z)-configuration double bonds at C(4) and C(15). Their crystal structures show considerable similarity to that of bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds. The interplanar angle of the spirocyclohexyl ridge-tile is nearly the same (94°) as that of bilirubin (95°), but the interplanar angle of the spirofluorenyl ridge-tile is noticeably smaller (84°). The hydrogen bond distances of both spiro-rubin crystal structures are generally longer by 0.1–0.2Å than those in bilirubin. Both new pigments exhibit excellent lipophilicity and, unlike bilirubin, are soluble in methanol. |
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Keywords: | , Linear tetrapyrroles, Stereochemistry, Hydrogen bonding, |
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