Abstract
Bilirubin analogs have been prepared with a methylene group forming an N to N bridge. When the bridge is in the same dipyrrinone, only one propionic acid can hydrogen bond intramolecularly (to the unbridged dipyrrinone), but the pigment still adopts a ridge-tile conformation. Molecular dynamics calculations predict global energy minima for two enantiomeric ridge-tile conformations separated by an energy barrier of ∼14.5 kcal/mole. In contrast, when the bridge connects two dipyrrinones through their lactam nitrogens, that pigment is held in a porphyrin-like shape, neither propionic acid groups can participate in intramolecular hydrogen bonding, and the pigment is much more polar and hydrophilic than bilirubin. Molecular dynamics calculations predict an interconversion barrier of ∼25 kcal/mole for this isomer.
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