Novel Linear Tetrapyrroles: Hydrogen Bonding in Diacetylenic Bilirubins

Summary.

Bilirubin congeners with dipyrrinones conjoined to a diaceteylene unit (–C≡C–C≡C–) rather than to –CH₂– were synthesized and examined spectroscopically. This new class of rubrified linear tetrapyrroles cannot easily fold or bend in the middle, but the dipyrrinones can rotate independently about the diacetylene unit. Thus, unlike bilirubin, which is bent in the middle and has a ridge-tile shape, the diacetylene unit orients the attached dipyrrinones along a linear path, and intramolecular hydrogen bonding between the dipyrrinones and opposing carboxylic acids preserves a twisted linear molecular shape when the usual propionic acids are replaced by hexanoic. In a bis-hexanoic acid rubin, the extended planes of the dipyrrinones intersect along the –(C≡C)₂– axis at an angle of 102° for the conformation stabilized by intramolecular hydrogen bonding. With propionic acid chains, however, neither CO₂H can engage an opposing dipyrrinone in intramolecular hydrogen bonding, and the energy-minimum conformation of this linear pigment, shows nearly co-planar dipyrrinones, with an intersection of an angle of ∼180° of the extended planes of the dipyrrinones. Spectroscopic evidence for such linearized and twisted (bis-hexanoic) and planar (bis-propionic) structures comes from the pigments’ NMR spectral data and their exciton UV-Vis and induced circular dichroism spectra.