Resonance Raman spectroscopy of bilirubins: band assignments and application to bilirubin/lipid complexation.

Resonance Raman spectra of bilirubins IX alpha, III alpha, and XIII alpha and mesobilirubin XIII alpha in alkaline aqueous and chloroform solutions are reported. Partial band assignments of bilirubin IX alpha are proposed. The model compounds confirm assignments of bands of the Raman spectrum of bilirubin IX alpha to each of the two different pyrromethenones. Resonance Raman spectra of mesobilirubin IV alpha, vinylneoxanthobilirubinic acid, and vinylisoneoxanthobilirubinic acid in alkaline aqueous solution and of the tetra-n-butylammonium salt of bilirubin IX alpha are used to define markers for the presence or absence of internal hydrogen bonds. Interaction of bilirubin dianion and sphingomyelin liposomes is studied. The Raman evidence suggests that in the bilirubin dianion/liposome complex the intramolecular hydrogen bonds between the propionate groups and the lactam NH/CO are ruptured. It is proposed that in the complex the bilirubin propionates form ion pairs with the quaternary ammonium ion of the choline moiety of sphingomyelin.