A high-pressure, infrared spectroscopic study of the solvation of bilirubin in lipid bilayers

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Abstract
The location of bilirubin IXa in lipid bilayers of dimyristoylphosphatidylcholine or dioleoylphosphatidylcholine was studied by determining the effects of bilirubin on the infrared spectra of the lipids as a function of pressure. It was found for both bilayers that bilirubin intercalated into the polymethylene chain region of the bilayer, being located between the carbonyl region and the methylene group two carbons from the methyl terminus. Small amounts of bilirubin interacted with the choline region of dioleoylphosphatidylcholine. Lesser amounts interacted with the choline region of dimyristoylphosphatidylcholine. This difference between the two types of bilayers was attributed to the degradation of small amounts of bilirubin IXa to more polar isomers in the presence of dioleoylphosphatidylcholine. In dioleoyl- but not dimyristoylphosphatidylcholine, bilirubin interacted with the C.dbd.O region, probably indicating that bilirubin in the latter type of bilayer was intercalated into the polymethylene chains above and below the double bond. Bilirubin decreased the pressure required for the liquid-crystal to gel-phase transition in both bilayers at 28.degree. C. Bilirubin was not forced out of either bilayer at pressures as high as 20 kbar.