State of aggregation of bilirubin in aqueous solution: principal component analysis approach

Abstract

Principal component analysis (PCA) of the absorption spectra of the solutions of bilirubin with increasing its concentration at a fixed BSA concentration definitely shows that the pigment in aqueous solution remains as a mixture of tetramer and monomer in equilibrium. The association constant among the bilirubin molecules is as high as ∼10 4. We have been able to resolve the observed spectra of aqueous bilirubin into component spectra absolutely characteristic of monomer and tetramer, showing peaks at 480 and 425 nm, respectively. The tetrameric form of bilirubin in solution fits well with the model of unit cell packing of four bilirubin molecules in solid crystalline powdered state. Hence, it has been speculated that bilirubin in solution persists as liquid crystal.