SULFOXIDES AS CHIRAL COMPLEXATION AGENTS. CONFORMATIONAL ENANTIOMER RESOLUTION AND INDUCED CIRCULAR DICHROISM OF BILIRUBINS

Article abstract of DOI:10.1016/S0040-4020(01)81462-9

Bilirubin-IXα, the end product of porphyrin metabolism in mammals and the neurotoxic yellow-orange pigment of jaundice, exhibits a strong tendency to fold like a book into either of two interconverting enantiomeric conformations, which are further stabilized by intramolecular hydrogen bonding.Bilirubin exhibits optical activity in R(-)-ethylmethylsulfoxide solvent, as seen by moderately strong bisignate circular dichroism Cotton effects (Δε^max₄₅₂=+10.9, Δε^max₄₀₄=-4.5), and in dichloromethane solution in the presence of 2M R-(+)-methyl-p-tolylsulfoxide its circular dichroism spectrum (Δε^max₄₆₃=+11.2, Δε^max₄₁₂=-7.1) is comparably strong.As observed earlier for chiral recognition of bilirubin by optically active amines and serum albumines, the optically active sulfoxide acts as a chiral complexation agent to induce an asymmetric transformation of bilirubin, whose bisignate circular dichroism spectra are characteristic of an exciton splitting arising from interaction of the two component dipyrrinone chromophores.