ABSOLUTE-CONFIGURATION OF BILIRUBIN CONFORMATIONAL ENANTIOMERS

Bilirubin, the yellow pigment of jaundice, is a bichromophoric tetrapyrrole formed in mammals by heme catabolism. It readily adopts either of two enantiomeric folded conformations which are shaped like ridge tiles and are stabilized by a network of intramolecular hydrogen bonds. Interconversion of the conformational enantiomers is rapid at room temperature and may be displaced toward either enantiomer by complexation with chiral agents. Intramolecular steric effects may also affect enantioselection. Thus, introduction of a methyl group at each of the beta- and beta'-carbons of the propionic acid chains on the symmetric bilirubin analog, mesobilirubin XIIIalpha, can shift the conformational equilibrium toward either the M- or the P-chirality intramolecularly hydrogen-bonded conformer, depending only on the R or S stereochemistry at beta and beta'. With the appropriate R or S configurations, intense bisignate circular dichroism (CD) may be detected for the approximately 430-nm transition(s) that is characteristic of exciton coupling between the component dipyrrinone chromophores. The absolute configuration of the beta,beta'-dimethylmesobilirubin XIIIalpha exhibiting a negative chirality CD exciton couplet (DELTAepsilon434max -337, DELTAepsilon390max + 186 in CHCl3) is firmly established as betaS,beta'S by X-ray crystallography of the brucine salt of a monopyrrole synthetic precursor, (+)-(S)-3-[2,4-dimethyl-5-(ethoxycarbonyl)-1H-pyrrol-3-yl]butanoic acid (5). Molecular dynamics calculations on the betaS,beta'S enantiomer confirm a strong preference (approximately 20 kcal/mol stabilization) for intramolecularly hydrogen-bonded conformational enantiomers in which the M-chirality is favored over the P-chirality (by -4 kcal/mol). For the first time, the absolute configuration of a bilirubin ridge-tile conformational enantiomer has been unequivocally established.