ALLOSTERIC REGULATION OF CONFORMATIONAL ENANTIOMERISM - BILIRUBIN

(4Z,15Z)-Bilirubin IX-alpha, the cytotoxic yellow tetrapyrrole pigment of jaundice, readily adopts either of two interconverting, enantiomeric conformations, which are stabilized through complementary intramolecular hydrogen bonding between the pyrrole and lactam N-H and C = O residues of one dipyrrinone moiety and the CO2H group of propionic acid side chains on the second dipyrrinone. One conformational enantiomer can be destabilized relative to the other through allosteric action by judicious placement of methyl groups in the propionic acid side chains. Thus, insertion of a methyl group at the pro-R site on the alpha-carbon of the propionic acid destabilizes the M-chirality intramolecularly hydrogen-bonded conformational enantiomer by introducing a severe nonbonded CH3\CH3 steric interaction with a pyrrole methyl substituent. In contrast, introduction of a methyl group at the pro-S site destabilizes the P-chirality enantiomer. When resolved into enantiomers, the corresponding alpha-methylated derivatives of a symmetric bilirubin analogue, mesobilirubin XIII-alpha, gave intense, bisignate circular dichroism Cotton effects (DELTA-epsilon-436max = +/- 246, DELTA-epsilon-392max = -/+ 135) for the long-wavelength exciton transition near 433 nm (epsilon-433max = 56 000) measured in chloroform and DELTA-epsilon-425max = +/- 121, DELTA-epsilon-379max = -/+ 87 in pH 7.4 phosphate buffer (epsilon-418max = 46 000). Thus, for the first time a bilirubin has been separated into its conformational enantiomers by a forced resolution originating from internal steric effects.