The bichromophoric pigment bilirubin acts as a molecular exciton in its UV-visible and circular dichroism (CD) spectroscopy. The optically active analogue, (beta R,beta'R)-dimethyllmesobilirubin-XIII alpha exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV-vis absorption near 400 nm, with Delta epsilon(434)(max) +337, Delta epsilon(389)(max) -186 in the nonpolar solvent CHCl3, and nearly as intense Cotton effects in the polar, hydrogen bonding solvent CH3OH: Delta epsilon(431)(max) +285, Delta epsilon(386)(max) -177. Addition of amines to Cotton effect sign inversions: in isopropylamine Delta epsilon(436)(max) -605, Delta epsilon(392)(max) +375, due to an inversion of molecular chirality.