INTERMOLECULAR HYDROGEN-BONDING IN PI-FACIAL DIPYRRINONE DIMERS AS MOLECULAR CAPSULES

(4Z)-Dipyrrinones, which are the component chromophores of yellow pigment of jaundice, are known to self-associate strongly in nonpolar solvents (K-assoc similar or equal to 25 000 M at 25 degrees C in CHCl3), forming planar dimers in which the monomers are linked tightly by four intermolecular hydrogen bonds. When the chromophore has an attached propionic or longer chain acid group, it forms a new type of stacked dimer through a network of six hydrogen bonds in which the carboxyl,stoup of one dipyrrinone is tethered to the other dipyrrinone. Thus, xanthobilirubic acid and its homologs strongly self-associate as stacked dimers in contrast to its methyl ester, which forms planar dimers. The stacked dimers are recognized by large (1 ppm) shieldings of their NH resonances in their H-1-NMR spectra, as compared with planar dimers. They are also recognized by unusually large optical rotations and exciton coupling in the circular dichroism spectra when a stereogenic center is present in the alkanoic acid chain. In CHCl3, (beta S)- methylxanthobilirubic acid (1) has [alpha]D-20 = -314 degrees and Delta epsilon(434)(max) = -10.9, Delta epsilon(388)(max) = +5.7, whereas its methyl ester (6) has [alpha]D-20 = +62 degrees; Delta epsilon(370)(max) < 1.