CHIRAL RECOGNITION BY CYCLIC OLIGOSACCHARIDES - ENANTIOSELECTIVE COMPLEXATION OF BILIRUBIN WITH BETA-CYCLODEXTRIN THROUGH HYDROGEN-BONDING IN WATER

The mechanism for enantioselective complexation of bilirubin (BR) with beta-cyclodextrin (beta-CDx) in water (pH 10.8) has been studied by means of CD spectroscopy. The (-) to (+) bisignate CD signals observed for BR bound to beta-cyclodextrin are enhanced upon addition of co-existing guests such as cyclohexanol and cyclooctanol which are completely included in the beta-CDx cavity. Non-cyclic oligosaccharides such as maltoheptaose also induce the bisignate CD signals of BR. These results clearly indicate that a lipophilic cavity of beta-CDx does not play an important role for enantioselective complexation of BR with beta-CDx. The CD signals disappear in water at pH 13.0 where an electrostatic repulsion is expected between BR and beta-CDx because either the carboxy groups of BR or the secondary hydroxy group of beta-CDx are dissociated under these conditions. BR complexed with heptakis(6-deoxy)-beta-CDx also shows the (-) to (+) bisignate CD Cotton effect. On the basis of these results, it has been concluded that optically active BR is bound to the rim of the beta-CDx cavity and hydrogen bonding between the carboxylate ions of BR and the secondary hydroxy groups of beta-CDx participates in the enantioselective complexation of BR with beta-CDx.