Chiral recognition by cyclic oligosaccharides. Enantioselective complexation of bilirubin with β-cyclodextrin through hydrogen bonding in water
The mechanism for enantioselective complexation of bilirubin (BR) with β-cyclodextrin (β-CDx) in water (pH 10.8) has been studied by means of CD spectroscopy. The (–) to (+) bisignate CD signals observed for BR bound to β-cyclodextrin are enhanced upon addition of co-existing guests such as cyclohexanol and cyclooctanol which are completely included in the β-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 β-CDx does not play an important role for enantioselective complexation of BR with β-CDx. The CD signals disappear in water at pH 13.0 where an electrostatic repulsion is expected between BR and β-CDx because either the carboxy groups of BR or the secondary hydroxy group of β-CDx are dissociated under these conditions. BR complexed with heptakis(6-deoxy)-β-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 β-CDx cavity and hydrogen bonding between the carboxylate ions of BR and the secondary hydroxy groups of β-CDx participates in the enantioselective complexation of BR with β-CDx.