Chiral Recognition of d-Tryptophan by Confining High-Energy Water Molecules Inside the Cavity of Copper-Modified β-Cyclodextrin

We report a Cu 2+ -modified β-cyclodextrin, Cu 2 -β-CD, that was self-assembled on poly( l -glutamic acid) (P- l -Glu) for chiral recognition of d -tryptophan ( d -Trp). Cu 2+ formed binuclear hydroxy-bridge (Cu(OH) 2 Cu) at the wider opening of the cavity of β-CD and acted as a cap to prevent high-energy water molecules from being released while forcing Trp isomers to enter through its narrower opening. Because H-bonds were favored to form between the high-energy water molecules and the amino groups of d -Trp inside the cavity of β-CD, d -Trp was thus recognized. The recognition capability of the Cu 2 -β-CD appeared to depend on the way the Cu 2 -β-CD self-assembled onto P- l -Glu, preferring via H-bonds rather than via electrostatic interactions, because electrostatic interactions between P- l -Glu and Cu(OH) 2 Cu could alter the configuration of the Cu 2 -β-CD cavity, resulting in weakened H-bonds between high-energy water molecules and d -Trp.