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

We report a Cu2+-modified B-cyclodextrin, Cu-2-beta-CD, that was self-assembled on poly(l-glutamic acid) (P-l-Glu) for chiral recognition of d-tryptophan (d-Trp). Cu2+ formed binuclear hydroxy-bridge (Cu(OH)(2)Cu) at the wider opening of the cavity of beta-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 beta-CD, d-Trp was thus recognized. The recognition capability of the Cu-2-beta-CD appeared to depend on the way the Cu-2-beta-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-beta-CD cavity, resulting in weakened H-bonds between high-energy water molecules and D-Trp.