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

We report a Cu2+-modified β-cyclodextrin, Cu2-β-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)2Cu) 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 Cu2-β-CD appeared to depend on the way the Cu2-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)2Cu could alter the configuration of the Cu2-β-CD cavity, resulting in weakened H-bonds between high-energy water molecules and d-Trp.