Cyclic hexapeptide of D,L-α-aminoxy acids as a selective receptor for chloride ion

Abstract

Cyclic hexapeptide 2, prepared from linear hexapeptide 1 of alternating D- and L-α-aminoxy acids, was found to adopt a C 3 symmetric and bracelet-like conformation with consecutive eight-membered-ring hydrogen bonds (N-O turns) in nonpolar solvents, similar to that of valinomycin, a cyclodepsipeptide that binds cations selectively. However, 2 showed affinities for halide ions with selectivity following the order of Cl - ≫ F - ≫ Br -. The observed higher selectivity for Cl - (K a = 11880 M -1) over F - (K a = 30 M -1) in CD 2Cl 2 suggested that the selectivity of 2 for halide ions is mainly governed by the size complementarity rather than the hydrogen-bonding strength. Upon Cl - ion binding, the original bracelet-like conformation of 2 turned into a rather flat conformation with all six amide NHs pointing inward to form hydrogen bonds with Cl -. Copyright © 2002 American Chemical Society.