A cyclic hexapeptide comprising alternating alpha-aminoxy and alpha-amino acids is a selective chloride ion receptor

In nonpolar solvents, the cyclic hexapeptide 2, which comprises alternating D-alpha-amino and D-alpha-aminoxy acids, adopts a C3-symmetric conformation with alternating eight (N--O turns)- and seven (gamma turns)-membered-ring hydrogen bonds. A series of anion-binding studies has suggested that 2 can function as an effective anion receptor that not only displays a high selectivity for chloride ions, but also the capability to extract chloride ions from aqueous solutions into organic phases.