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1.D.156.  The Alpha-Aminoxy Acid Channel (AAAC) Family 

Cyclic hexapeptide 2 (2) was prepared from linear hexapeptide 1 of alternating d- and l-alpha-aminoxy acids, and was found to adopt a C3 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 (Yang et al. 2002). However, 2 showed affinities for halide ions with selectivity following the order of Cl- > F- > Br-. The observed higher selectivity for Cl- (Ka = 11880/M) over F- (Ka = 30/M) in CD2Cl2 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-.

Chemists have developed peptidomimetic foldamers, unnatural oligomeric molecules that fold into rigid and well-defined secondary structures, mimicking the structures and biological functions of these natural peptides. Li et al. 2008 designed peptidomimetic foldamers that give predictable, backbone-controlled secondary structures, irrespective of the nature of the side chains.  α-aminoxy acid units were used to construct a synthetic Cl- channel that mediates the passage of Cl- ions across cell membranes (Li et al. 2008). Structural modifications of α-aminoxy peptides on both their intestinal absorption and their transport mechanisms have been reported (Li et al. 2008). Structural modifications of α-aminoxy peptides on both their intestinal absorption and their transport mechanisms have been reported (Ma et al. 2011).

References associated with 1.D.156 family:

Li, X., Y.D. Wu, and D. Yang. (2008). Alpha-aminoxy acids: new possibilities from foldamers to anion receptors and channels. Acc Chem Res 41: 1428-1438. 18785763
Ma, B., H. Zha, N. Li, D. Yang, and G. Lin. (2011). Effect of structural modification of α-aminoxy peptides on their intestinal absorption and transport mechanism. Mol Pharm 8: 1073-1082. 21630669
Yang, D., J. Qu, W. Li, Y.H. Zhang, Y. Ren, D.P. Wang, and Y.D. Wu. (2002). Cyclic hexapeptide of D,L-α-aminoxy acids as a selective receptor for chloride ion. J. Am. Chem. Soc. 124: 12410-12411. 12381172