α/β-Peptide 11/9-helix is an unconventional helical structure in which 11- and 9-membered ring hydrogen bonds alternate along the helical axis. We have examined the interplay and the relative strength of these two hydrogen bonding types by IR, NMR, and X-ray crystallographic methods. A pair of two adjacent hydrogen bonds with opposite directionality cooperatively stabilized each other in non-hydrogen-bonding solvents. In contrast, an unpaired hydrogen bond was unstable to promote helical folding. The IR and the NMR data of α/β-depsipeptides suggested that a 9-membered ring hydrogen bond is favored over an 11-membered ring hydrogen bond.
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