The 1 H NMR spectra of 2a-c and 3a-c provided evidence of a rigid and well-defined major conformation for each peptide. All peptides in each series exhibit similar spectral characteristics, suggesting that the position of the AA in the chain has a larger impact on peptide structure than the nature of the AA (i.e., small, medium-sized, or bulky). Monitoring of the NH moieties of NOE-restrained structural calculations using the program CYANA 2.1 35 provided insights into the solution structures of 1 and its analogs. First, ROESY spectra were acquired for each compound with a mixing time of 200 ms. The resulting cross-peaks were then converted to upper-distance constraints. A standard simulated annealing calculation was then performed for 100 conformers of each peptide with the upper-limit file and experimentally determined hydrogen bonding pattern as input. The ten structures with the lowest objective target function were then superimposed to create a final ensemble for each peptide, which was then analyzed based on structure and RMSD values using the molecular visualization software VMD. These ensembles are shown in Figure . The parent peptide 1 exhibits the highest RMSD value of 1.154 Å, suggesting that it has more conformational freedom than its analogs. As anticipated from the intramolecular hydrogen bonding data, the peptides with a ΔAA in place of Aib-1 (i.e., 2a-c) form incipient 310-helical structures and the peptides with a ΔAA in place of Aib-3 (i.e., 3a-c) resemble βturns. These structural data demonstrate that the position of the ΔAA plays an important role in determining the conformation preferred by each analog.