Date : 2011. 5. 12, 5:00 PM Place : Int"l conf. room, Bldg.25-1 -Abstract- Oligomers containing both α- and β-amino acid residues ("α/β-peptides") are intriguing as potential foldamers. The four new types of helical secondary structures were characterized crystallographically for α/β-peptides with three α:β-residue repeat patterns (1:1, 2:1 and 1:2). The 14 crystal structures of 1:1 α/β-peptides displayed two types of distinct helical conformations: the 11-helix and the 14/15-helix arising from (i, i+3) and (i, i+4) hydrogen bonds, respectively. Various statistical analyses suggest that the relationship between the two 1:1 α/β-peptide helices is analogous to the relationship between the 3-helix and the α-helix in α-peptides. The 13 crystal structures of 2:1 or 1:2 α/β-peptides displayed only a single type of helix with (i, i+3) hydrogen bonds, although previous two-dimensional NMR data suggest the two helices with (i, i+3) or (i, i+4) hydrogen bonds. In light of these results, we proposed the relationship between the α:β ratio in α/β-peptide backbones and the propensity to adopt helical conformations with either hydrogen boding types.