Evidence that the β-Peptide 14-Helix is Stabilized by β3-Residues with Side-Chain Branching Adjacent to the β-Carbon Atom



Oligomers of β-substituted β-amino acids (‘β3-peptides') are known to adopt a helical secondary structure defined by 14-membered ring hydrogen bonds ('14-helix'). Here, we describe a deca-β3-peptide, 1, that does not adopt the 14-helical conformation and that may prefer an alternative secondary structure. β3-Peptide 1 is composed exclusively of residues with side chains that are not branched adjacent to the β-C-atom (β3-hLeu, β3-hLys, and β3-hTyr). In contrast, an analogous β-peptide, 2, containing β3-hVal residues in place of the β3-hLeu residues of 1, adopts a 14-helical conformation in MeOH, according to CD data. These results illustrate the importance of side-chain branching in determining the conformational preferences of β3-peptides.