Helix Nucleation by the Smallest Known [alpha]-Helix in Water

Cyclic pentapeptides (e.g. Ac-(cyclo-1,5)-[KAXAD]-NH2; X=Ala, 1; Arg, 2) in water adopt one [alpha]-helical turn defined by three hydrogen bonds. NMR structure analysis reveals a slight distortion from [alpha]-helicity at the C-terminal aspartate caused by torsional restraints imposed by the K(i)-D(i+4) lactam bridge. To investigate this effect on helix nucleation, the more water-soluble 2 was appended to N-, C-, or both termini of a palindromic peptide ARAARAARA (≤5% helicity), resulting in 67, 92, or 100% relative [alpha]-helicity, as calculated from CD spectra. From the C-terminus of peptides, 2 can nucleate at least six [alpha]-helical turns. From the N-terminus, imperfect alignment of the Asp5 backbone amide in 2 reduces helix nucleation, but is corrected by a second unit of 2 separated by 0-9 residues from the first. These cyclic peptides are extremely versatile helix nucleators that can be placed anywhere in 5-25 residue peptides, which correspond to most helix lengths in protein-protein interactions.