Linear oligopeptides. 401. In search of a peptide 3(10)-helix in water

The terminally blocked, sequential oligopeptides of general formula Ac-[L-Lys-(Aib)(2)](n)-OtBu (n = 1-4) have been synthesized by segment condensation in solution and fully characterized. Using CD technique we determined the preferred solution conformation of these hydrosoluble oligomers under a variety of experimental conditions. Low populations of alpha-helical structure are shown by the nonamer and dodecamer in water. The amount of alpha-helix can be increased by increasing the pH to 12, adding 2,2,2-trifluoroethanol up to 50%, and decreasing the temperature to 3 degrees C. The effect of peptide concentration on conformation is negligible. Under membrane-mimetic conditions the two highest oligomers appear to fold largely into a 3(10)-helix. The observation of a peptide 3(10)-helix in water remains elusive. Only the dodecamer shows antibacterial activity, although modest.