Peptide design. Helix–helix motifs in synthetic sequences

Two centrally positioned Æ-aminoisobutyryl (Aib) residues have been used to stabilize distinct heptapeptide helical segments in the 15-residue synthetic sequence Boc-Met-Ala-Leu-Aib-Val-Ala-Leu- Acp-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe. The helices are connected by the flexible linker Â-aminocaproic acid (Acp). NMR studies in CDCl3 establish helical conformations for both independent segments as evidenced by NH-NH nuclear Overhauser effects (NOEs). The peptide strongly aggregates in CDCl3 with the NH groups of Met(1) and Ala(2) participating in intermolecular hydrogen bonds. In (CD3)2SO two solvated helical segments are supported by NMR results. Solvent dependent breakdown of aggregates on addition of (CD3)2SO to CDCl3 solutions is suggested by analysis of chemical shifts and temperature coefficients of NH protons. The observation of several interhelical NOEs in CDCl3, relatively few NOEs in 10% (CD3)2SO-CDCl3 and their absence in (CD3)2SO provides a means of inferring helix orientations. While an antiparallel arrangement resulting in closed aggregate formation is suggested in CDCl3, a parallel solvated arrangement is favoured in (CD3)2SO.