Inversion of 3 10 ‐helix screw sense in a ( D ‐αMe)Leu homotetrapeptide induced by a guest D ‐(αMe)val residue

The terminally blocked tetrapeptide p BrBz‐[ D ‐(αMe)Leu] 2 ‐ D ‐(αMe)Val‐ D ‐(αMe)Leu‐O t Bu is folded in the crystal state in a left‐handed 3 10 ‐helical structure stabilized by two consecutive 1 ← 4 CO ⃛HN intramolecular H‐bonds, as determined by X‐ray diffraction analysis. A CD study strongly supports the view that this conformation is also that largely prevailing in MeOH solution. A comparison with the published conformation of p BrBz‐[ D ‐(αMe)Leu] 4 ‐O t Bu indicates that incorporation of a single internal β‐branched (αMe)Val guest residue into the host homo‐tetrapeptide from the γ‐branched (αMe)Leu residue is responsible for a dramatic structural perturbation, i.e. an inversion of the 3 10 screw sense from right to left‐handed.