Stabilisation of the type I β-turn conformation by a bicyclic analogue of proline

A highly constrained analogue of l-proline, (1S,2S,4R)-2-phenyl-7-azabicyclo[2.2.1]heptane-1-carboxylic acid, has been incorporated into a model dipeptide. X-Ray diffraction analysis has shown that, in the solid state, this constrained peptide adopts a type I β-turn whereas the analogous dipeptide sequence incorporating l-proline has been shown to accommodate a βII-turn disposition. Attractive interactions involving the middle NH group and either the aromatic rings or the pyramidalised bicycle nitrogen seem to play a role in the stabilisation of the βI-turn conformation observed.