First observation of a helical peptide containing a chiral residue without a preferred screw sense

The molecular and crystal structure of the fully blocked, chiral pentapeptide Ac-(Aib)2-S-Iva-(Aib)2-OMe was determined by X-ray diffraction. It was found that the two crystallographically independent molecules (A and B), alogned in an antiparallel arrangement in the asymmetric unit, differ essentially by the handedness if their 3(10)-helical structure, left-handed for A while right-handed for B. To our knowledge, this is the first observation of a helical peptide containing a chiral residue without a preferred screw sense. Conformational energy computations on the same peptide support the view that various helical structures, including two close to those found in the crystal state, have comparable stabilities.