Conformation of Boc‐ L ‐Ala‐Aib‐ L ‐Ala‐OMe in the Crystal and in Solution

Boc‐L‐Ala‐Aib‐L‐OMe (1) crystallizes in the space group P21 with a = 11.732(1), b = 6.013(1), c = 14.195(2) Å, β = 91.76(1)→, and Z = 2(R value for 3089 reflexions: 0.047). The peptide adopts a new type of β‐turn with a very wide 4→1 hydrogen bond distance of 3.621 Å. As for Ac‐L‐Ala‐Aib‐L‐Ala‐Me this distortion can be attributed to strong intermolecular hydrogen bonds forming a two‐dimensional network in the bc plane. Temperature and solvent dependent 1H and 13C NMR reveal a hydrogen bond from Boc–CO to NH–3Ala in solution, E/Z isomerism of the Boc urethane bond, and a large magnetic nonequivalence of the two geminal Aib methyl groups. The unusual conformation of 1 is reflected also in its CD spectrum, which differs from most of comparable Aib containing tripeptides.