Conformation of Boc‐<scp>L</scp>‐Ala‐Aib‐<scp>L</scp>‐Ala‐OMe in the Crystal and in Solution

Abstract

Abstract Boc‐ L ‐Ala‐Aib‐ L ‐OMe ( 1 ) crystallizes in the space group P 2 1 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 1 H and 13 C NMR reveal a hydrogen bond from Boc–CO to NH– 3 Ala 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.