Ac‐L‐Ala‐Aib‐L‐Ala‐OMe: X‐Ray Analysis of a Distorted β‐Bend and Magnetic Nonequivalence of Aib‐Methyl Groups

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Abstract
Ac‐L‐Ala‐L‐Ala‐OMe (1) crystallizes in the space group P21 with a = 10.796(2), b = 10.788(3), c = 14.625(6) Å, β = 92.74(3)°, and Z = 4 (R value for 1590 independent reflexions 0.05). In contrast to comparable Aib oligopeptides the two independent tripeptide molecules 1 in the asymmetric unit form very distorted β‐turns. The 41 intramolecular hydrogen bridges are extremely weak (3.34 and 3.67 Å, respectively) in the solid state. This widening of the β‐turn seems to result from strong intermolecular hydrogen bonds (2.94 and 2.85 Å) linking the two independent molecules 1 to form dimeric units. These units are linked with further hydrogen bonds (3.01 and 2.83 Å) to a two‐dimensional network parallel to the bc‐plane. In solution the tripeptide 1 exhibits a particularly large magnetic nonequivalence of the two Aib‐Cβ atoms (Δδ = 2.6 ppm). This indicates a highly populated conformation with a preferred orientation of the pro‐S‐Aib‐Cβ relative to the Aib peptide carbonyl group.