Crystal structure of Boc‐Ala‐Aib‐Ala‐Aib‐Aib‐methyl ester, a pentapeptide fragment of the channel‐forming ionophore suzukacillin

Abstract t‐Buthyoxycarbonyl‐L‐alanyl‐α‐aminiosobutyryl‐L‐alanyl‐α‐aminoisobutyryl‐α‐aminoisobutyric acid methyl ester (t‐Boc‐L‐Ala‐Aib‐L‐Ala‐Aib‐Aib‐OMe), C24H43N5O8, an end‐protected pentapeptide with a sequence corresponding to the 6th through the 10th residues in suzukacillin, crystallizes in the orthorhombic space group P212121 with a = 11.671, b = 14.534, c = 17.906 Å and z = 4. The molecule exists as a right‐handed 310‐helix with a pitch of 6.026 Å. The helix is stabilized by three 4 → 1 hydrogen bonds with the NH groups of Ala(3), Aib(4), and Aib(5) hydrogen bonding to the carbonyl oxygens of t‐Boc, Ala(1), and Aib(2), respectively. The helical molecules arrange themselves in a head‐to‐tail fashion along the a direction in such a way that the NH groups of Ala(1) and Aib(2) hydrogen bond to the carbonyl oxygens of Aib(4) and Aib(5), respectively, of a translationally related molecule. The helical columns thus formed close‐pack nearly hexagonally to form the crystal.