Crystal structure of Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBu, a hexapeptide fragment of trichotoxin.

The crystal structure of Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBu, an end-protected hexapeptide with a sequence corresponding to residues 7-12 of several trichotoxin A-50 sequence analogues has been determined by X-ray crystallography. The hexapeptide adopts a right-handed 3(10)-helical conformation consisting of four consecutive beta-turns of type III. The helix is stabilized by four intramolecular hydrogen bonds. In the crystal the molecules are connected head to tail with intermolecular hydrogen bonding interactions among translationally related molecules thus forming infinitely long helical columns. The column-column interactions in the crystal are hydrophobic and occur predominantly between antiparallel directed columns.