Crystal structure of L-Pro-L-Leu-Aib-Aib-L-Glu-L-Valol, the C-terminal hexapeptide fragment of trichotoxin.

The crystal structure of L-prolyl-L-leucyl-alpha-aminoisobutyryl-alpha- aminoiso-butyryl-alpha-L-glutamyl-L-valinol (L-Pro-L-Leu-Aib-Aib-L-Glu-L-Valol), the C-terminal hexapeptide fragment of trichotoxin, has been determined by X-ray crystallography. The hexapeptide forms a right-handed 3(10)-helix consisting of two 10-atom hydrogen-bonded beta-turns of type III and one beta-turn of type I. Two of the intramolecular hydrogen-bonds are particularly weak, thus suggesting that the title compound may adopt non-helical conformations in solution, as observed by circular dichroism. In the crystal the molecules are hydrogen-bonded head-to-tail, forming infinitely long helical columns.