Crystal structure of Boc‐Leu‐Aib‐Pro‐Val‐Aib‐Aib‐Glu(OBzl)‐Gln‐Phl × H₂O, the C‐terminal nonapeptide of the voltage‐dependent ionophore alamethicin

Abstract

Abstract Boc‐ L ‐Leu‐Aib‐Pro‐Val‐Aib‐Aib‐Glu(OBzl)‐Gln‐Phl (Boc = t ‐butyloxycarbonyl, Aib = α‐aminoisobutyric acid, Bzl = benzyl, Phl = phenylalaninol), C 59 H 90 N 10 O 14 , the protected C‐terminal nonapeptide with the sequence 12–20 of alamethicin, crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with a = 15.666, b = 16.192, c = 26.876 Å, and Z = 4. The molecular conformation is right‐handed helical with three α‐(5 → 1 hydrogen bonds) and three β‐turns (4 → 1 hydrogen bonds). All but two of the hydrogen bonds are significantly longer than the usual value and show bifurcation to some extent. The α/3 ‐helical nonapeptide molecules are arranged head‐to‐tail along the a direction. The resulting linear antiparallel chains are linked by a weak intermolecular hydrogen bridge, thus forming a two‐dimensional layer structure in the ab plane. The conformation of this nonapeptide is almost identical with that of the corresponding C‐terminal part found by x‐ray crystallography of the eicosapeptide alamethicin.