Unusual conformational preferences of β‐alanine containing cyclic peptides. VII

In the present paper we describe the synthesis, purification, and single crystal x‐ray analysis of the cyclic pentapeptide cyclo‐(Pro‐Phe‐Phe‐β‐Ala‐β‐Ala). This compound crystallizes in the orthorhombic space group P2 1 2 1 2 1 from methanol and adopts in the solid state an unusual conformation characterized by a cis β‐Ala 5 ‐Pro 1 peptide bond and by an intramolecular hydrogen bond stabilizing a C 11 ‐ and a C 12 ‐ring structure. The C 11 , structure contains the Phe 3 and the β‐Ala 4 at the corner position of the turn; it is the first observation of a type II β‐turn enlargement due to the insertion of an extra methylene group of the β‐alanine residue. The rest of the molecule participates in a newly characterized C 12 ‐ring structure, which incorporates a β‐Ala residue at position i of the turn. © 1996 John Wiley & Sons, Inc.