Unusual conformational preferences of beta-alanine containing cyclic peptides .7.

In the present paper we describe the synthesis, purification, and single crystal x-ray analysis of the cyclic pentapeptide cyclo-(Pro-Phe-Phe-beta-Ala-beta-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 beta-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 beta-Ala(4) at the corner position of the turn, it is the first observation of a type II beta-turn enlargement due to the insertion of an extra methylene group of the beta-alanine residue. The rest of the molecule participates in a newly characterized C-12-ring structure, which incorporates a beta-Ala residue at position i of the turn. (C) 1996 John Wiley & Sons, Inc.