Conformations of chicle(l- or d-Phe-l-Pro-Aca) and chicle(l-Pro-l- or d-Phe-Aca)

Conformational analyses on four cyclic model peptides of the β-bend, chicle( l - or d -Phe- l -Pro-ϵ-aminocaproyl(Aca)) and cyclo( l -Pro- l - or d -Phe-Aca), were carried out both experimentally and theoretically. Chicle( d -Phe- l -Pro- Aca) was shown to exist as a single conformer taking the type II′ β-bend. The comparison of its CD spectra with those of chicle ( l -Ala- l -Ala-Aca) revealed that type I and II′ β-bends, both with α-helix-like CD spectra, can be distinguished. Chicle ( l -Phe- l -Pro-Aca) was shown to exist as a single conformer with a cis l -Phe- l -Pro peptide bond, taking the type VI β-bend. Its CD spectrum has thus been observed for the first time for the bend containing a cis peptide bond. Chicle( l -Pro- l -Phe-Aca) was shown to exist as a mixture of two conformers, the major one taking the type I β-bend with a trans Aca- l -Pro peptide bond and the minor one with a cis Aca- l -Pro peptide bond. Chicle( l -Pro- d -Phe-Aca) was suggested to exist as a mixture of two conformers, the major one taking the type II β-bend with a trans Aca- l -Pro peptide bond and the minor one with a cis Aca- l -Pro peptide bond.