The cyclic, imido acid containing pentapeptides cyclo(Asp-Trp-(NMe)Nle-Asp-Phe) (cpp[NMeNle(3)]) and cyclo(Asp-Trp-Pro-Asp-Phe) (cpp[Pro(3)]) have been investigated by 1H-NMR spectroscopy in DMSO and by restrained molecular dynamics methods. The spectra indicate the existence of at least four cis/trans isomers for cpp[NMeNle(3)] and two cis/trans isomers for cpp[Pro(3)]. In addition to the imido peptide bonds, cpp[NMeNle(3)] shows cis/trans isomerization of the Asp4-Phe5 and Phe5-Asp1 peptide bonds whereas only the Phe5-Asp1 peptide bond isomerizes in the Pro-containing peptide. In cpp[Pro(3)] all cis bonds are centred in betaVIb turns. Also, cpp[NMeNle(3)] prefers backbone angles around the cis bonds which are rather similar to the angles of a betaVIb turn. The higher number of cis/trans isomers and slight deviations in the backbone angles of comparable isomers of both peptides are caused by an enhanced flexibility of cpp[NMeNle(3)] due to the possibility of the phi-(NMe)Nle rotation.