AbstractIn order to restrain the flexibility of the peptide molecules and reduce their biodegradation, modifications of the main chain are now introduced in pseudopeptide analogues. Surprisingly, there is very little data on the conformational properties of these derivatives. We have examined pseudopeptide analogues of RCO‐X‐Y‐NHR′ model dipeptides in the depsi, N‐methylated, reduced, retro, α, β‐dehydro, β‐amino acid, and hydrazino series, in the solid state by x‐ray diffraction, and in solution by ir and1H‐nmr spectroscopy. This study provides us with accurate dimensions of the peptide surrogates, and gives some information on the conformational tendencies induced by these substitutions, with reference to those of the related