Modifications of the amide bond and conformational constraints in pseudopeptide analogues

We have investigated the conformational effects of modifying the amide group in model dipeptides. The N‐methyl amide ψ[CO‐NMe], N‐hydroxy amide ψ[CO‐N(OH)], N‐amino amide ψ[ CO‐N (NH2)], retro amide ψ[ NH‐CO], reduced amide in the neutral ψ[CH2‐NH] and protonated ψ[CH2‐N + H2] state, and hydrazide ψ[CO‐NH‐NH] have been introduced as surrogates of the amide link in pseudopeptide derivatives of the Pro‐Gly or Ala‐Gly model dipeptides protected on both termini by an amide group. These compounds have been studied in solution by proton nmr and ir spectroscopy, and in the solid state by x‐ray diffraction, giving an extended data set of experimental structural and conformational information on pseudopeptide sequences. The conformational effects depend both on the nature and the position of the modified amide link. Some modifications appear to have no intrinsic conformational induction (N‐amino and retro amide), but destabilize any local folded structure by hydrogen‐bond breaking. Because of the formation of strong intramolecular interactions, others are capable of stabilizing a β‐turn (for example protonated reduced amide), or of inducing a particular local conformation such as a β‐ or γ‐like turn (for example N‐hydroxy amide). The particular geometry of the cis N‐methyl amide and of the “hydrazino” proline favors the formation of a sharp turn of the main chain. All these structural data are of interest to the design of bioactive peptide mimics.