Rǒle des electrons π des fonctions amide et ester dans les peptides et depsipeptides

The IR data for the R¹ CO-O-CHR² -CO-NHR³ derivatives are interpreted in terms of a H…π interaction involving the NH bond and the π orbitals of the ester function and giving rise to a high ν(CO) frequency and a low ν frequency. The resulting molecular conformation corresponds to the angular values ϕ # -90°, ψ # 0°. The H…π interaction in MeCO-L-Lac-NHMe is highly destabilized by water and aprotic solvents but is retained in methanol. Considering the high ν(CO) ester or amide frequency of the middle function in β-folded depsipeptide or peptide sequences, it may be supposed that the residue indexed i + 2 in β turns experiences a H…π interaction which has a stabilizing effect on β turns. Some examples concerning valinomycin and some model compounds are discussed.