R??le des electrons ? des fonctions amide et ester dans les peptides et depsipeptides

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Abstract
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.