The effect of hydrogen bonding on the peptide linkage

On the basis of the results obtained in a series of structure investigations and a series of theoretical studies the effect of hydrogen bonding on the peptide linkage (the NCO moiety) is discussed. The structure determinations., using X-ray diffraction methods, comprise eight molecules which contain the NCO moiety. The theoretical studies have been carried out using ab initio methods with hydrogen bonded complexes of formamide as model systems. The effect of hydrogen bonding on the peptide linkage, i.e. the shortening of the CN bond and the lengthening of the CO bond, is well reflected by the theoretical calculations. These calculations indicate that the readily delocalized π-electronic system, which can counteract perturbations in the σ-electronic system, is one of the major factors in the ability of the peptide linkage to form hydrogen bonds. On the basis of the structural parameters obtained in the crystal structure determinations and those found in the gaseous state for some simple amides, it is found that the formation of one intermolecular NH… O hydrogen bond, i.e. such that both the nitrogen and oxygen atoms of the NCO moiety are involved, leads to a shortening of the CN bond by ∼ 0.025 Å and a lengthening of the CO bond by ∼ 0.014 Å