Refinement of the angular dependence of the peptide vicinal NHCαH coupling constant

Abstract

The refined dependence of the peptide NHCαH vicinal coupling constant on the dihedral angle θ have been derived on the basis of the accumulated experimental data. The mean permissible values (in Hz) are approximated by 3 J NHCH = 9·4 cos 2 θ - 1·1 cos θ + 0·4 An analogous relationship for the sum of two vicinal NHCαH 2 coupling constants in the glycyl residue have been calculated from the above dependence. Measurements on N-methylacetamide in various solvents and in the presence of an alkali salt showed the vicinal constant NHCH to vary by not more than ± 3%. Some of the other proposed 3 J NHCH(θ) dependencies give too low values for the cis-oriented NH and CαH bonds. This may be due to the fact that in these correlations the data for compounds with cis-amide bonds have been used for 0° ⩽ θ ⩽ 90° region of the dependence.