Variation of the NH–CαH coupling constant with dihedral angle in the NMR spectra of peptides

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Abstract
Proton magnetic resonance data and conformational calculations of a series of model compounds containing a NH‐CαH group substituted as in peptides have been used to generate a proton–proton coupling constant–dihedral angle relation for the peptide unit. For those substances used in which the dihedral angle about the N‐Cα bond is not fixed, the angle distribution was calculated from conformational theory. Using eight examples in which the number of theoretical assumptions were least, the best values of the coefficients A, B, and C in the expression J(θ) = Acos2θ + B cosθ + Csin2θ were found by a least‐squares procedure to be 7.9, −1.55, and 1.35, respectively. This relation gives reasonable values for the dihedral angles ϕ in cyclic oligopeptide structures for which the availability of both NMR data and other structural information allow comparison. When applied to N‐acetylamino acid N‐methylamides having side chains extending beyond Cβ, however, agreement with the calculated conformational distribution was found for Leu, Met, and Trp, but observed values of J were larger than expected for Val, He, Phe, and Tyr, These disagreements are considered to be the result of interactions not yet taken into account in the usual conformational calculations.