Correlation between ¹⁵N NMR chemical shifts in proteins and secondary structure

An empirical correlation between the peptide ¹⁵N chemical shift, δ¹⁵N_i, and the backbone torsion angles φ_i, ψ_i-1 is reported. By using two-dimensional shielding surfaces Δ(φ_iψ_1-1), it is possible in many cases to make reasonably accurate predictions of ¹⁵N chemical shifts for a given structure. On average, the rms error between experiment and prediction is about 3.5 ppm. Results for threonine, valine and isoleucine are worse (∼4.8 ppm), due presumably to χ₁-distribution/γ-gauche effects. The rms errors for the other amino acids are ∼3 ppm, for a typical maximal chemical shift range of ∼15-20 ppm. Thus, there is a significant correlation between ¹⁵N chemical shift and secondary structure.