Large Variations in One-Bond <sup>13</sup>C<sup>α</sup>−<sup>13</sup>C<sup>β</sup> <i>J</i> Couplings in Polypeptides Correlate with Backbone Conformation

One-bond <sup>1</sup><i>J</i><sub>C</sub><sub>α</sub><sub>C</sub><sub>β</sub> scalar couplings, measured in the protein ubiquitin, exhibit a strong dependence on the local backbone conformation. Empirically, the deviation from the <sup>1</sup><i>J</i><sub>C</sub><sub>α</sub><sub>C</sub><sub>β</sub> value measured in the corresponding free amino acid, can be expressed as Δ<sup>1</sup><i>J</i><sub>C</sub><sub>α</sub><sub>C</sub><sub>β</sub> = 1.3 + 0.6 cos(ψ − 61°) + 2.2 cos[2(ψ − 61°)] − 0.9 cos[2(φ + 20°)] ± 0.5 Hz, where φ and ψ are the intraresidue polypeptide backbone torsion angles obtained from ubiquitin's X-ray structure. The relation between <sup>1</sup><i>J</i><sub>C</sub><sub>α</sub><sub>C</sub><sub>β</sub> and backbone torsion angles is confirmed by density functional theory (DFT) calculations on the peptide analogue Ace-Ala-NMe.