Determination of ψ Torsion Angle Restraints from <sup>3</sup><i>J</i>(C<sub>α</sub>,C<sub>α</sub>) and <sup>3</sup><i>J</i>(C<sub>α</sub>,H<sup>N</sup>) Coupling Constants in Proteins

Homonuclear <sup>3</sup><i>J</i>(C<sub>α</sub>,C<sub>α</sub>) and heteronuclear <sup>3</sup><i>J</i>(C<sub>α</sub>,H<sup>N</sup>) coupling constants have been determined in the protein ubiquitin. Despite the fact that all amide bonds in ubiquitin have a <i>trans</i> conformation, considerable spread in the size of the coupling constants can be observed. The <sup>3</sup><i>J</i>(C<sub>α</sub>,H<sup>N</sup>) coupling constants vary from 0.0 to 1.0 Hz, and the <sup>3</sup><i>J</i>(C<sub>α</sub>,C<sub>α</sub>) coupling constants that could be determined vary from 1.1 to 2.2 Hz. Interpretation of the coupling constants reveals a non-Karplus-type dependence and suggests that vicinal homonuclear <sup>3</sup><i>J</i>(C<sub>α</sub>,C<sub>α</sub>) and heteronuclear <sup>3</sup><i>J</i>(C<sub>α</sub>,H<sup>N</sup>) depend on the ψ<sub>i</sub><sub>-</sub><sub>1</sub> torsion angle. The proposed sensitive E.COSY-type HNCO[C<sub>α</sub>] experiment for the measurement of vicinal <sup>3</sup><i>J</i>(C<sub>α</sub>,H<sup>N</sup>) coupling constants can be used in protonated and deuterated proteins, and the quantitative <i>J</i> correlation experiment HN(COCA)CA can be carried out on perdeuterated proteins for the measurement of <sup>3</sup><i>J</i>(C<sub>α</sub>,C<sub>α</sub>) that provide unique torsion angle information in these proton sparse proteins.