Determination of ψ Torsion Angle Restraints from 3J(Cα,Cα) and 3J(Cα,HN) Coupling Constants in Proteins

Homonuclear 3J(Cα,Cα) and heteronuclear 3J(Cα,HN) coupling constants have been determined in the protein ubiquitin. Despite the fact that all amide bonds in ubiquitin have a trans conformation, considerable spread in the size of the coupling constants can be observed. The 3J(Cα,HN) coupling constants vary from 0.0 to 1.0 Hz, and the 3J(Cα,Cα) 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 3J(Cα,Cα) and heteronuclear 3J(Cα,HN) depend on the ψi-1 torsion angle. The proposed sensitive E.COSY-type HNCO[Cα] experiment for the measurement of vicinal 3J(Cα,HN) coupling constants can be used in protonated and deuterated proteins, and the quantitative J correlation experiment HN(COCA)CA can be carried out on perdeuterated proteins for the measurement of 3J(Cα,Cα) that provide unique torsion angle information in these proton sparse proteins.