Determination of ψ torsion angle restraints from 3J(Cα,Cα) and 3J(Cα,HN) coupling constants in proteins

Homonuclear (3)J(C-alpha,C-alpha) and heteronuclear (3)J(C-alpha,H-N) 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 (3)J(C-alpha,H-N) coupling constants vary from 0.0 to 1.0 Hz, and the (3)J(C-alpha,C-alpha) 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 (3)J(C-alpha,C-alpha) and heteronuclear (3)J(C-alpha,H-N) depend on the phi(i-1) torsion angle. The proposed sensitive E.COSY-type HNCO[C-alpha] experiment for the measurement of vicinal (3)J(C-alpha,H-N) 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 (3)J(C-alpha,C-alpha) that provide unique torsion angle information in these proton sparse proteins.