Pairwise NMR experiments for the determination of protein backbone dihedral angle Phi based on cross-correlated spin relaxation.

Novel cross-correlated spin relaxation (CCR) experiments are described, which measure pairwise CCR rates for obtaining peptide dihedral angles Phi. The experiments utilize intra-HNCA type coherence transfer to refocus 2-bond JNCalpha coupling evolution and generate the N(i)-Calpha(i) or C'(i-1)-Calpha(i) multiple quantum coherences which are required for measuring the desired CCR rates. The contribution from other coherences is also discussed and an appropriate setting of the evolution delays is presented. These CCR experiments were applied to 15N- and 13C-labeled human ubiquitin. The relevant CCR rates showed a high degree of correlation with the Phi angles observed in the X-ray structure. By utilizing these CCR experiments in combination with those previously established for obtaining dihedral angle Psi, we can determine high resolution structures of peptides that bind weakly to large target molecules.