Refinement of the protein backbone angle psi in NMR structure calculations.

Cross-correlated relaxation rates involving the Calpha-Halpha dipolar interaction and the carbonyl (C') chemical shift anisotropy (CSA) have been measured using two complementary 3D experiments. We show that the protein backbone angle psi can be directly refined against such cross-correlated relaxation rates (gammaHalphaCalpha,C') and the three-bond H/D isotope effect on the Calpha chemical shifts (3 deltaCalpha(ND)). By simultaneously using both experimental parameters as restraints during NMR structure calculations, a unique value for the backbone angle psi is defined. We have applied the new refinement method to the alpha-Spectrin SH3 domain (a beta-sheet protein) and to the Sgs1p HRDC domain (an alpha-helical protein) and show that the quality of the NMR structures is substantially improved, judging from the atomic coordinate precision and the Ramachandran map. In addition, the psi-refined NMR structures of the SH3 domain deviate less from the 1.8 A crystal structure, suggesting an improved accuracy. The proposed refinement method can be used to significantly improve the quality of NMR structures and will be applicable to larger proteins.