NMR chemical shift tensors (CSTs) in proteins, as well as their orientations, represent an important new restraint class for protein structure refinement and determination. Here, we present the first determination of both CST magnitudes and orientations for 13Cαand 15N (peptide backbone) groups in a protein, the β1 IgG binding domain of protein G from Streptococcus spp., GB1. Site-specific 13Cα and 15N CSTs were measured using synchronously evolved recoupling experiments in which 13C and 15N tensors were projected onto the 1H- 13C and 1H- 15N vectors, respectively, and onto the 15N- 13C vector in the case of 13Cα. The orientations of the 13Cα CSTs to the 1H- 13C and 13C- 15N vectors agreed well with the results of ab initio calculations, with an rmsd of approximately 8° . In addition, the measured 15N tensors exhibited larger reduced anisotropies in α-helical versus β-sheet regions, with very limited variation (18 ± 4° ) in the orientation of the z-axis of the 15N CSTwith respect to the 1H- 15N vector. Incorporation of the 13Cα CST restraints into structure calculations, in combination with isotropic chemical shifts, transferred echo double resonance 13C- 15N distances and vector angle restraints, improved the backbone rmsd to 0.16 Å(PDB ID code 2LGI) and is consistent with existing X-ray structures (0.51 Å agreement with PDB ID code 2QMT). These results demonstrate that chemical shift tensors have considerable utility in protein structure refinement, with the best structures comparable to 1.0-Å crystal structures, based upon empirical metrics such as Ramachandran geometries and χ 1χ 2 distributions, providing solid-state NMR with a powerful tool for de novo structure determination.
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Oct 11 2011|
- Cross validation
- Dihedral angles
- Magic-angle spinning
- Quantum chemistry