Figure 1. Solid‐state NMR assignment strategy. (a) BMRB distribution for Thr, Ala, and Val, and an idealized spectrum for each residue. (b) SPECIFIC CP transfer pathways for inter‐ and intraresidue correlations mapped on a molecular fragment. (c) N‐CA/CO‐CX polarization transfer pathway. NCACX proceeds along the arrows indicated by 1 and 2, while NCOCX proceeds along arrow 3. (d) A representative backbone walk from I133 to L131 using NCO/CACX experiments on αB‐crystallin oligomers.

Figure 2. Solid‐state NMR three‐ and four‐dimensional assignment experiments. (a) Pulse sequence schematic for three‐dimensional NCACX or NCOCX experiments for sequential assignments. (b) Pulse sequence schematic for four‐dimensional CANCOCX or CONCACX experiment for sequential assignments.

Figure 3. Dipolar recoupling schemes. (a) The pulse sequence schematic for the REDOR pulse sequence element. The I spin is the observed spin, and the S spin is the recoupled nucleus. (b) A generalized two‐dimensional N–C TEDOR pulse sequence schematic with z‐filters before and after the indirect dimension evolution time. In this case, the I spin is and the S spin is .

Figure 4. Molecular fragment and backbone torsion angles. (a) Idealized molecular fragment with all atoms and the torsion angles ψ and Φ indicated. (b) Heavy atom definition of the torsion angles ψ and Φ.

Figure 5. Approaches for torsion angle measurements. (a) A doubly dephased experiment where the pseudo‐atoms are generated by a mixing scheme. The cross‐peaks will have angle‐dependent trajectories. One of the dipolar recoupling periods could be exchanged for a CSA period to report the projection of the dipolar interaction on the CSA. (b) A double‐quantum state is excited, creating a pseudo‐atom that produces a trajectory that is dependent on the pseudo‐bond angle.

Figure 6. paramagnetic enhancements to R ₁ (a) and R ₂ (b) for the various metal ions as a function of the typical values of the electron relaxation time, at 850 MHz of proton Larmor frequency and a metal–nucleus distance of 10 Å (HS = high spin, LS = low spin).

Figure 7. The nucleus experiences the PCS induced by both the intramolecular metal (solid arrow) and the metals in surrounding molecules (dashed arrows).

Figure 8. Comparison of structure quality of microcrystalline proteins determined by MAS solid‐state NMR. (a) Distribution of an overall quality score reported by MolProbity [ 98] for X‐ray (filled gray), solution NMR (solid black), and solid‐state NMR (red) structures. High‐quality structures have a low score, while high scores generally illustrate a low‐quality structure. (b) Comparison of the overall quality of MAS solid‐state NMR structures of microcrystalline proteins deposited in the Protein Data Bank (PDB). For each solid‐state NMR structure, the height of the bar indicates the percentage of lower quality X‐ray (gray) or solution NMR (blue) structures. A percentage larger than 50% (dotted line) indicates that the quality is better than the “average” structure from the other techniques.

Figure 9. Superposition of the – cross‐polarization/MAS PDSD spectra of the diamagnetic zinc MMP‐12 (blue) with the paramagnetic CoMMP‐12 (red). Green arrows indicate the paramagnetic shifts.