Recording data • TIMING ~11 days
1. Record 3D TROSY-HNCA, 3D MQ-CCH-TOCSY and 4D 13C, 15N-edited NOESY for resonance assignment. Use half-point delays for the evolution periods of Cα spins (t1) in the HNCA, 13C spins (bonded with the directly detected 1H, t2) in the MQ-CCH-TOCSY, and both 13C and 15N spins (t2 and t3) in the 4D NOESY. In this way, the peaks aliased with odd times have opposite signs to those aliased with even times. Record a 2D 1H-15N TROSY-HSQC with enough spectral width in the 15N dimension to facilitate the identification of peaks aliased in the 15N dimension in 3D HNCA and 4D 13C, 15N-edited NOESY.
Processing data • TIMING ~1 day
2. Process the data with NMRPipe. Use linear prediction to improve the resolution of all indirectly detected dimensions.
Covert the processed spectra into ucsf format for further analysis with Sparky.
Picking peaks and grouping peaks into clusters • TIMING ~1-2 days
3. Use peak picking functions in Sparky to pick HNCA peaks automatically. Group them into clusters according to (HN, N) chemical shifts. Usually one cluster contains two peaks: one from the intraresidue Cα, another from the Cα of the preceding residue. However, some clusters contain only one peak due to degenerate Cα chemical shifts, while some clusters may contain more than two peaks due to the overlap of amide chemical shifts. Identify peaks from NH2 groups according to their paired patterns and unusual “Cα” chemical shifts. These peaks can be excluded during backbone assignment.
Check the number of clusters identified and the expected number and ensure that there is not a large difference.
4. Pick peaks in 4D 13C,15N-edited NOESY manually or automatically. Group NOE peaks into their corresponding clusters according to (HN, N) chemical shifts of individual clusters. Due to lower resolution of the 4D NOESY, some peaks cannot be grouped into only one cluster unambiguously. These peaks will remain ungrouped.
5. Correct chemical shifts of the aliased peaks in 3D TROSY-HNCA and 4D NOESY with Sparky.
6. Pick peaks in CCH-TOCSY.
Identifying and classifying spin-systems • TIMING ~5-10 days
7. Each cluster is characterized by a pair of (HN, N) chemical shifts. Typically, one cluster contains two HNCA peaks and a number of HC-NH NOE peaks. First, initialize a spin-system from one HNCA peak in a given cluster. Second, among the NOE peaks in this cluster, identify the peak that matches the HNCA peak in 13C chemical shift within a certain tolerance (e.g., ~0.3 ppm). If only one peak is identified, this peak is considered as a sequential or intra-residue CαHα-NH NOE10. Note that there are two α protons for glycine residues and thus two peaks with identical Cα shifts may be observed. Third, display the TOCSY slice defined by the (H, C) chemical shifts of each NOE peak in this cluster and check if the CH spin-pair of the NOE peak correlates with the Cα spin of this initial spin-system. If yes, the NOE peak is regarded as a sequential or intraresidue side chain NOE in this spin-system. The 13C spin of the CH spin-pair should also correlate with CαHα of this spin-system via TOCSY. In some cases, this correlation cannot be detected because it is too weak or close to the intense water signal. For spin-systems of long side chains, a few more CH spin-pairs may be added into the spin-systems according to their TOCSY correlations with side chain CH spin-pairs that are already identified.
8. In cases where only one HNCA peak is picked for one cluster due to degenerate Cα chemical shifts and two NOE peaks match this HNCA peak, build two spin-systems using the two NOE peaks. The NOE peaks belonging to these two spin-systems can be separated because side chain 13C spins correlate with two non-degenerate CαHα spin-pairs in CCH-TOCSY, provided that 13C-CαHα correlations are detectable.
9. For each spin-system, check whether the CH spin-pairs correlate with each other in CCH-TOCSY and whether the correlations can be explained by one of the 20 amino acids in terms of number of peaks and chemical shifts.
10. Classify a spin-system by residue type according to its Cα chemical shift and NOE peaks. Typically it is possible to unambiguously recognize most glycine, alanine, threonine, valine, isoleucine, and leucine, which account for ~42% amino acid residues in a protein. Glycine can be identified according to its small Cα chemical shift, and sometimes two Hα peaks. Alanine can be identified based on its strong intraresidue CβHβ-NH NOE peak, strong Cα-CβHβTOCSY peak, and characteristic Cα and Cβ chemical shifts. For threonine, valine, isoleucine and leucine, classification can be easily done based on their characteristic side chain chemical shifts. In particular, the methyl groups of these residues always give rise to well resolved TOCSY correlations with other 13C spins. Some serine, lysine, arginine, and proline can be distinguished, provided that the CβHβ spin-pair for serine, CβHβ, CγHγ and CδHδ spin-pairs for lysine, arginine and proline can be identified. Classify other residues into three groups: (Asp, Asn, Phe, Tyr), (Gln, Glu, Met) and (Cys, Trp, His) according to both 1H and 13C chemical shifts.
If a spin-system contains too few peaks to be classified, leave it without classification. Usually classification of about ~35-50% spin-systems is sufficient for obtaining backbone assignment.
Matching spin-systems to form fragments and Mapping fragments • TIMING ~1-3 days
11. Link clusters to build fragments through matching spin-systems. Two spin-systems match each other when they have i). identical Cα chemical shifts, ii). the largest number of matched NOE peaks, and iii). no inconsistent (C, H) chemical shifts.
12. Compare spin-system types of a connectivity fragment with the protein sequence. If only one segment in the sequence matches the fragment in residue type at every amino acid position, map this fragment onto the segment of the sequence. Note that it is unknown which end of the fragment is the N-terminus since intraresidue and sequential spin-systems are not distinguished. The intraresidue HNCA peak is usually stronger than the sequential one. If a mapping is inconsistent with this, it may contain errors and need to check more carefully.
13. Use common NOEs shared by two clusters to resolve ambiguities in sequential connectivities for unassigned spin-systems6. If two spin-systems have a true sequential relationship, no mismatch of intra-residue and sequential correlations should exist. On the basis of this fact, some ambiguities can be removed too.
14. Repeat steps 11-13 and assign the unassigned fragments and clusters in an iterative way.
15. Note that some initial errors may be introduced during the processes of identifying, classifying and matching spin-systems and mapping fragments to the sequence. Most errors can be recognized during the process of assignment and will not lead to wrong final assignments. Those lead to wrong initial assignments should be recognized when some other clusters can’t be assigned6.
Side chain assignments • TIMING ~2-7 days
16. Most side chain chemical shifts are ready to be extracted according to the identified spin-systems and backbone assignments. Assign some more side chain resonances by comparing two 13C,15N-edited NOESY planes defined by two adjacent amides, and by exploring the MQ-CCH-TOCSY more extensively10.
17. Assign NH2 chemical shifts according to i) Cβ-NH2 and Cγ-NH2 correlations in the HNCA spectrum for asparagine and glutamine respectively, ii) HβCβ-NH2 and HγCγ-NH2 /HβCβ-NH2 correlations in the 4D 13C,15N-edited NOESY for asparagine and glutamine respectively.
18. Assign CδHδ groups of aromatic residues according to their NOE correlations with intraresidue and sequential amides11. Other aromatic resonances can be assigned later with a 4D 13C,13C-edited NOESY ( ~7 days for data collection) or a 3D 15N/13C-simutaneous NOESY ( ~3 days for data collection).
NOE assignment and structure determination • TIMING ~2-6 weeks
19. With a relatively complete chemical shift list, assign other NOE peaks in the 4D 13C,15N-edited NOESY and 13C,13C-edited NOESY. Follow the conventional procedure for NOE assignment and structure calculation. The chemical shifts of a few more side chain atoms may be assigned during this process.
D. Marion, P.C. Driscoll, L.E. Kay, P.T. Wingfield, A. Bax, A.M. Gronenborn and G.M. Clore (1989) Biochemistry28 6150-6156. (Link to Article)
D. Marion, L.E. Kay, S.W. Sparks, D.A. Torchia and A. Bax (1989) J. Am. Chem. Soc.111 1515-1517. (Link to Article)
E.R.P. Zuiderweg and S.W. Fesik (1989) Biochemistry28 2387-2391. (Link to Article)
To start with, magnetisation is exchanged between all hydrogens using the NOE. Then the magnetisation is transferred to neighbouring 15N nuclei and back to 1H for detection.
This spectrum can be used to obtain restraints for structure calculations. In this case the NOESY mixing time should probably be around 80ms. It can also be used to help assignment, and for small to medium-sized proteins, assignment using this and 15N-TOCSY-HSQC only is possible. In this case it may be useful to use a slightly longer NOESY mixing time.