Orientation restraints

The starting point of each structure elucidation is the collection of experimental data like NOE/ROE-derived distances, angular restraints from J-couphngs or CCRs and RDCs as orientational restraints (1). 

Since the energy landscape of a simulated annealing procedure in which only dipolar couplings are included tends to have many local minima, these orientational restraints are usually introduced once a preliminary structure has been calculated from NOE data. This procedure has also the advantage of identifying misassigned NOEs and possible conflicts among the various types of input data. 

Figure 4 EROS ensemble of human ubiquitin determined using NOE and orientational restraints. Drawn from structure 2K39.102...

Orientational restraints, which are derived by paramagnetic labeling of a single domain of a protein, are especially important for the analysis of domain-domain interactions (141). The paramagnetic tag induces anisotropic alignment, which is scaled for the two domains of the protein according to their relative domain mobility. 

Marassi FM, Opella SJ. Simultaneous assignment and structure 45. determination of a membrane protein from NMR orientational restraints. Protein Sci. 2003 12 403-411. 

Mobley, D.L., Chodera, J.D., Dill, K.A. On the use of orientational restraints and symmetry corrections in alchemical free energy calculations, J. Chem. Phys. 2006,125, 084902. 

RDCs within a protein that correspond to bond directions within the alignment tensor frame, providing orientation restraints for protein stmcture determination. 

Skrynnikov NR, Kay LE (2000) Assessment of molecular structure using frame-independent orientational restraints derived from residual dipolar couplings. J Biomol NMR 18 239-252... 

Structure elucidation requires still the very time consuming analysis of distance information from NOESY spectra. Therefore, it would be highly desirable to obtain information from different sources. This is possible by the measurement of dipolar couplings in partially oriented samples. The use of orientational restraints can enhance the speed of stmcture determination quite dramatically, hi Fig. 54, the amount of time for the different steps in structure elucidation are summarized. 

MAS solid-state NMR methods for resolving and assigning resonances. The unique feature of this method is the measurement of orientation restraints that enable the protein structure and orientation to be detected in unoriented proteoliposomes. ... 

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