Cross-Correlated Relaxation CCR

A second way of measuring angular information is based on CCR processes. In contrast to empirical relations like the Karplus equation, angular information out of CCR measurements can be directly extracted from fundamental principles. 

The beauty of CCR measurements is that the nuclei i and k do not need to be neighboring spins, but can be very distant as long as they can be correlated. Based 

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Elucidation of the stereostructure - configuration and conformation - is the next step in structural analysis. Three main parameters are used to elucidate the stereochemistry. Scalar coupling constants (mainly vicinal couplings) provide informa-hon about dihedral bond angles within a structure. Another way to obtain this information is the use of cross-correlated relaxation (CCR), but this is rarely used for drug or drug-like molecules. 

Since the discovery of the nuclear Overhauser effect (NOE, see previous section) [4, 5] and scalar coupling constants [36, 37] decades ago, NMR-derived structure calculations of biomolecules largely depended on the measurement of these two parameters [38]. Recently it became possible to use cross-correlated relaxation (CCR) to directly measure angles between bond vectors [39] (see also Chapt 7). In addition, residual dipolar couplings of weakly aligned molecules were discovered to measure the orientation of bond vectors relative to the alignment tensor (see Sect 16.5). Measurement of cross-correlated relaxation was described experimentally earlier for homonuclear cases [40, 41] and is widely used in solid-state NMR [42 14]. 

CCR Cross-correlated relaxation CSA Chemical shift anisotropy... 

Measurement of cross-correlated relaxation has been described for homo-nuclear cases [10,11], and is widely used in soUd-state NMR [12-14]. It is the availability of isotopically labelled biomolecules and its appHcation to solution-state NMR that makes the method so interesting. The first application of CCR in solution-state NMR with a N, C labelled protein, was the determination of the torsion angle in the small protein rhodniin [7]. This torsion angle is difficult to obtain by traditional methods. 

Two conformations of EpoA in complex with tubulin have been proposed on the basis of EC [26] and NMR [76, 96] data, respectively (Fig. 11). The tubulin-bound conformation of EpoA was determined by solution NMR spectroscopy [96] before the EC structure of EpoA bound to tubulin was available. The observation that, in a 100 1 mixture with tubulin, NOE cross-peaks of EpoA have negative sign, indicated that there is a fast exchange equilibrium in solution. This offered the opportunity to measure transferred NMR experiments, that report on the bound conformation of the ligand. A total of 46 interproton distances were derived from cross-peak volumes in tr-NOE spectra. However, these distance restraints did not suffice to define a unique conformation, as several distinct structures were consistent with them. Transferred cross-correlated relaxation (Sect. 2.2.1.3) provided the additional dihedral restraints that were crucial to define the bound conformation [96, 97], One requirement to measure CH-CH dipolar and CH-CO dipolar-CSA CCR rates is that the carbon atoms involved in the interaction are labeled with 13C. The availability of a 13C-labeled sample of EpoA offered the opportunity to derive seven of these dihedral angle restraints from tr-CCR measurements (Fig. 12). 

The individual lines of the doublet signals relax with different rates due to the phenomenon of cross-correlated dipole-CSA relaxation. The CCR-rate can be extracted from the ratio of the intensity of the peaks /dipoie-csA = l/2tln (fhighfieldAowfieid)> where t is the time the selected coherences experience relaxation, and I denotes the signal intensities of the spin states. 

Cross correlation (CCR) contribution to the paramagnetic relaxation of a nucleus arising from the rotational average of the cross-term between the Curie relaxation of that nucleus and its dipole-dipole relaxation with another nucleus (eg. in a... 

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