Transferred cross-correlated relaxation

W. Transferred cross-correlated relaxation complements transferred NOE structure of an IL-4R-derived peptide bound to STAT-6. J. Am. Chem. Soc. 1999, 121, 1949-1953. 

Fig. 7.27 a Transferred NOESY, b cross and c reference traces from the transferred rcCH CH-HCCH experiment that display the dependence of the rate of the transferred cross-correlated relaxation... 

In the situation where the interaction is weak, one of the traditional methods that can be applied to obtain structm-al information (internuclear distances) of the bound ligand is the so-called transferred NOE (trNOE) method. Recently, it became possible to use transferred cross-correlated relaxation (trCCR) to directly measure dihedral angles. The combined use of these two techniques significantly improves the precision of the structure determination of ligands weakly bound to macromolecules. 

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). 

Felli et al. [23] and Pelupessy et al. [24] have developed a method that measures dipole, dipole cross-correlated relaxation by transferring an initial operator to another operator by the evolution of the desired cross-correlated relaxation rate. 

Therein, cross-correlated relaxation T qHj c h °f the double and zero quantum coherence (DQ/ZQ) 4HizCixCjj generated at time point a creates the DQ/ZQ operator 4HjzCjJCiy. In the second part of the experiment, the operator 4HJZCjxQy is transferred via a 90° y-pulse applied to 13C nuclei to give rise to a cross peak at an(i... 

By combining the quantitative approach [23] to extract cross-correlated relaxation with resolution enhancement methods using restricted coherence transfer in a so-called forward directed TOCSY [27], Richter et al. could determine the ribose sugar conformation for all but two residues in a uniformly 13C,15N labeled 25mer RNA [28] and compare them to 3J(H, H) values determined using a forward-directed HCC-TOCSY-CCH-E.COSY experiment [29]. 

Due to the fact that cross-correlated relaxation depends linearly on the correlation time, it can be used to determine the conformation of ligands when bound to target molecules, provided that the off rate is fast enough to enable detection of the cross-correlated relaxation rate via the free ligand [33, 34]. The conditions under which such an experiment can be performed are similar to those found for transferred NOEs [35], and, for Kd values... 

The angle Qsin is that between the IN axis and the IS axis. The expression (3cos 0—1)/2 in Eq. (28) is characteristic for the cross-correlated relaxation effects. An analogous and somewhat more general expression for the case of anisotropic susceptibility was given by Bertini et al. (56). The crosscorrelation-driven coherence transfer phenomena between nuclear spins in paramagnetic systems with anisotropic susceptibility were even earlier considered by Desvaux and Cochin (65). 

For a spin-1/2 nucleus, such as carbon-13, the relaxation is often dominated by the dipole-dipole interaction with directly bonded proton(s). As mentioned in the theory section, the longitudinal relaxation in such a system deviates in general from the simple description based on Bloch equations. The complication - the transfer of magnetization from one spin to another - is usually referred to as cross-relaxation. The cross-relaxation process is conveniently described within the framework of the extended Solomon equations. If cross-correlation effects can be neglected or suitably eliminated, the longitudinal dipole-dipole relaxation of two coupled spins, such... 

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