Paramagnetic cross-correlation

B. Paramagnetic cross-correlation and interference phenomena TV. The general (slow-motion) theory... 

As another example, the three-dimensional structure of Cytochrome c has been determined on the basis of structural information from pseudocontact paramagnetic chemical shifts, Curie-Dipolar cross-correlation, secondary structure constraints, dipolar couplings and 15N relaxation data [103]. This protein has a paramagnetic center, and therefore the above-mentioned conformational restraints can be derived from this feature. Dipolar couplings do not average to zero because of the susceptibility tensor anisotropy of the protein. The structure determination of this protein without NOE data gives an RMSD (root... 

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). 

In the case of T measurements we have mentioned that cross relaxation provides multiexponential magnetization recovery (Sections 1.7.4 and 7.2.2). A far less known analogy may occur in the linewidths, as already discussed (Section 8.8) when two protons are dipole-dipole coupled and cross correlation occurs between Curie relaxation and proton-proton dipolar relaxation. In this case, we are in the presence of two overlapping signal components with different linewidths, i.e. of biexponentiality in T2 [35], Pulse sequences are available to remove the effects of cross correlation [36]. Such effects are common in paramagnetic metalloproteins where Curie relaxation is usually relevant (in principle, such cross correlation effects can be operative also in the case of 7i, although only to the extent that Curie relaxation on T is effective). 

With respect to the previous books, there is an attempt here to project the field of paramagnetic molecules into the domains of cross correlations and partially oriented systems. 

In Chapters 7 and 8, one-dimensional NOE experiments and a few two-dimensional experiments are presented. Strategies to minimize adverse paramagnetic effects are discussed, as well as ways to exploit such effects to extract structural and dynamic properties. Partial orientation and cross correlation between the Curie magnetic moment relaxation and nuclear dipolar relaxation are also discussed. Chapter 9 deals with the experimental strategies necessary to achieve the highest level of performance in NMR of paramagnetic compounds in solution. 

One important source of distance constraints for paramagnetic proteins are cross correlations between Curie spin relaxation and H-X dipolar relaxation (X = H, C, or N). These cross correlations can in principle be detected by the same pulse sequences as are commonly used in diamagnetic systems to detect cross correlation between chemical shift anisotropy and H- N dipolar relaxation. However, in paramagnetic systems, rapid transverse self-relaxation tends to quench the build-up of relaxation allowed coherence transfer. To overcome this problem, Kateb and Piccioli have proposed a modified HSQC experiment to observe and quantify relaxation-allowed coherence transfer before it is quenched by a short T2. 

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... 

Homonuclear correlation spectroscopy (COSY) experiments (see Chapter 9) substantiate the theoretical predictions, based on molecular orbital calculation, of the pattern of spin delocalization in the 3e orbitals of low-spin Fe(III) complexes of unsymmetrically substituted tetraphenylporphyrins [46]. Furthermore, the correlations observed show that this n electron spin density distribution is differently modified by the electronic properties of a mono-orf/io-substituted derivative, depending on the distribution of the electronic effect over both sets of pyrrole rings or only over the immediately adjacent pyrrole rings [46]. No NOESY cross peaks are detectable, consistently with expectations of small NOEs for relatively small molecules and effective paramagnetic relaxation [47]. 

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