Cross-relaxation biomolecules

As a result of relaxation, the nuclear Overhauser effect (NOE) is a phenomenon predicted by Albert Overhauser in 1953, which is the fractional change in intensity of one NMR resonance when another resonance is irradiated. It is the transfer of nuclear spin polarisation between nuclei by cross-relaxation and has become indispensable for the determination of the liquid structure of macromolecules, particularly biomolecules, since the first 2D methods were developed by K. Wiithrich, who was awarded the Nobel Prize in Chemistry in 2001 for his work [28]. It was first shown, theoretically, that saturating the electron magnetic resonance in a metal would cause the nuclear resonance intensity to increase by three orders of magnitude (Feiectron/ynuciei) Similar, albeit much less, enhancement was caused between two nuclei... 

Overlap of peaks along the diagonal, as in the spectra of large biomolecules, prevents the use of such an equation, as does the presence of several mechanisms of exchange or cross-relaxation pathways. Alternatively, analysis of the t dependence of the... 

Brutscher, B. Prindples and applications of cross-correlated relaxation in biomolecules. Concepts. Magn. Reson. 2000, 12, 207-229. 

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]. 

Cross-correlated dipolar relaxation can be measured between a variety of nuclei. The measurement requires two central nuclear spins, each of which is directly attached to a remote nuclear spin (Fig. 16.4). The central spin and its attached remote spin must be connected via a large scalar coupling, and the remote spin must be the primary source of dipolar relaxation for the central spin. The two central spins do not need to be scalar coupled, although the necessity to create multiple quantum coherence between them requires them to be close together in a scalar or dipolar coupled network. In practice, the central spins will be heteroatoms (e.g. 13C or 15N in isotopically enriched biomolecules), and the remote spins will be their directly attached protons. 

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. 

After extraction, the fluorescent indicator was in the unbound state and gave input to the radiative relaxation. Therefore, the fluorescence lifetime increased and, consequently, the intensity as well. After MIP contacting with the analyte, the non-radiative processes were again efficient compared to the radiative processes and, subsequently, fluorescence was quenched. With steady-state fluorescence spectroscopy the cross-reactivity test towards structurally similar biomolecules was performed that yielded selectivity factors for guanosine, cAMP and cCMP of 1.5, 2.5 and 5.1, respectively. 

R268 B. Brutscher, Principles and Applications of Cross-Correlated Relaxation in Biomolecules , Concepts Magn. Reson., 2000,12, 207... 

R 191 M. Ranee, Cross-Correlated Relaxation Effects in Biomolecules , p. 354 R 192 G. Navon, H. Shinar and U. Eliav, Double Quantum Filtered NMR in Ordered Biological Tissues , p. 374... 

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