Biomolecule, nuclear spins

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. 

Finally, as tabulated below, many elements having useful natural abundance and nuclear spin are naturally present in biomolecules. Still others can be substituted into biological molecules to provide a diverse range of opportunities. For example, fluorine can be substituted for hydrogen in many cases. 

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... 

In the above, we have seen that a certain interpolymer interaction is required for different polymers to be miscible. Here, we will see that high resolution NMR enables us to locate interacting regions in component polymers. One of the most useful methods is the nuclear Overhauser effect (NOE) between H— H and H—NOE can be observed between spins whose distances are less than about 0.5 nm. The one- (ID) and two-dimensional (2D) NOE experiments have been used to reveal the spatial structure of biomolecules in solutions. Of course, these can be applied to locate interacting regions in a blend in solution and in solids [3]. For example, Crowther et al. [22] and Mirau et al. [23] applied NOE experiments to polystyrene/poly(vinyl methyl ether) (PS/PVME) in a toluene solution, and show that the interpolymer NOE signals between the aromatic protons of PS and the methoxy protons of PVME can be observed at polymer concentrations higher than 25 wt%. In the solid state, Heffner and Mirau [24] measured 2D H— H NOESY (NOESY nuclear Overhauser effect spectroscopy) spectra of 1,2-polybutadi-ene and polyisoprene (1,2-PB/PI) and observed NOE cross-peaks between these component polymers. White and Mirau observed interpolymer NOE interactions between the H spins of PVME and the spins of deuterated... 

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