Nuclear magnetic resonance three-spin effects

NMR Spectroscopy The three dimensional structures of small proteins containing about as many as 200 amino acids can be studied with nuclear magnetic resonance (NMR) spectroscopy. In this technique, a concentrated protein solution Is placed in a magnetic field and the effects of different radio frequencies on the spin of different atoms are measured. The behavior of any atom Is Influenced by neighboring atoms In adjacent residues, with closely spaced residues being more perturbed than distant residues. From the magnitude of the effect, the distances between residues can be calculated these distances are then used to generate a model of the three-dimensional structure of the protein. 

In conclusion, although it has been demonstrated that a three-spin effect exists, it is usually unimportant unless the radical concentration is low. This is readily understandable, since the magnetic moment of the electron is much larger than that of any nucleus so that nuclear-electron interactions are the dominant relaxation terms, except at low concentrations where nuclear—nuclear interactions become important. The presence of a three-spin effect can be revealed most easily either by observation of the transient relaxation behaviour of the nuclear resonance or by triple irradiation experiments. In the latter case, account must be taken of the collapse of any multiplet structure in the interpretation of the results. 

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... 

TTie TOCSY 2D NMR experiment correlates all protons of a spin system, not just those directly connected via three chemical bonds. For the protein example, the alpha proton, Ft , and all the other protons are able to transfer magnetization to the beta, gamma, delta, and epsilon protons if they are connected by a continuous chain—that is, the continuous chain of protons in the side chains of the individual amino acids making up the protein. The COSY and TOCSY experiments are used to build so-called spin systems—that is, a list of resonances of the chemical shift of the peptide main chain proton, the alpha proton(s), and all other protons from each aa side chain. Which chemical shifts correspond to which nuclei in the spin system is determined by the conventional correlation spectroscopy connectivities and the fact that different types of protons have characteristic chemical shifts. To connect the different spin systems in a sequential order, the nuclear Overhauser effect spectroscopy... 

The excess (magnetic) heat capacity may be represented to a good approximation as the sum of (a) the electronic transitional (or Schottky) heat capacity, (b) the effects of interaction between the electrons and the nuclear spin of the paramagnetic ion, (c) the dipolar interaction between these ions, and (d) interactions for other types of interionic coupling. The last three terms are often small above 2°K. and in some instances can be obtained from paramagnetic relaxation data. In principle, the second and third can also be obtained from paramagnetic resonance data. 

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