Fast nuclear relaxation rate

The structural information derived from relaxation enhancement studies depends somewhat on the model chosen to describe the interaction of solvent protons with the protein molecules. For example even if the experiments indicated that the dispersion of Tfpr were essentially determined by the correlation time for rotational tumbling of the protein the tumbling of the hydration waters would not necessarily have to be restricted to that of the entire hydrated protein. Evidence was found that fast intramolecular tumbling about an axis fixed with respect to the surface of the hydrated species reduced the proton and O17 nuclear relaxation rates even in extremely stable aquocomplexes of Al3+ and other metal ions (Connick and Wiithrich (21)). The occurrence of similar... 

However this is not the only source of CIDNP time dependence that can arise from such a reaction. Consider what happens in the limit of very fast nuclear relaxation in A (i.e. no cancellation). Exchange now transfers recombination polarization from A to unpolarized A where it is able to relax very rapidly. This may be called a "relaxation sink" mechanism, and should occur at a maximum rate of k [A ], being most effective when the A radicals are a e to relax completely prior to reconversion to A i.e. 

An idealized temperature dependence of the relaxation rates is shown in Figure 10.4. The parameter plotted, is the difference between the relaxation rate in the presence of the exchanging species and the rate for the pure solvent, divided by the metal ion concentration. In the high-temperature limit at the left of Figure 10.4, exchange is fast and relaxation is controlled by the nuclear relaxation rate in the inner coordination sphere of the metal ion, T2m - temperature is lowered, exchange becomes... 

Relaxation is the process by which the spins in the sample come to equilibrium with the surroundings. At a practical level, the rate of relaxation determines how fast an experiment can be repeated, so it is important to understand how relaxation rates can be measured and the factors that influence their values. The rate of relaxation is influenced by the physical properties of the molecule and the sample, so a study of relaxation phenomena can lead to information on these properties. Perhaps the most often used and important of these phenomena in the nuclear Overhauser effect (NOE) which can be used to probe internuclear distances in a molecule. Another example is the use of data on relaxation rates to probe the internal motions of macromolecules. 

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