Noe In Multi-Spin Systems

3. Noe In Multi-Spin Systems:

Real molecules have more than two spins. Fortunately, almost all relaxation processes only involve pairs of spins; processes involving three spins are called cross-correlation processes and can usually be ignored. This means that we can understand what happens in real molecules by a simple extension of the two-spin case. Perturbation of X leads to an effect on the intensity of A; in turn this leads to an effect on As neighbors. Therefore, NOE’s can propagate throughout the molecule. This has several consequences.

In large molecules, the homonuclear steady-state NOE is -1 (figure 5), meaning that saturation of X leads to complete saturation of A also. This in turn passes its saturation on to other spins in a process known as spin diffusion. The consequence is that all nuclei in the molecule eventually become saturated, and the steady-state experiment is uninformative. Consequently, all NOE experiments in large molecules (e.g. proteins) are carried out as transient experiments, usually by versions of the 2D NOESY experiment.

In small molecules, saturation of X leads to an increase in the intensity of A, with a maximum of +50%, though in practice it is usually much smaller than this. An increase in A is passed on to its neighbors as a decrease in their intensity. Thus, in small molecules multi-spin effects lead to alternations in the sign of the NOE along a chain. In practice the effect is not observed for more than three spins in a row. It is therefore possible to carry out steady-state NOE experiments on small molecules, and the effects are approximately related to distance. However, calculation of sizes of NOEs requires detailed knowledge of geometries (and usually of dynamics as well), and steady-state NOEs are not (and should not be) normally interpreted in more than a qualitative manner. Nevertheless, they are extremely useful for the assignment of geometries around double bonds, and for stereochemical assignments, particularly in rigid systems such as fused ring systems. NOEs have in the past been much used to connect molecular fragments together in natural product identification. However, with the advent of heteronuclear correlation experiments such as HMBC (heteronuclear multiple bond correlation), most problems of this type can now be addressed more simply, leaving only a few stereochemical details to be addressed by specific NOE experiments. Although exact calculation of steady-state NOEs in multi-spin systems is generally not worthwhile, a few guidelines are useful for their interpretation. Protons that are very close together (for example, methyl and methylene protons) relax each other very efficiently. This means that NOEs into such groups are usually small. (The NOEs are small on a fractional basis. In absolute terms, an NOE to a methyl group can still be easily observed because there are three protons in a methyl group, and because the signal is often sharp.) NOEs to isolated protons can be often be misleadingly large.