Measurements of NOE

The nuclear Overhauser effect was predicted in 1953 [37], experimentally demonstrated in 1955 [38], and widely used since then to obtain structural and conformational information in diamagnetic small molecules. As it appears from 

In our experience, the best experimental scheme is to choose two off-resonance positions symmetrical with respect to the irradiated signal and typically offset by one to two times the linewidth of the irradiated signal (Fig. 9.8). 

It often happens that there is at least one signal too close to the signal to be irradiated to allow for the setting of the two off-resonance frequencies in the optimal position. In fact, if another signal is close to one of the two off-resonance frequencies, in the difference spectrum this signal appears as strongly positive. 

Approaches of this kind are also particularly important when the NOE to be observed is on a signal close to the irradiated one. In the case of the two P-CH2 protons of an Asp residue bound to a high spin cobalt(II) in the protein superoxide 

It is often necessary to obtain a profile of NOE intensities vs. the irradiation time, called NOE buildup. A series of truncated NOEs are thus needed (Section 7.3). The only caveat with respect to buildup experiments in a diamagnetic case is that there is a lower limit in the irradiation time because even a truncated NOE needs saturation of the irradiated signals. Since instantaneous saturations are 

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Fig. 2.22. Measurement of NOE enhanced coupled 13C NMR spectra sample 2,4,6-trichloro-pyrimidine. 75% by vol. in hexadeuteriobenzene, 25 °C, 20 MHz ...

Fig. 2.24. Measurement of NOE enhancement factors by pulsed proton decoupling sample menthol, 200 mg in 1 mL of deuteriochloroform, 25 DC, 20 MHz ...

When direct (A-B) NOEs are negative, indirect effects are also negative and so cannot be distinguished they spread rapidly and have high intensities. This process is referred to as spin diffusion and is fatal for the steady-state NOE since it causes a loss of specificity and hence provides no information on molecular geometry. In this regime, it usually becomes necessary to use kinetic (transient) methods based on the measurement of NOE growth rates. 

This method is widely used to measure internuclear distances, particularly in macromolecules. It is simple to apply, and does not require very accurate measurement of NOE intensities, because of the sixth root, which reduces the error in the calculated distance. However, it is only accurate at very short times after the initial perturbation of S, as described below, and so is usually applied with wide error margins, by imposing distance restraints as ranges rather than exact distances. 

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