Truncated NOE

Non-selective Ti and NOE values in bovine Q12C02SOD at 200 MHz [16]. The distances calculated from the NOE values using the two-spin approximation agree with available X-ray distances 

Up to now steady state NOEs have been considered, i.e. when one signal is saturated for a long time with respect to T of the nucleus on which NOE is going to be measured. Let s consider here what happens when the saturation time is short and variable. The resulting NOE is called truncated NOE [17] because not enough time is left for full polarization transfer. These experiments are of fundamental importance for the measurement of pi, for evaluating cross relaxation, and to avoid or to measure spin diffusion. 

In Appendix VII, where the steady state NOE has been derived, the equation for the NOE as a function of the irradiation time is also derived. In the case of homonuclear NOE, it is 

For irradiation times of J short with respect to the relaxation time of / the NOE extent is independent of the relaxation time of the nucleus and provides a direct measurement of ri(j)- If the time required to saturate signal J is not negligible compared with t, the response of the system is not linear [18]. The truncated NOE is independent of paramagnetism as it does not depend on p/, which contains the electron spin vector S in the R[m term, and only depends on gkj), which does not contain S. If then the steady state NOE is reached, the value of p/ can also be obtained. This is the correct way to measure p/ of a nucleus, provided saturation of J can be considered instantaneous. In general, measurements at short t values minimize spin diffusion effects. In fact, in the presence of short saturation times, the transfer of saturation affects mainly the nuclei directly coupled to the one whose signal is saturated. Secondary NOEs have no time to build substantially. As already said, this is more true in paramagnetic systems, the larger the R[m contribution to p/. 

Measuring the build up of NOE is necessary every time the NOE is provided by a signal buried in an envelope of many other signals. Such a procedure allows one to estimate, besides the shift, p/ of such buried signal. From the initial part of the slope the distance between the two nuclei can be obtained. If p/ is dominated by R[m, the distance from the metal can also be guessed. 

---

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... 

The equation for truncated NOE (Section 7.3) can also be derived from Eq. (VII. 13), again generalized by substituting Phj) with the total relaxation rate pi, by setting (Jz(t)) = 0 (instantaneous saturation). Integration then gives... 

In the following sections we will go through the various classical experiments like steady state, truncated and transient NOE, as well as ROE. The presentation has the twofold purpose of sketching (or refreshing) the basic theory to... 

In this type of experiment the NOE buildup tends to disappear with p/. In the steady state case, the saturation time is always long enough to allow spin / to cross-relax with other spins, even if pr is large. In transient experiments, the cross relaxation with spin / is by itself limited in time and, pj being the same, cross relaxation with other spins is drastically limited. In any case, spin diffusion is limited in that region of time in which NOE is growing (Fig. 7.6). Truncated and transient NOEs performed with short NOE buildup times are efficient in quenching spin diffusion. 

Fig. 7.7. Comparison between truncated or steady state (a) and transient (b) NOE R u = 50 s-1. Other conditions as in Figs. 7.5 and 7.6. The dotted lines represent the initial slope (aHJ) or 2o, J)) in the two cases, respectively.

A series of 11-demethyl analogues (Fig. 24) with modification or truncation of the side chain displayed reduced antimitotic potency [134], The same kind of conformational analysis was applied to these derivatives based on molecular modeling restrained by NMR J and NOE data. Interestingly, the conformation of the macrocycle was not perturbed by these chemical modifications on the side chain, highlighting the essential role of the side chain for activity (Fig. 25). 

Figure 8.26. The control ID spectrum (a) and NOE difference spectra (b and c) of 8.9 in MeOD. The difference spectra show only NOE enhancements and the truncated difference signal of the saturated resonance (arrowed). The observed enhancements are consistent with the indicated 2,5-dr geometry across the ring oxygen of 8.9.

Figure 8.40. Selected ID gradient NOESY spectra of the bicyclic lactam 8.13 recorded with a mixing time of 800 ms. Spectrum (a) is the conventional ID spectrum and the strong geminal H4 -H4 NOE in (c) has been truncated.

---