NOE in the rotating frame ROE

The ROE dependence on the spin lock time has the same profile as that of transient NOE, with the difference that the limiting values are 0.385 and 0.675 at the condition that o itc 1 (see Fig. 7.10). It appears that the ROE is less convenient than the transient and steady state NOEs in the sense that the expected effect is smaller when all other conditions are the same. Another disadvantage in paramagnetic molecules is that it is difficult to spin lock all the signals in a 

---

Molecules in the transition area of molecular weight (2000-4000 Da depending on molecular shape, rigidity, and solvent viscosity) show little or no NOE. For these molecules an alternative experiment called ROES Y (rotating-frame Overhauser effect spectroscopy, Chapters 8 and 10) is effective. 

The best way to avoid the l-cox problem (and weak nOes) is to use a rotating frame experiment, for instance ROESY (Rotating frame Overhauser Effect Spectroscopy) also named CAMELSPIN by its inventors (77). The (oXg dependence of rOes is complex and one may simply remember that rOes are always positive, never null. The ROESY sequence is similar to the sequence of HOHAHA the main difference is the power of the spinlock which is generated by a long soft pulse rather than by a WALTZ sequence. In ROESY experiments, rOe cross peaks may be accompanied by Hart-mann-Hahn correlations which are easily distinguished by their opposite sign (in phased experiments) (78). 

Method of measuring rotating-frame nOes (or rOes). Of importance for mid-sized molecules, for which nOes can be zero whereas the rOes will still be measurable. Gives approximately the same information (though more care is required in interpretation). 

Which one of these two approaches is adopted in the laboratory may be dictated by the motional properties of the molecule(s) under study and more specifically the rates at which the molecules tumble in solution. Pre-empting what is to follow, it will be shown that the steady-state experiments are appropriate only for molecules that tumble rapidly in solution (we shall also see what defines rapidly in this context). Such measurements have traditionally been the home territory of small organic molecules in relatively non-viscous solutions. In contrast, very much larger molecules that tumble slowly in solution (or smaller molecules in very viscous solutions) can be meaningfully studied only with the transient NOE techniques, which may also be suitable for small-molecule studies. Between these two extremes of molecular tumbling rates, the conventional NOE can become weak and vanishingly small, a condition most likely to occur for those molecules with masses of around 1000-2000 daltons. It is here that rotating-frame NOE (ROE) measurements play a vital role, and these shall also be described. 

---