97 The steady state NOE resulting from this balance now does not relly depend on the distance between the two spins, only so far that the observation of any effect proves the distance to be shorter than ca 5-6 A. The intensity of the observed NOE can vary widely depending on the neighbourhood of other protons which act as a relaxation source. a few examples will show this(from: Sanders/Hunter, Modern NMr Spectroscopy, Oxford University press, Oxford 1987) two-spin system I-12 saturated 50% for small molecules(W2 dominating) saturated saturated -100% for large molecules(Wo -100% aturated There is no distance dependence for these steady-state NOes! linear three-spin system 1-12-1(r23=r12)* saturated 25% -11.5% 49.2% saturated 49.2% -11.5% only shown for positive NOEs, with W2>> Wo Generally, the NOEs are weaker, because they now have to compete with relaxation from two neighbouring protons. This can be seen esp for the first case, where 12 gets an NOE contribution from the saturated spin I, but now has two equally close neighbours causing T, relaxation(I' and 13) Since the NOE on 12 leads to an increased population difference for this spin, it causes itself(being97 The steady state NOE resulting from this balance now does not relly depend on the distance between the two spins, only so far that the observation of any effect proves the distance to be shorter than ca. 5-6 Å. The intensity of the observed NOE can vary widely, depending on the neighbourhood of other protons which act as a relaxation source. A few examples will show this (from: Sanders / Hunter, Modern NMR Spectroscopy, Oxford University press, Oxford 1987): two-spin system I1 – I2 I 1 I 2 saturated 50 % for small molecules (W2 dominating) 50 % saturated saturated -100 % for large molecules (W0 dominating) -100 % saturated There is no distance dependence for these steady-state NOEs! linear three-spin system I1 – I2 – I3 (r23 = r12) * I 1 I 2 I 3 saturated 25 % -11.5 % 49.2 % saturated 49.2 % -11.5 % 25 % saturated *only shown for positive NOEs, with W2 >> W0 ! Generally, the NOEs are weaker, because they now have to compete with relaxation from two neighbouring protons. This can be seen esp. for the first case, where I2 gets an NOE contribution from the saturated spin I1 , but now has two equally close neighbours causing T1 relaxation (I1 and I3 ). Since the NOE on I2 leads to an increased population difference for this spin, it causes itself (being