核磁共振光譜與影像導論

1. 核磁共振光譜與影像導論 Introduction to NMR Spectroscopy and ImagingLecture06Nuclear Overhauser Effect (NOE) and NOESY(Spring Term, 2011)Department of ChemistryNational Sun Yat-sen University

2. Nuclear Overhauser Effect (NOE) and NOESY

3. Population Transfer In population manipulations, the most commonly used technique is selective population transfer (SPT):

4. N N S S Nuclear Overhauser Effect (NOE) r The distance between the two spins therefore can be determined by disturbing one of them and observing how other is affected.

5. electron 23 Na

6. =422+422=844 99 98 547 =1+5=6 520 100 550 W2 W0 547 995 577 999 1000 550 Nucleus-Electron OE: Mechanism W2>>W0

7. 99 =-410-410=-820 547 517 =1+5=6 100 107 550 W2 W0 547 995 990 1000 580 550 Nucleus-Electron OE: Mechanism W2<<W0

9. =32+32=64 18 24 54 =2+10=12 56 20 60 W2 W0 54 90 58 90 100 60 Nucleus-Nucleus OE (Nuclear OENOE): Mechanism W2>>W0

10. =-26-26=-52 18 50 54 =2+10=12 24 20 60 W2 W0 54 90 90 64 100 60 Nucleus-Nucleus NOE: Mechanism W2<<W0

11. Example: C-H NOE

12. Whenever a polarization or a transition of a spin is inverted or saturated, the polarization or transition of the other spins that are coupled to it will be affected. Perturbation on a Spin (Saturation/Inversion) + Cross Relaxation  The Polarization of Another (Coupled) Spin Is Altered. Depending on the relative magnitudes of W2 and W0, NOE factor can be Larger or smaller than 1 and can be both negative and positive.

13. w2 w0

14. =15+15=30 18 19 54 =2+10=12 34 20 60 W2 W0 54 90 80 95 100 60 Longitudinal Relaxation RatesAlso Affect NOE R1S R1I R1S W2,W0, R1I, R1S all affect overall NOE. Here W2 > W0, R1I, R1S Positive NOE

15. =6+6=12 18 18 54 =2+10=12 24 20 60 W2 W0 54 90 90 96 100 60 Longitudinal Relaxation RatesAlso Affect NOE R1S R1I R1S W2,W0, R1I, R1S all affect overall NOE. Here W2 > W0, R1I, but R1S>W2. No NOE

16. =3-3=0! 18 19 54 =2+10=12 22 20 60 W2 W0 R1S 54 90 95 92 R1I 100 60 R1S Longitudinal Relaxation RatesAlso Affect NOE W2,W0, R1I, R1S all affect overall NOE. Here W0> W2, but R1I>W0, R1S Negative NOE

17. W0 logW W2 W1 4 2 -4 -3 -2 -1 0 1 3 Relaxation Rates and Motion Fast motion Slow motion For slow motions, W0 is dominant and NOE tends to negative.

19. =12+12=24 18 24 54 =2+10=12 36 20 60 W2 W0 54 90 78 90 100 60 Homonuclear Steady State NOE NOE factor depends on W2, W0

23. NOE Difference Spectrum(NOEdif)

28. Red: Saturated peak Black: NOE affected peak

31. 85 90 81 R1 R1 90 W2 100 90 88 W0 81 90 R1 98 90 100 1D Homonuclear Transient NOE 180o A single spin is inverted and the spin system response is read using a 90° pulse after a “mixing” time delay of variable duration. In the transient mode, the NOE builds up due to cross-relaxation of nearby spins by the inverted spin as the entire spin system. Neither have to be steady Nor have to be equilibrium

32. N N S S NOE: Essence Whenever the polarization of one of two coupled spins deviates from its equilibrium value, the polarization of the other spin is affected by cross relaxations. The NOE factor (the extent that the polarization of the unperturbed spin is affected) depends on cross relaxation rates and longitudinal relaxation rates. r

34. A B When the distance between spins A and B is smaller than ~ 5 Å, NOE cross peaks are observable.

38. 3 6 7 8 10 9 4 5 2 1 3 1 2 6 10 5 8 7 4 9

39. 3 6 7 8 10 9 4 5 2 1 3 1 2 6 10 5 8 7 4 9

40. 180o 90o 1D Homonucelar ROE A single transition is inverted using a selective 180° pulse (along the x axis), and then a hard 90° x pulse is immediately applied to the spin system. This has the effect of placing the “inverted” magnetization along the -y axis while the rest of the magnetization is aligned along +y. Then, a low-power rectangular pulse is applied long the y-axis. This pulse is applied parallel to the magnetization (in the rotating frame) and effects no net rotation. Instead, it “locks” the magnetization along the y axis, and is referred to as a spin lock pulse. The magnetization is said to be spin locked because it doesn’t precess about B0, but the spins now precess aboutB1(the spin lock pulse). Therefore, under these conditions, the magnetization can be considered to being analogous to alignment along the z axis in the presence of B0 alone. Finally, the spins will relax towards a new equilibrium in the presence of B1; the characteristic time constant for this decay is called T1ρ forT1in the rotation frame.

41. Y 81 R1rho R1rho W2rho 100 W0,rho 90 R1rho 90 76 81 92 90 95 90 98 100 ROE Mechanism:All relaxation rates are changed into rotating frame. Z Rotating frame Note that both W2 and W0 promote ROE!

44. For homonuclear systems W2 promotes NOE while W0 blocks NOE Both W2 and W0 promote ROE