NOESY spin-diffusion

ROESY Rotating frame NOESY, detection of NOE in the HH COSY format with suppressed spin-diffusion, detects closely spaced protons also in smaller molecules... 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

NOESY varied from 12 h to 48 h, depending on the ligand concentration (between 500 pM and 2 mM). Occasional ambiguities due to proton overlap among Thr and Met residues were resolved by recording a 3D [13C, HJ-resolved [ H, HJ-NOESY experiment. QUIET-NOESY (Quenching Undesirable Indirect External Trouble in NOESY) experiments [34] were also performed to avoid artificial NOE cross peaks arising from spin diffusion. (Taken from Ref. [4]). 

The latter, in contrast to nuclear Overhauser enhancement and exchange spectroscopy (NOESY), always feature positive NOEs (negative cross-peaks with respect to diagonal), eliminating known problems of NOEs vanishing or spin diffusion, depending on correlation time, when high field spectrometers are used for measurements of medium-size compounds. 

As illustrated on this example, interpretation of NOESY spectra of polysaccharides might be complicated by the presence of spin-diffusion and higher order elfects. ID NOESY-NOESY experiment can provide some assistance in identification and assignment of spin-diffusion signals and thus prolong networks of spins experiencing NOE contacts beyond immediate neighbours normally detected in NOESY experiments. 

Therefore, for a complete characterization of the cross-relaxation network, it is best to do all three experiments and to extract from each of them information that can be interpreted unambiguously. If the number of possible experiments is limited, then in the extreme-narrowing and the spin-diffusion regime it is best to record NOESY and in the intermediate motional regime ROESY or T-ROESY spectra. 

In conclusion, if temperature can be chosen freely, the best one is around the high-temperature maximum of a". Then, the NOESY spectrum has the highest possible sensitivity but is still free of spin diffusion. Low-temperature spectroscopy can increase sensitivity immensely, but quantitative data analysis requires either the full matrix or the buildup curve analysis. 

In the spin diffusion motional regime (small molecules at low temperatures or macromolecules at all temperatures) the cross-relaxation is so efficient that it can hardly be limited to a single-step magnetization transfer. The multistep magnetization transfer is known as spin diffusion. It manifests differently in NOESY and ROESY spectra, as can be illustrated by writing eq. (29b) explicitly for the process of cross relaxation ... 

Heimer, N. E., Del Sesto, R. E., and Carper, W. R., Evidence for spin diffusion in a H,H-NOESY study of imidazolium tetrafluoroborate ionic liquids, Magn. Reson. Chem., 42, 71-75,2004. 

In contrast to the 1D experiment, where steady-state NOEs may be obtained, only the less intense transient NOE.s are measured in the NOESY experiment. ROEs can only be obtained as transient effects in both the ID and the 2D experiment. Furthermore the intensities of the NOESY and ROESY cross peaks depend upon the molecular size as well as the length of the mixing period. In the case of large molecules, e.g. polypeptides, rather short mixing times are usually chosen to avoid spin diffusion. 

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

In practice, these methods offer more than just reliable interpretation of distances. Because the procedure can be used to model NOESY cross-peaks with contributions from spin diffusion, they suggest that cross-peaks from spectra with long mixing times, and thus better signal-to-noise, can be reliably used. As Nilges et al.67 point out, a cross-peak may arise entirely via indirect magnetization transfer, but may contain useful structural information. 

The NOESY experiment provides information about the proximity of protons and hence is used primarily for distinguishing structures that have clear stereochemical differences. For larger molecules, the ROESY experiment may offer some advantages because of its lower tendency to exhibit spin diffusion. The related EXSY experiment is used only when chemical exchange is being investigated. 

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