Relayed nOe Experiments

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

The pioneering work in this field, a two-dimensional relayed-NOE experiment proposed by Wagner [7], was quickly followed by the appearance of several related NMR techniques [8-17]. Application of isotropic mixing during the J-transfer period yielded the 2D TOCSY-NOESY [11, 15] and NOESY-TOCSY [12, 14] experiments. When spin-lock sequences were applied to both J and NOE-transfers, the 2D TOCSY-ROESY and ROESY-TOCSY experiments [10, 16, 17] emerged. 

Two-dimensional relayed NOESY experiments (Wagner, 1984 Kessler et al., 1988) give cross-peaks that could be a superposition of nOe s resulting from different relay nuclei. This complicates the extracting of crossrelaxation rates. The 1D NOESY experiment (Kessler et al., 1989a), however, allows the path of the magnetization transfer to be clarified. 

An interesting possibility for assignment of NOEs from two or more overlapping anomeric protons is provided by a 1D RELAY-NOESY experiment [38] (fig. 13(d)). In this experiment the magnetization is transferred from H-1 protons to H-2 protons first and after a chemical-shift-selective... 

The i N H -NOEs can also be presented in a two-dimensional spectrum, using tbe [ N, H]-COSY-relayed i N H -NOE experiment. Positive and negative cross-peaks may then be displayed in separate subspectra (Fig. 6, B and C), which affords a nice illustration of the different dispersion of the chemical shifts in the unfolded and folded parts of the protein (Wuthrich, 1986). The spectrum of Fig. 6B is typical for a globular domain, with the backbone amide proton chemical shifts spread out from 7 to 10 ppm, and the spectrum of the tail in Fig. 6C contains backbone amide proton peaks only between 8.0 and 8.7 ppm, wbicb is typical for an extended random coil polypeptide chain. 

In contrast to the NOE experiment the cross-peaks of the ROE experiment are of opposite sign with respect to the chemical-exchange peaks. This is a particularly valuable property in studies of complex formation where peaks arise both from cross-relaxation and chemical exchange. Furthermore, the assignment of stereo-specific restraints to the methylene protons that are often subject to spin-diffusion can be facilitated by the ROESY measurement. The ROE cross-peaks that stem from the relayed transfer have opposite sign with respect to the direct transfer and thus the methylene ROE signals do not tend to equal values. 

Improvements to the basic TROSY scheme include the optimisation of intraresidual pulse sequence elements that provide improved sensitivity and suppression of sequential cross peaks. Nietlispach has provided a simple modification of the TROSY pulse transfer scheme that suppresses unwanted anti-TROSY lines without loss in sensitivity. Lin el al. has provided a modified SEA-TROSY experiment that eliminates longitudinal relaxation effects and exchange-relayed NOE contributions from fast exchanging hydroxyl or amine protons. ... 

COSY = homonuclear chemical shift correlation spectroscopy NOESY = 2D nuclear Overhauser effect (NOE) spectroscopy HMQC = heteronuclear mulltiple-quantum coherence RELAY = relayed coherence transfer COLOC = correlation via long-range coupling HMBC = heteronuclear long-range coupling INADEQUATE = incredible natural abundance double quantum transfer experiment. 

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