Relayed coherence transfer

Two-dimensional1 H/Y-correlations with relayed coherence transfer... 

The COSY sequence for proton-proton shift correlation (HA — Hx relationships, Fig. 2.56) can be combined with a two-dimensional carbon-proton shift correlation (CaHa and CXHX bonds, Fig. 2.53). This experiment, referred to as two-dimensional relayed coherence transfer or RELAY [IQ], permits direct identification of CAHA —HXCX... 

The appearance of a 2-D TOCSY experiment resembles in all aspects a COSY. The FI and F2 axes are for proton the diagonal contains 1-D information and even the cross peaks have the same appearance. The difference here is that the cross peaks in a COSY are due to coupled spins while the cross peaks in the TOCSY spectrum arise from relayed coherence transfer. For long mixing times in a TOCSY spectrum, all spins within a spin system appear to be coupled. To appreciate the advantages of TOCSY, we continue with the disaccharide lactose, which has three distinct (i.e., separate) spin systems. 

The pulse methods rely on selective irradiation of a particular resonance line with a radio frequency (rf) and observation of the resulting effects in the rest of the spectrum. Among commonly employed methods are 2D correlated spectroscopy (COSY), 2D spin-echo correlated spectroscopy (SECSY), 2D nuclear Overhauser and exchange spectroscopy (NOESY), 2D J-resolved spectroscopy (2D-J), and relayed coherence-transfer spectroscopy (RELAYED-COSY) (Wutrich, 1986). 

Figure 6-17 Diagram of COSY and relayed coherence transfer (RCT) experiments for two three-spin systems (AMX and A M X ) whose M and M portions overlap. (Adapted from F. J. M. van de Yen, Multidimensional NMR in Liquids, VCH, New York,...

Figure 6-28 Left The and spectra of the dimethyl acetal of acrolein. Right The normal HETCOR (called HSC, bottom) and the H-H-C relay coherence transfer (top) experiments for the same molecule. (Reproduced from A. E. Derome, Modern NMR Techniques for Chemistry Research, Pergamon Press, Oxford, UK, 1987, p. 257.)...

TOCSY spectra have been reported often [2,27,30-40], and have been valuable in finding minor structures in lignins [34,41]. The older relayed coherence transfer experiments (where magnetization is transferred sequentially from one proton to the next coupled proton) provided similar data, but is seldom used today these experiments were useful for assignments in the eight isomers of trimeric lignin 3-ether models [11]. 

Lemer, L., and Bax, A. (1986) Sensitivity-enhanced two-dimensional heteronuclear relayed coherence transfer NMR spectroscopy. J. Magn. Reson. 69(2), 375-380. 

The C NMR spectra of disaccharide 3,2 -0-isopropylidene acetals (31) were assigned through DEPT and heteronuclear correlated experiments (HETCOR and H-H-C RELAYED coherence transfer) and those of the eight-membered ring carbons are shown in Table 7 <89JCS(P2)1867>. 

The individual proton spin systems for each sugar residue were delineated by homonuclear and heteronuclear correlations as detected from COSY and multistep homonuclear relayed coherence transfer (RCT) experiments [9], In this way triple relayed correlations were detected from anomeric protons (Hi) to H5 via the intervening H2 ( direct coupling), H3 (single relayed coherence transfer) and H4 (double relayed coherence transfer). Thus, beginning from the anomeric proton (5 6.13, Glc Hi 5.68, Rha. Hi 4.89, Glc . Hi 4.70, Ara . Hi 4.68, Ara. Hi ), all 2-position protons of sugar moieties were easily located in the... 

Figure 2 (a) Single RCT spectrum of 1. Labelled peaks indicate the relayed coherence transfers between anomeric protons of sugar residues with their respective 3-position protons, (b) Double RCT Sj tnim of 1. Labelled peaks indicate the relayed coherence transfers between ancmieric protons with their respective 4-position protons, and the coherence transfer between Rha.Hi with Rha.H3. 

The inverse-detected 2D NMR experiments that have been discussed to this point have all been discrete, single-purpose experiments, e.g. correlating protons with their directly bound heteronucHde (typically or N). There are another class of inverse-detected 2D NMR experiments that are generally referred to as hyphenated 2D experiments. These are experiments that first establish one type of correlation, followed by an additional experiment segment that then pursues a further spectroscopic task. Predecessors of the inverse-detected variants of these experiments were the HC-RELAY (proton—carbon heteronuclear relayed coherence transfer) experiments pioneered by Bolton [151—155]. Examples of these include, but are by no means hmited to HXQC-COSY and -TOCSY [156—158], -NOESY [159], -ROESY [160], and more recent gradient variants [161] etc., where X = S (single) or M (multiple) quantum variants of the experiments. 

C/13C in natural abundance, while longer-range connectivities, such as can be probed by the RCT (relayed coherence transfer)... 

Bax A (1983a) Two-dimensional heteronuclear relayed coherence transfer spectroscopy. J Magn Reson 53 149-153... 

Bax A, Davis DG (1985) MLEV-17 Based two-dimensional homonuclear magnetization transfer spectroscopy. J Magn Reson 65 355-360 Bax A, Drobny G (1985) Optimization of two-dimensional homonuclear relayed coherence transfer NMR spectroscopy. J Magn Reson 61 306-320 Bax A, Marion D (1988) Improved resolution and sensitivity in H-detected heteronuclear multiple-bond correlation spectroscopy. J Magn Reson 78 186-191 Bax A, Subramanian S (1986) Sensitivity-enhanced two-dimensional heteronuclear chemical shift correlation NMR spectroscopy. J Magn Reson 67 565-569 Bax A, Summers MF (1986) and Assignments from sensitivity-enhanced detection of heteronuclear multiple bond connectivity by 2D multiple-quantum NMR. J Am Chem Soc 108 2093-2094... 

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