Relayed Coherence Transfer TOCSY

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 common theme so far in our correlation experiments has been to allow spins to evolve during q under the influence of directly coupled nuclear spins. We have seen the power of COSY, HMQC, HMBC, and INADEQUATE to provide us with detailed structural information for ipsenol, caryophyllene oxide, and lactose. In this section, we will develop another method for showing correlations and apply it to molecules with distinct, isolated proton spin systems such as carbohydrates, peptides, and nucleic acids. 

Our goal is to relay or to transfer magnetization beyond directly coupled spins thus enabling us to see correlations among nuclei that are not directly coupled 

Our goal is to relay or to transfer magnetization beyond directly coupled spins, thus enabling us to see correlations among nuclei that arc not directly coupled but within the same spin system. The experiment is called TOCSY (Totally Correlated SpectroscopY) and we will consider both the 2-D and 1-D versions. The pulse sequence for a 2-D TOCSY resembles our prototype 2-D experiment but, instead of a second 7r/2 pulse, we insert a mixing period during which the 

FIGURE 6.23. The 300 MHz H NMR spectrum of /3-lactose in DzO. The inset shows severe overlap of the ring protons from both the glucose and the galactose rings. 

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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]. 

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. 

The most commonly used 2D nmr technique applied to through-bond interactions is termed (7 ) correlated spectroscopy (COSY), others are total correlation spectroscopy (TOCSY), which allows somewhat longer-range through-bond connectivities to be observed than with COSY, spin echo coherence transfer spectroscopy (SECSY), relayed coherence transfer spectroscopy (RELAY), double quantum spectroscopy (DQNMR) and homonuclear Hartmann-Hahn spectroscopy (HOHAHA). The most commonly used 2D nmr technique applied to through-space interactions is termed nuclear Overhauser effect spectroscopy (NOESY) also used is the closely related rotating-frame NOESY (ROE-SY). 

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) ]. 

The relayed-COSY experiment shall be considered only very briefly because it has essentially been superseded by the far superior TOCSY experiment described below, although one may still encounter references to this experiment in older literature however. The relayed-COSY experiment (Fig. 5.57) attempts to overcome problems caused by coincidental overlap of crosspeaks in COSY that can lead to a breakdown in the stepwise tracing of coupling networks within a molecule. It incorporates an additional coherence transfer step in which that transferred from a spin. A, to its partner, M, as in the standard COSY, is subsequently relayed onto the next coupled spin X in the sequence. This produces a crosspeak in the spectrum between spins A and X even though there exists no direct coupling between them, by virtue of them sharing... 

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