Relayed- COSY

The relayed-COSY experiment shall be considered only very briefly because it has essentially been superseded by the far superior TOCSY experiment described below, although one may still encounter references to this experiment in older literature however.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 

---

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

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) ]. 

Figure 7.2 Pulse sequences for 1D COSY and 1D relayed COSY. A soft 90° Gaussian pulse serves as an excitation pulse for these experiments. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Nymphaea caerulea, for seven natural anthocyanins stabilizing a DNA triplex, etc. Sequential analysis of the oligosaccharide structures of the flavonol tamarixetin-7-O-rutino-side has been performed by ID multistep-relayed COSY-ROESY experiments. Selective excitation was performed by Gaussian-shaped soft pulses. 

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). 

Relayed-COSY Provides two (or more) step transfers and can reduce ambiguities arising from crosspeak overlap. Typically has low sensitivity and responses show mixture of lineshapes so magnitude-mode presentations may be required. TOCSY preferred. 

Figure 5.57. The relayed-COSY sequence in which an additional transfer step has been appended to the basic COSY experiment.

Relayed COSY experiments are used to establish a link between separate homonuclear spin systems either via a heteronuclear relay step (left side of Fig. 5.24) or in H, H spin systems a homonuclear relay step (right side of Fig. 5.24). In the latter case correlation peaks are generated for protons which are not directly coupled together but form part of the same spin systems. Correlation s can arise using the small value long-range coupling constants " J(H, H) and J(H, H). 

Even though IH coherences are assigned to both dimensions, heteronuclear relay COSY experiments can be helpful in the indirect detection of heteronuclear functional group. Often from the relay cross peak the functional group acting as a bridge between two spin systems can be deduced. 

Crotonaldehyde has been used in the literature to test the homonuclear and heteronuclear relayed COSY experiment [5.142]. In the following Check its a spin system based on crotonaldehyde will also be used but because the maximum number of coupled spins in a cluster is restricted to nine, see section 1.2, and to speed up the calculation the CH3 group has been replaced by a CHX2. Two spin systems have also been created for this crotonaldehyde type molecule one for a pure isotopomer and a second with the natural ratio of and DC isotopomers the latter being used for pulse sequence with relay transfer through the DC nucleus. 

Fig. 5.25 Molecular structure of the relay COSY experiment test spin system.

The test spin system corresponds to an IH AKMX system. H-1 has a single coupling to H-2 while H-2, H-3 and H-4 couple with each other with an observable scalar coupling. So in the homonuclear relay COSY experiment proton H-1 should exhibit connectivity to either H-3 or H-4 or both. 

The H-X-H relay COSY experiment enables the verification that two apparently isolated IH spin systems belongs to the same molecule. The experiment is based upon the detection of a heteronuclear coupling interaction between at least two protons from each spin system to the same bridging heteronucleus such as a l C atom in a carbonyl functional group. By default the experiment is not very sensitive, the relay cross peak depending on the magnetization transfer to a 1 C atom with low natural abundance. 

Load the configuration file ch54113.cfg an6 run the simulation of the 2D 130-1H relay COSY experiment of the crotonaldehyde type spin system. In this modified spin system the coupling between H-1 and H-2 is excluded to show the relay transfer from H-1 to H-2 trough C-1. Compare the result with the basic 1H COSY spectrum. Repeat both simulations using the spin system relcspsy.ham. Note however that in this spin system the 13C nucleus is 100% abundant. 

As shown in the list of relay COSY experiments heteronuclear correlation experiments are possible. In Check it 5.4.1.14 the H-H-X relay IR, DC COSY experiment for the crotonaldehyde type spin system is calculated. Implementing a relay step to a heteronucleus enables complex IR COSY spectrum to be disentangled by including a heteronuclear polarization transfer to link the IR signals to the heteronuclear chemical shift dimension. Fig. 5.26 illustrates this schematically for two spin systems. Since 8(Ra) = 5(Rd) and 5(Rt>) = 5(Rg) at least two IR, IR relay cross peaks which belong to two different relayed spin systems overlap in the IR, IR spectrum. Rowever because 5(Cc) 4 5(Cf) these peaks may be separated if the correlation peaks can be related to the heteronucleus which has a different chemical shift for each peak. 

Fig. 5.26 IR-IR-Dc relay COSY spectrum to separate IR, IR correlation peaks with similar chemical shift.

---