Total correlation spectroscopy TOCSY

An essentially identical experiment has also been referred to as Homonuclear Hartmann-Hahn spectroscopy [50,51] or HOHAHA (the two differ only in some technical details in the originally published sequences). This name arises from its similarity with methods used in solid-state NMR spectroscopy for the transfer of polarisation from proton to carbon nuclei (so-called crosspolarisation), which are based on the Hartmann-Hahn match described below. For the same reason, the transfer of magnetisation during the TOCSY sequence is sometimes referred to as homonuclear cross-polarisation. Throughout this text the original TOCSY terminology is used, although TOCSY and HOHAHA are now used synonymously in the chemical literature. 

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HOMONUCLEAR HARTMANN-HAHN SPECTROSCOPY (HOHAHA), OR TOTAL CORRELATION SPECTROSCOPY (TOCSY)... 

The total correlation spectroscopy (TOCSY) techniques, which come in both 1- and 2-D versions, offer an alternative to 1-D spin decoupling and COSY methods for establishing through-bond connectivities. The important difference between the two is that TOCSY methods allow easy identification of isolated spin systems. For example, using our trusty morpholine compound once more, you can see that it is possible to identify the -CH2-CH2- spin system between the nitrogen and the oxygen atoms, these hetero-atoms, effectively isolating the protons from all others in the molecule. 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

We have characterized a resin-bound pentasaccharide by HR-MAS techniques. A comparison of the solution spectrum of the resin-cleaved pentasaccharide with the HR-MAS spectrum of the resin-bound pentasaccharide is shown in Figure 8.5. It is immediately obvious that the HR-MAS technique provides data of a quality similar to that of the solution technique, but in both cases, only four of the five anomeric protons are visible. However, a 2D homonuclear total correlation spectroscopy (TOCSY) spectrum (Fig. 8.6) of the resin-bound pentasaccharide allowed us to clearly observe the overlapped anomeric protons (demonstrating a resolution of 4.4 Hz). 

The vast literature associated with flavanoid chemistry precludes a discussion here but two valuable reviews have been published. The first reviews a number of spectroscopic techniques used for flavonoid analysis, with a strong emphasis on NMR spectroscopy (plus also mass spectrometry, vibrational spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray crystallography, and circular dichrosim (CD)) . The second review deals with NMR methods that have been successful in the characterization of phenolic acids and flavonoids from plant extracts that have not been separated or isolated as single components. The emphasis of the article is 2-D NMR methodology and a variety of experiments such as total correlated spectroscopy (TOCSY), COSY, nuclear Overhauser enhancement spectroscopy (NOESY) and heteronuclear multiple quantum correlation (HMQC) are discussed . 

High resolution MAS techniques of 13C, DEPT, correlated spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear chemical shift correlation (HETCOR) were used to examine conventional CBS and efficient TMTD vulcanisation of polybutadiene [37]. In conventional CBS vulcanisation, the major vulcanisate 13C NMR peak occurred at 44.9 ppm and was assigned to a trans allylic structure (-C=C-C-Sx with X=3 or 4). The efficient TMTD vulcanisation yielded as main product a 13C NMR peak at 54.0 ppm and was assigned to a cis allylic vulcanisate (-C=C-C-Sx x=l). While cyclic sulfur by-products were observed in both vulcanisation systems, the CBS formulations gave rise to a higher percentage postulated to be formed via a episulfide intermediate. 

By way of example, useful 2-D techniques are homonuclear correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear multiple quantum coherence (HMQC) spectroscopy, and heteronuclear multiple quantum coherence/total... 

All the spectroscopic approaches applied for structural characterization of mixtures derive from methods originally developed for screening libraries for their biological activities. They include diffusion-ordered spectroscopy [15-18], relaxation-edited spectroscopy [19], isotope-filtered affinity NMR [20] and SAR-by-NMR [21]. These applications will be discussed in the last part of this chapter. As usually most of the components show very similar molecular weight, their spectroscopic parameters, such as relaxation rates or selfdiffusion coefficients, are not very different and application of these methodologies for chemical characterization is not straightforward. An exception is diffusion-edited spectroscopy, which can be a feasible way to analyze the structure of compounds within a mixture without the need of prior separation. This was the case for the analysis of a mixture of five esters (propyl acetate, butyl acetate, ethyl butyrate, isopropyl butyrate and butyl levulinate) [18]. By the combined use of diffusion-edited NMR and 2-D NMR methods such as Total Correlation Spectroscopy (TOCSY), it was possible to elucidate the structure of the components of this mixture. This strategy was called diffusion encoded spectroscopy DECODES. Another example of combination between diffusion-edited spectroscopy and traditional 2-D NMR experiment is the DOSY-NOESY experiment [22]. The use of these experiments have proven to be useful in the identification of compounds from small split and mix synthetic pools. 

In this chapter multiple-pulse sequences for homonudear Hartmann-Hahn transfer are discussed. After a summary of broadband Hartmann-Hahn mixing sequences for total correlation spectroscopy (TOCSY), variants of these experiments that are compensated for crossrelaxation (clean TOCSY) are reviewed. Then, selective and semiselective homonudear Hartmann-Hahn sequences for tailored correlation spectroscopy (TACSY) are discussed. In contrast to TOCSY experiments, where Hartmann-Hahn transfer is allowed between all spins that are part of a coupling network, coherence transfer in TACSY experiments is restricted to selected subsets of spins. Finally, exclusive TACSY (E.TACSY) mixing sequences that not only restrict coherence transfer to a subset of spins, but also leave the polarization state of a second subset of spins untouched, are reviewed. 

Heteronuclear Hartmann-Hahn sequences also effect homonuclear Hartmann-Hahn transfer, resulting in (heteronuclear and homonuclear) total correlation spectroscopy (TOCSY Bearden and Brown, 1989 Zuiderweg, 1990 Brown and Sanctuary, 1991 Ernst et al., 1991). Simultaneous heteronuclear and homonuclear magnetization transfer can be beneficial in relayed transfer experiments (Gibbs and Morris, 1992 Tokles et al., 1992 Majumdar et al., 1993). However, as pointed out by Ernst et al. 

ROESY-TOCSY experiment Selective inverse detection of C-H correlation Scalar heteronuclear recoupled interaction by multiple pulse Simulation program one Selectively inverted soft PICSY Singular value decomposition Tailored correlation spectroscopy Triple-resonance J cross-polarization Total correlation spectroscopy TOCSY-ROESY experiment TOCSY without NOESY... 

P i and P 2 overlap at w,. So, we may not merge P, and P 2 unless we can prove this merge by additional evidence. Total Correlation SpectroscopY (TOCSY) is one of the NMR experiments that can provide the additional evidence. 

Stop-flow requires the calibration of the delay time, which is the time required for the sample to travel from the UV detector to the NMR flow cell, which depends, in turn, on the flow rate and the length of the tubing connecting HPLC with NMR. Because the chromatographic run is automatically stopped when the chromatographic peak of interest is in the flow cell, the amount of sample required for the analysis can be reduced and 2-D NMR experiments, such as correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), and others, can be obtained because the sample can remain inside the flow cell for days. It is possible to obtain NMR... 

Traditionally, homonuclear 2D double quantum filtered correlation spectroscopy (DQF-COSY) and total correlated spectroscopy (TOCSY) spectra are valuable in the identification of resonances of individual monosaccharide units. In the presence of small couplings, through space connectivities detected by NOESY/ROESY (nuclear Overhauser effect spectroscopy/ rotational nuclear Overhauser effect spectroscopy) experiments are also useful in completing the resonance assignment. When the H NMR spectra of complex oligosaccharides are too crowded to fully elucidate the structure by homonuclear correlation methods, it is efficient to use 2D heteronuclear correlation methods, such as heteronuclear single quantum correlation... 

Two-dimensional NMR methods that yield a two-dimensional frequency spectrum have not yet been attempted successfully to study metal interactions with HS, although instances of successful applications of these approaches can be found in the study of metalloproteins (Kingery et al., 2001). Mononuclear ( H) two-dimensional NMR experiments, such as total correlation spectroscopy (TOCSY), can show H- H coupling throughout the complete spin system, and exchange protons can provide information on sites to which metal attach. For example, N-containing units in HS that bind the metal can be identified since the amido protons from these structures will exchange and disappear from the spectrum. 

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