TOCSY sequence

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. 

Fig. 8. Schematic representation of heteroatom-containing structural elements in polymers that are disposed for characterisation by 1H/X/Y triple resonance experiments where X = 13C and Y = 19F, 31P, 29Si, 119Sn, with possible coherence transfer pathways being indicated by single and double headed arrows.36 39 Selective observation of the correlations of the building blocks in (a)-(c) requires experiments involving out-and-back coherence transfer via Vc.h/ -A.x (a), Vc.h/ cx (b), or / . (c), whereas the simultaneous observation of all correlation signals originating from a chain of an isotope labelled sample (d) is feasible by means of a HCa(Y)-CC-TOCSY sequence.39...

EXSY cross peaks are also obtained in TOCSY experiments (see later) because scalar interactions in the rotating frame are not separable from exchange interactions [7]. An EXSY experiment, performed using a TOCSY sequence (see Section 8.6) is reported in Fig. 8.7 relative to the complex 5Cl-Ni-SAL-MeDPT [5]. This complex, as shown in Fig. 8.8, displays a chemical equilibrium in which the two salicylaldiminate moieties exchange their non-equivalent positions [8]. It is interesting to learn that such complex interconversion occurs with times of the order of the spin-lock time (20 ms) or shorter. 

Fig. 9.1. (A) Gaussian (a) and sine (b) excitation profiles. (B) Composite (G3) Gaussian pulse. (C) Train of soft pulses modified after the DANTE sequence to achieve selective off-resonance excitation. (D) Redfield 21412 sequence. (E) Binomial 11, 121, 1331, 14641 sequences. (F) JR (a) and compensated JR (or 1111) (b) sequences. (G) Watergate sequence. (H) Weft (Superweft) sequence. (I) Modeft sequence. (J) MLEV16 sequence. (K) NOESY sequence with trim pulse. (L) MLEV17 sequence with trim pulses. (M) Clean-TOCSY sequence.

For example, in H spin systems, the MLEV-17 sequence (Bax and Davis, 1985b) is commonly used for TOCSY experiments. However, the same rf sequence may act as a TACSY sequence when applied to C spin systems (Eaton et al., 1990). If a spin system consists of both H and C spins, the MLEV-17 sequence acts as an E.TACSY sequence if it is applied at the H frequency. In this case, magnetization is transferred only within the subsystem consisting of H spins, whereas the polarization of the C spins is not affected. Therefore, if a rf irradiation scheme like MLEV-17 is called a TOCSY sequence, it is tacitly assumed that it is applied to spin systems with resonance frequencies that fall within the active bandwidth of the sequence for a given rf amplitude. 

The main difference between approaches that use delays during the basis sequence (Methods C and D) or after completed basis sequences (Methods A and B) is the efficiency of Hartmann-Hahn transfer. In Methods A and B, no Hartmann-Hahn transfer occurs during the compensating delays. If Methods A or B are used, the total mixing time (including the compensating delays) must be increased by 50% compared to a partially compensated multiple-pulse TOCSY sequence with roe/4> in order to obtain the same Hartmann-Hahn transfer. 

Existing homonuclear Hartmann-Hahn mixing sequences that have been converted to clean TOCSY sequences by the introduction of delays using Method D include MLEV-17 (see Fig. 26A Griesinger et al., 1988), DIPSI-2 (see Fig. 26B Cavanagh and Ranee, 1992), and WALTZ-16 (Kerssebaum, 1990). Method C was applied to WALTZ-16, DIPSI-2, and FLOPSY-8 (Briand and Ernst, 1991). 

Even larger usable bandwidths can be obtained for a given average rf power if clean homonuclear Hartmann-Hahn sequences are optimized from scratch (Briand and Ernst, 1991 Quant, 1992 Kadkhodaei et al., 1993 Mayr et al., 1993), rather than modifying existing uncompensated TOCSY sequences. The clean CITY sequence (see Fig. 26C, Table 3), which was developed by Briand and Ernst (1991), is still one of the most efficient broadband Hartmann-Hahn sequences with cross-relaxation compensation. The sequence is constructed using Method C and is based on the computer-optimized symmetric composite pulse R = SS with S = 48° 138° (see Fig. 22F, sequence 5g). The TOWNY (TOCSY without... 

Both limitations can be avoided if tailor-made multiple-pulse sequences are used for band-selective Hartmann-Hahn transfer. The so-called tailored TOCSY sequences TT-1 and TT-2 (see Table 4) were the first crafted band-selective Hartmann-Hahn sequences to be reported in the literature (Glaser and Drobny, 1989). Both phase-alternated sequences do not use any supercycling scheme. The TT-1 sequence with vf = 10 kHz was developed for band-selective coherence transfer between the offset ranges R- (-2.5 kHz < < —1.5 kHz) and Rj (1.5 kHz < Vj < 2.5 kHz). 

Fig. 30. TOCSY (A,B) and HNHA-TACSY (C,D) spectra of the peptide Gln-Lys-Leu-Glu-Ala-Met-His-Arg-Gln-Lys-Tyr-Pro are shown for mbdng times of 45 (A, C) and 85 (B, D) ms. The expansions show the region (0.75 ppm 6, < 4.85 ppm, 7.43 ppm < 62 8.38 ppm) that contains the (H, H ) fingerprint signals as well as the cross-peaks between and side chain protons. The experimental TOCSY sequence was DIPSI-2 with = 5 kHz and the HNHA-TACSY sequence was CABBY-1 with — 2.661 kHz. At a spectrometer frequency of 400 MHz with the carrier at 6.15 ppm, the range of offsets and Vj l is region is —2.156 kHz < —0.516 kHz and 0.520 kllz < i>2 < 0.9 kHz. The...

Heteronuclear multiple-quantum correlation Experiment for tailored correlation spectroscopy of H and H resonances in peptides and proteins Homonuclear Hartmann-Hahn spectroscopy Heteronuclear quadruple-quantum coherence Heteronuelear triple-quantum coherence Heteronuclear single-quantum coherence TOCSY sequences developed at the Indian Institute of Chemical Technology Insensitive nucleus enhancement by polarization transfer... 

TTie TOCSY sequence (ii) [85], supplemented with trim pulses and z-filter [24],... 

Fig. 23. Excitation-sculpting TOCSY sequence. The shaped pulses have SEDUCE profiles.

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. 

Figure 5.59. The TOCSY sequence. The spin-lock mixing time, im, replaces the single mixing pulse of the basic COSY experiment.

Figure 5.69. Gradient-selected TOCSY. Sequence (a) is suitable for absolute-value presentations with a 1 1 gradient combination selecting the N-type spectrum. Sequence (b) provides phase-sensitive data sets via the echo-antiecho method for which separate P- and N-type data are collected through inversion of the first gradient.

Figure 6.20. The schematic HMQC/HSQC-TOCSY sequence and the coupling pathway it maps. Direct correlations are produced for the proton bound to the spin- /2 heteroatom (Ha) as in the basic shift correlation sequence, and further relayed correlations are produced for those protons receiving magnetisation through the TOCSY transfer (Hr).

An interesting alternative to three dimensional NMR techniques is to suppress one of the evolution times while retaining the basic 3D pulse sequence. The spectral resolution is no longer increased, which is usually not a problem with smaller molecules, but the extra information is still available. The gs-HMQC-TOCSY experiment represents one such experiment. The combination of the HMQC method with the TOCSY sequence leads, in principle, to a 3D technique. However, if the tj evolution period of the TOCSY part is omitted, a 2D sequence is obtained which provides a 13C edited TOCSY spectrum. Starting from each HMQC cross-signal, one finds in the same row additional signals which are caused by a TOCSY transfer. When the structure elucidation is difficult, this experiment can fruitfully complement the HMBC experiment. 

For samples of sufficient strength to require only a single scan per increment, gradient versions of TOCSY may be attractive alternatives for the rapid recording of spectra. By analogy with the previous COSY discussions, the absolute-value TOCSY sequence simply requires equal gradients to be placed on either side of the spin-lock sequence (Fig. 5.69a) to... 

Kramer and Glaser analysed the transfer efficiency of cross-relaxation compensated (Clean) TOCSY sequences for applications to residual dipolar couplings. Surprisingly most conventional Clean TOCSY sequences are very inefficient for dipolar transfer. It is shown theoretically, that this is a general property of all phase-alternating mixing sequences, i.e., for such sequences the suppression of cross-relaxation excludes dipolar transfer in the spin-diffusion limit. A new family of clean dipolar TOCSY sequences is derived which provides excellent transfer efficiencies for a broad range of offset frequencies. 

A desired loss in resolution was the driving force in a recent study on the macrohde antibiotic clarithromycin 7 at 400 MHz [89]. A H—H TOCSY sequence with a Zangger—Sterk module [90] for suppressing homonuclear J-evolution in the indirect dimension formed the basis for a doubly pure-shift TOCSY. The Fourier transform in the direct dimension was followed... 

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