TOCSY clean

Gradient-enhanced 2D TOCSY spectrum of 10 mMof sucrose in D..,0 is shown in figure 7.26. The clean spectrum obtainable without any noise and without the nece.ssity of any phase cycling illustrates the power of this new technique in modern NMR spectroscopy. 

These spectra demonstrate that with the multiselective method a clean separation of the subspectra of the three independent spin systems may be achieved. They furthermore prove that - compared to the basic ID TOCSY experiment - spectra of the same quality with respect to the suppression of residual signals originating from the other spin systems and with respect to the signal-to-noise ratios can be measured. 

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.

Homonudear Hartmann-Hahn sequences with delays were developed for clean TOCSY experiments (see Section X.B). Examples are delayed MLEV-17 (Griesinger et al., 1988), delayed DIPSI-2 (Cavanagh and Ranee, 1992), and clean CITY (computer-improved total-correlation spectroscopy Briand and Ernst, 1991). The MGS sequences (Schwendinger et al., 1994) are examples of broadband heteronuclear Hartmann-Hahn mbdng sequences with delays and variable rf amplitudes. 

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. 

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... 

Figure 5 Cross-peaks in homonuclear 2D TOCSY spectra arising due to ROESY effects. Clean TOCSY spectra were acquired with the MLEV-17 spin-lock sequence, (a) Base proton H6-to-methyl correlations in a 27-nt AT-rich DNA stem-loop structure 93 the spectrum was recorded with the 50-ms mixing sequence, (b) and (c) TOCSY spectra acquired for a 31 -nt stem-loop RNA (unpublished data), (b) H5-H6 cross-peaks in pyrimidines and a H1 -H8 cross-peak (boxed) in the syn guanine from the tetraloop UACG the spectrum was recorded with the 30-ms mixing sequence, (c) Sequential H2 -H6/H8 cross-peaks the spectrum was recorded with the 90-ms mixing sequence.

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. 

Kramer and Glaser " analysed the transfer efficiency of cross-relaxation compensated (Clean) TOCSY sequenees is analyzed for applications to residual dipolar couplings. Surprisingly most conventional Clean TOCSY sequences are... 

New Pulse Schemes. A general method for constructing optimized coherence-transfer pulse-sequence elements has been proposed. " As an illustrative application, the method is used to quantitatively measure H- H and H- C residual dipolar eouplings in a 17 kDa protein weakly aligned by means of Pfl phages. The transfer efficiency of cross-relaxation compensated Clean-TOCSY sequences has been analyzed for applications to residual dipolar couplings and it is observed that most conventional sequences are very... 

Some of these differences are therefore utilized for differentiating the TOCSY and ROESY peaks, in particular the strength of the spin-locking field and the mixing time, as well as the sign of the cross-peak. Experiments have also been designed for obtaining clean TOCSY as well as clean ROESY spectra. 

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