MLEV-17 TOCSY mixing sequence

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). 

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spectrum was recorded with the 50-ms mixing sequence, (b) and (c) TOCSY spectra acquired for a 31 -nt stem-loop RNA (unpublished data), (b) H5-H6 cross-peaks in pyrimidines and a H1 -H8 cross-peak (boxed) in the syn guanine from the tetraloop UACG the spectrum was recorded with the 30-ms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.

Figure 5.68. Two practical schemes for implementing TOCSY based on (a) the MLEV-17 mixing scheme and (b) the DIPST2 isotropic mixing scheme. The MLEV sequence is bracketed by short, continuous-wave, spin-lock trim pulses (SL) to provide pure-phase data. In scheme (b) this can be achieved by phase-cycling the 90° z-filter pulses that surround the mixing scheme. This demands the independent inversion of each bracketing 90° pulse with coincident receiver inversion, thus (p =x, —X, X, —x (j) = X, X, —X, —X and (j)r = x, —X, —X, X. The S periods allow for the necessary power switching.

Fig. 7. HMQC-TOCSY pulse sequence described by Lerner and Bax (1986). Proton magnetization is manipulated and labeled with the chemical shift of the directly attached in a fashion identical to the HMQC experiment (see Fig. 1). After a refocusing period, A, proton magnetization is propagated from the directly attached proton to its vicinal neighbors using an MLEV-17-based isotropic mixing period. In the original work, the receiver was enabled and broadband heteronuclear decoupling initiated after a fixed delay, A = 1/2( Jch) to suppress the direct responses. Alternative considerations regarding direct responses in an HMQC-TOCSY spectrum are discussed in the text...

There are essentially two approaches based on composite-pulse methods in widespread use for the practical implementation of the TOCSY experiment (Fig. 5.68). The first of these [51] (Fig. 5.68a) is based on the so-called MLEV-17 spin-lock, in which an even number of cycles through the MLEV-17 sequence are used to produce the desired total mixing period. To ensure the collection of absorption-mode data, only magnetisation along a single axis should be retained, so it is necessary to eliminate magnetisation not parallel to this before or after the transfer sequence. In this implementation, this is achieved by the use of trim-pulses applied for 2-3 ms along the chosen axis. 

---