MLEV-17 pulse sequence

Since the seminal paper of Braunschweiler and Ernst (1983), many experimental mixing schemes have been proposed for broadband homonuclear Hartmann-Hahn transfer. The most important mixing sequences are summarized in alphabetical order in Table 2. The listed names of the sequences are either acronyms that were proposed in the literature or acronyms composed from the initials of the authors who introduced them. For each sequence, the expansion scheme that is applied to the basic (composite) pulse R is indicated. For symmetric composite pulses R that can be decomposed into a composite pulse 5 and its time-reversed variant 5, only S is specified in Table 2 for simplicity and classification. For example, the composite 180° pulse R = 90 180 90 (Levitt and Freeman, 1979), which forms the basis of the MLEV-16 sequence, consists... 

The addition of a 17th pulse j3y with rf amplitude to a MLEV-16 sequence of duration IdTj go results in an average spin-lock field... 

Because many isotropic-mixing sequences have been extended by adding an additional pulse (see Table 2) to make them insensitive to experimental imperfections, the reduction of the bandwidth that is induced by such an incompensated pulse is a general problem. Equation (246) can be generalized for arbitrary multiple-pulse sequences by replacing the duration of the MLEV-16 sequence (tMlev-i6 with the duration of the... 

Figure 24A -D shows the offset dependence of the corresponding MLEV-16 and MLEV-17 sequences. The reduction of the active bandwidth, which is induced by the additional pulse, can be limited by reducing the flip angle B of this pulse (Sklenaf and Bax, 1987 Bax, 1988a see Fig. 24B and C). For example, for a MLEV-17 sequence with pf = Pj = 10 kHz and B = 180° (Bax and Davis, 1985b), the effective fields for two spins i and j with offsets p, = 0 kHz and Vj = 3 kHz are mismatched by about 13 Hz [psl(f,) = 303 Hz and Psl(f,) 316 Hz], which significantly reduces the efficiency of Hartmann-Hahn transfer for coupling...

If pulse shaping devices and linear amplifiers are available, then rapid, phase-coherent changes of the rf amplitude can be conveniently implemented. In this case, the Hartmann-Hahn mismatch that is created by the additional pulse can be further reduced by increasing the rf amplitude of the additional pulse (see Fig. 24D and D ). The offset dependence of the coherence-transfer efficiency of a boosted MLEV-17 sequence (MLEV-17b) with I f = 10 kHz and v = 20 kHz is shown in Fig. 24D. 

Generalized MLEV-16 sequences that consist of symmetric composite pulses R = l yO x have been investigated by Glaser and Drobny (1990). A systematic variation of the flip angles a and j8 provided a map of this sequence space. This map showed that the composite pulse R = 90° 180° 90° is by no means unique. In fact, in the offset range of +0.4i f, Hartmann-Hahn transfer is much more efficient for R = 90° 240° 90° (Levitt et al., 1983 Fujiwara and Nagayama, 1989). However, for 0° < a 360° and 0° < 13 < 360°, the best transfer properties were found for the GD-1 sequence (Table 2) with R = 260° 80° 260°. 

In addition to multiple-pulse sequences that were derived from heteronuclear decoupling experiments, a number of rf sequences have been specifically developed for homonuclear Hartmann-Hahn transfer. A systematic search for phase-alternated composite 180° pulses R expanded in an MLEV-16 supercycle was reported by Glaser and Drobny (1990). Several clusters of good sequences were found for the transfer of magnetization in the offset range of 0.Av. However, substantially improved Hartmann-Hahn sequences were found after the condition that restricted R to be an exact composite 180° pulse on-resonance was lifted. For example, the GD-2 sequence is based on R = 290° 390° 290°, which is a composite 190° pulse on-resonance and is one of the best sequences based on composite pulses of the form R = (Glaser and Drobny, 1990). 

Fig. 25. Trajectory of an initial magnetization vector during the composite pulse R = 90° 180° 90, which forms the basis of the MLEV-16 sequence. The spin is assumed to be irradiated on resonance. (Adapted from Schleucher et al., 1996, courtesy of John Wiley Sons Ltd.)...

Fig. 27. Ninety-degree pulses from which the MLEV-17 sequence is obtained by expansion. The rf amplitude Bi(t) is given in units of the average rf amplitude ). (A) Delayed clean MLEV for 3. The delay (A/2) has the same duration as the 90° pulse.

The MLEV-16 sequence, which contains rf pulses with orthogonal phases, has poor heteronuclear transfer characteristics. The effective coupling tensors are neither planar nor isotropic. For two spins on-resonance, the average coupling tensor has the form... 

Only recently, new multiple-pulse sequences that were developed specifically for broadband heteronuclear Hartmann-Hahn experiments in liquids were reported. The SHR-1 sequence developed by Sunitha Bai et al. (1994) consists of a windowless phase-alternated composite pulse R, which is expanded according to the MLEV-8 supercycle. R was optimized based on a phase-distortionless single-spin 180° composite pulse and is related to the composite pulses used in DIPSI-1 (Shaka et al., 1988) and the composite pulses in the homonuclear IICT-1 sequence (Sunitha Bai and Ramachandran, 1993). The bandwidth of the SHR-1 sequence is comparable to the bandwidth of DIPSI-3, albeit with a slightly reduced transfer efficiency (Sunitha Bai et al., 1994 Fig. 33F). 

Fig. 4. Special pulse schemes for 2D- X, "Y H) correlations. The same notation as before is used A denotes a fixed delay of length ( /( H,Y(X)) l (a) HMQC sequence for indirect detection of spin-1 nuclei. (b) INEPT-HMQC. (c) INEPT-HETCOR. (d) HMQC-TOCSY si denotes an MLEV spinlock sequence of duration t which is framed by trim pulses. ...

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. 

Whilst the z-filter helps to eliminate experimental phase errors further spectral distortions can be suppressed by using two trim pulses and an additional pulse in the MLEV-17 pulse sequence. There are two main sources for these distortions ... 

In the MLEV-17 sequence a 180° pulse (or a 60° pulse) is appended to the MLEV-16 sequence. The additional 180° pulse inverts the magnetization that is not perfectly aligned with a particular axis of the rotating frame so that after an even number of MLEV17 cycles the magnetization is perfectly aligned. Any residual magnetization which is not perfectly parallel to the selected axis to which the spins are locked are defocused by the two trim pulses. 

Figure 5.48 The pulse sequence employed in 2D homonuclear Hartmann-Hahn spectroscopy. An extended MLEV-16 sequence is used. Trim pulses are applied before and after this pulse sequence in order to refocus the magnetization not parallel to the x-axis.

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