DIPSI sequences

Fig. 18. The pulse sequence of a ID ge-NOESY-TOCSY experiment, tnoe is the NOE mixing time, 5 are optional delays which can be used for z-filtration [81] or for suppression of ROE effects in macromolecules (2 x (5 + Tgrad) = 0.5 x mixing time). DIPSI-2 [78] sequence was used for isotropic mixing. Phases were cycled as follows 0i = 2x, 2(—x) (j)2 = X, —x Ip = X, 2 —x), X. Rectangular PFGs, G = 6 Gauss/cm and Gi = 7 Gauss/cm, were applied along the axis for Xpad = 1 ms.

Fig. 1. Basic pulse sequence and CP diagram for gradient-based spin-locked ID exf>eriments. A 1 (— 1) 2 gradient ratio selects N-type data (solid lines) while 1 (— 1) (—2) selects P-type data (dashed lines). When SL stands for a -filtered DIPSI-2 pulse train, a ge-lD TOeSY is performed. On the other hand, when SL stands for a T-ROESY pulse train, a GROESY experiment is performed. S stands for the gradient length.

Other strategies to achieve this task are provided by the broadband (BB) saturation sequences (like MLEV16 (Fig. 9.1J) [21], WALTZ16 [22], GARP [23], DIPSI [24], etc.). The power of the BB saturation pulse can be adjusted in such a way as to effectively saturate the slowly relaxing signals and only maiginally the... 

As stated in the introduction of this section, we use Hartmann-Hahn experiment as the generic term for transfer experiments that are based on the Hartmann-Hahn principle, that is, on matched effective fields. Because two vanishing effective fields are also matched, Hartmann-Hahn sequences need not have finite effective fields. Examples of Hartmann-Hahn sequences without effective spin-lock fields are MLEV-16 (Levitt et al, 1982), WALTZ-16 (Shaka et al., 1983b) and DIPSI-2 (Shaka et al., 1988). Note that the term Hartmann-Hahn sequence has also sometimes been used in the literature in a more restricted sense for experiments with matched but nonvanishing effective spin-lock fields (see, for example, Chandrakumar and Subramanian, 1985, and Griesinger and Ernst, 1988). 

Phase-modulated multiple-pulse sequences with constant rf amplitude form a large class of homonuclear and heteronuclear Hartmann-Hahn sequences. WALTZ-16 (Shaka et al., 1983b) and DIPSI-2 (Shaka et al., 1988) are examples of windowless, phase-alternating Hartmann-Hahn sequences (see Table II). 

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. 

A number of theoretical transfer functions have been reported for specific experiments. However, analytical expressions were derived only for the simplest Hartmann-Hahn experiments. For heteronuclear Hartmann-Hahn transfer based on two CW spin-lock fields on resonance, Maudsley et al. (1977) derived magnetization-transfer functions for two coupled spins 1/2 for matched and mismatched rf fields [see Eq. (30)]. In IS, I2S, and I S systems, all coherence transfer functions were derived for on-resonance irradiation including mismatched rf fields. More general magnetization-transfer functions for off-resonance irradiation and Hartmann-Hahn mismatch were derived for Ij S systems with N < 6 (Muller and Ernst, 1979 Chingas et al., 1981 Levitt et al., 1986). Analytical expressions of heteronuclear Hartmann-Hahn transfer functions under the average Hamiltonian, created by the WALTZ-16, DIPSI-2, and MLEV-16 sequences (see Section XI), have been presented by Ernst et al. (1991) for on-resonant irradiation with matched rf fields. Numerical simulations of heteronuclear polarization-transfer functions for the WALTZ-16 and WALTZ-17 sequence have also been reported for various frequency offsets (Ernst et al., 1991). 

With vf = 10 kHz, these sequences create only a small effective field that is on the order of a few hertz in the offset range between 11.5 kHz (MLEV-16) and between 9 kHz (DIPSI-2), respectively. Spins in this range of offsets have approximately matched effective fields Iv Kv,) and efficient Hartmann-Hahn transfer is possible,... 

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). 

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...

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). 

A markedly increased bandwidth of heteronuclear Hartmann-Hahn transfer for a given average rf power can be achieved with the MGS-1 and MGS-2 sequences developed by Schwendinger et al. (1994) (see Fig. 33G and H). The sequences are MLEV-4 and MLEV-8 expansions of new composite pulses R, which consist of square pulses with rf phases of 0 or 180° and different rf amplitudes that are separated by delays (see Fig. 34). Figure 35 shows HCCH-COSY spectra of a fully C-labeled protein using DIPSI-2 and MGS-2 for the initial polarization transfer from H to (prepolarization) as well as for back-transfer from to H (Majumdar et al., 1993). Note that the absolute bandwidth of MGS-2 is markedly increased compared to DIPSI-2, even though the average power... 

Broadband Hartmann-Hahn sequences, such as DIPSI-2 or WALTZ-16, can be made band-selective by reducing the rf amplitude of the sequences (Brown and Sanctuary, 1991). Richardson et al. (1993) used a low-amplitude WALTZ-17 sequence for band-selective heteronuclear Hartmann-Hahn transfer between N and in order to minimize simultaneous homonuclear Hartmann-Hahn transfer between and The DIPSI-2 sequence was successfully used by Gardner and Coleman (1994) for band-selective Hartmann-Hahn transfer between C d and H spins. So far, no crafted multiple-pulse sequences have been reported that were optimized specifically for band-selective heteronuclear Hartmann-Hahn transfer. Based on the results of Section X, it is expected that such sequences with well defined regions for coherence transfer and effective homonuclear decoupling will result in increased sensitivity of band-selective heteronuclear Hartmann-Hahn experiments. 

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