Jump-return

The jump-return or 1, 1 method is a very simple and elegant solution because rather than destroying the water signal it simply does not excite water in the first place. We saw in Chapter 8, Figure 8.19 that a null in excitation occurs at the center of the spectral window, and this can be adjusted to put the water peak exactly on-resonance. A jump-return NOESY spectrum of a small protein will be shown later in this chapter. Jump-return and some more complicated variations ( 1,1 - echo and binomial ) are not applicable to all experiments, however, and require some careful tuning and adjustment to work well. They also distort the peak intensities throughout the spectrum and greatly reduce the intensities near the water resonance. 

Figure 1 Excitation profiles of the jump-return (left), 11 (middle), and 1331 (right) pulse sequences used for suppressing the water resonance.

Figure 9.25. Simulated excitation profiles for (a) the jump-return sequence, (b) the 1-1 and (c) 1-3-3-1 binomial sequences. The jump-return sequence used X = 250 and the binomial sequences used x = 500 p,s.

Filter elements have been developed, not just for coupling evolution, but also for chemical shift selection [5.215 - 5.221]. An early example was the jump-return method for solvent signal suppression. Check it 5.2.3.3, whereby the resonances at a given chemical shift and related multiples were suppressed. In common with the z-filter for zero-quantum suppression, the CSSF (Chemical 5hift iSelective Filter) element uses different free precession periods to give varying degrees of chemical shift evolution for each scan in a multiple scan experiment. 

If we consider six directions for the atoms to move in instead of two as in Figure 10.13 the factor becomes or more generally a factor g, which also contains the correlation factor or the structure-dependent chance that the atom after jumping returns to its original position. For the fee lattice, g = 0.78 and for bcc, g = 0.73. Using Fick s first law, we find that D becomes... 

A wide range of other methods for solvent suppression has been developed which may collectively be classed as tailored excitation. These rely on the application of appropriate combinations of pulses to excite protons lying outside a narrow band of frequencies while leaving those within that band (i.e. the solvent) undisturbed. Examples of these are the Redfield pulse [38], the jump-return technique [39] and binomial sequences [40]. 

NMR experiments were carried out at 15 C on a Bruker AMX-500 spectrometer equipped with a 5 mm inverse detection probe and an X-32 computer. All ID spectra were recorded with a 5500 Hz spectral width and 8k data points. The water resonance was suppressed either by a pre-saturation irradiation or by using a tailored jump-return excitation pulse l. Phase-sensitive detection in the tl dimension of 2D experiments was achieved using the time-proportional phase incremental scheme. ... 

For natural abundance H, correlation experiments, the 2D HMQC pulse sequence as described by Bax et al. 3 s used. A proton spectral width of 5(X)0 Hz and a carbon-13 spectral width of 25000 Hz were used, with 2K X 512 data points in the t2 and tl dimensions respectively. H, P correlation experiments were obtained using a hetero-TOCSY sequence, with an isotropic mixing time of 67 ms. The spectral width was set to 4000 Hz for protons and 607 Hz for phosphorus, with typically 2K X 256 data points in the two dimensions respectively. The proton 2D DQF-COSY and TOCSY experiments were recorded with standard pulse sequences the data size was 2K X 512, with spectral width of 5500 Hz in both dimensions. The NOESY sequence with a jump-return excitation pulse was used for optimal imino proton detection . Mixing times from 100 to 300 ms were used. The spectral widths were 11000 Hz in both dimensions. 

A well-established nonexcitation method is the Jump-Return (JR) sequence. It is also known as the... 

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