Pulse sequence Watergate

Other even more cunning methods have been devised to suppress the water signal in samples that have a large water content (e.g., bio-fluid samples) such as the WET and the WATERGATE pulse sequences. Other sequences have been devised to cope with signals from carbon-bound hydrogens. Some of these actually collapse the 13C satellites into the main 12C peak prior to suppression. Such a sequence would be useful if you were forced to acquire a spectrum in a nondeuterated solvent. 

Figure 9 WATERGATE pulse sequences. The various evolutions of the WATERGATE sequences are shown. (A) shows the original WATERGATE, while (B) shows the 3919 modification, and...

Equivalently, the second gradient can be of the same sign of the first, provided that a 180° pulse is applied in between the two. In the so-called Watergate sequence [14], selective non-excitation is achieved by tailoring the 180° pulse in such a way that all frequencies but the unwanted frequency are irradiated. The simplest scheme employed a 90° x (sel) 180° (non-sel)90° JC (sel) pulse sequence (Fig. 9.1G). The selectivity increases with the length of the 90° pulses. 

Figure 2 Gradient-echo-based water suppression pulse sequences, (a) WATERGATE (b) water-flip-back (c) excitation sculpting (d and e) examples of the S pulse train that is sandwiched between the gradient echo (d) water-selective inversion...

One of the drawbacks of WATERGATE is that the water molecules have some transverse magnetization at the end of the pulse sequence, albeit dephased. This saturation can be transferred onto labile amide protons through chemical exchange, which, in a similar manner to presaturation, results in reduced sensitivity. 

The processes that occur during the evolution period are probably the most important in describing the effect of the complete pulse sequence. During this period coherence can evolve, coherence can be selectively manipulated or coherence transfer can occur. Coherence manipulation can be the inversion of the coherence order (WATERGATE experiment) or in a l S spin system a phase shift depending upon signal multiplicity (APT or SEMUT experiment). In the case of heteronuclear IS spin systems the creation of antiphase coherence and subsequent polarization transfer using a INEPT or a DEPT unit can be used in multiplicity edited experiments or heteronuclear 2D correlation experiments. In transient NOE experiments such as ROE and TROESY, coherence... 

In the original WATERGATE-1 experiment, using selective inversion element (a), a net rotation of zero for the solvent magnetization is achieved while the 180° pulse inverts all the other coherences. In Check it 52.3.5 the efficiency of the WATERGATE-1 sequence is demonstrated for the IH spectrum of uridine (1%), a nucleoside of uracil and ribose, in water (99%). 

Although the (3-9-19) sequence is the most commonly used binomial pulse sequence other sequences have been developed called W4 and W5 [5.19]. Only the W5 sequence will be considered here. The W5 sequence consists of a five hard pulse train, each pulse length being calculated to give the optimum excitation profile. In the first part of Check it 5.2.3.8 the spectrum with WATERGATE-3 (W5-binominal pulse sequence) is simulated and the results compared with Check it 5.2.3.7a. In the second part of this Check it the excitation profiles of three different WATERGATE sequences are calculated and compared. 

Most small molecule NMR will make use of highly deuterated solvents. However, there are times when protonated solvents have to be used, for example when examining intact biofluids, or in LC/NMR, and in these cases efficient suppression of the protonated solvent signals is imperative if the solutes are to be sensitively detected. This is a key use of PFGs and a series of pulse sequences have been devised, such as WATERGATE [28], WET [29], and excitation sculpting [30] to achieve solvent suppression. More will be said about these methods in relation to LC/NMR in Section 4.3.1.3. 

Solvent suppression is a particular problem in LC/NMR and has been a theme throughout its development. Early methods for suppression of the pro-tonated solvent signals which otherwise dominate the NMR spectrum made use of binomial pulse sequences [124-126]. Methods in use today either use fully deuterated solvents, or make use of solvent suppression schemes such as the NOESY presaturation technique [127], WATERGATE [28,128], WET [29,129], or excitation sculpting [30,130,131]. These methods have for some time made it possible to study relatively low-level (several %) impurities [132,133]. The need... 

In a related paper, WATERGATE and DANTE pulse sequences were used under MAS condition to selectively excite or suppress peaks in H solid state spectra. As known from solution NMR studies, signal selection or suppression... 

Figure 2 Water suppression pulse sequences using PFG to selectively dephase the solvent resonance, where the bar and open symbols represent 90° pulses. (A) Randomization approach to water suppression (RAW) sequence. (B) WATERGATE (the composition of the 3-9-19 pulse train and its variations are listed in Table 1). (C) Double WATERGATE echo method. It Is recommended that different echo times (/i y and different gradient strengths (G G ) are used.

Figure 4 500 MHz NMR spectra of human blood plasma obtained using pulse sequences of (A) the double WATERGATE echo method and of (B) NOESYPRESAT under identical conditions. The low-field region is expanded and plotted as the inset. The labile proton resonances (marked by arrows) are observable in (A) but suppressed in (B) by saturation transfer effects.

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.

Figure 3 Excitation profiles of the Watergate sequence obtained using either a selective 180° pulse sandwiched between two hard 90° pulses (left panel) or a 3-9-19 binomial sequence in place of the S element of the gradient echo (right panel) (see also Figure 2).

---