Coherence selection using pulsed

Coherence selection using pulsed field gradients... 

Coherence Selection Using Pulsed Field Gradients... 

Coherence transfer pathways (CT pathway) fall in the domain of spherical product operators instead of CARTESIAN operators. Before proceeding any further it is recommended to a necomer to read section 2.2.2 and for addition information references [2.20 - 2.31]. To illustrate the use of coherence transfer pathways in coherence selection, three pulse sequences will be examined. 

A second example of coherence selection using phase cycling is the suppression of the quaternary carbon signals in a polarization transfer spectrum. DEPT and INEPT type pulse sequences use a polarization transfer step to enhance the signal of an NMR insensitive nucleus such as which exhibits scalar coupling to a NMR sensitive nucleus such as IR. 

Figure 8 Basic pulse schemes to obtain F2-heterocoupled two-dimensional HSQC spectra (A) CLIP-HSQC, (B) perfect-CLIP-HSQC, and (Q PIP-HSQC experiments. Narrow and broad pulses represent 90° and 180° pulses, respectively, with phase x, unless specified explicitly. The interpulse delay A is set to 1/(2 J(CH)) and a basic two-step phase cycling is executed with i=x,-x and receiver (fi,=x—x. Gradients for coherence selection using the echo-antiecho protocol are represented by G1 and G2 and 8 stands for the duration and the gradient and its recovery delay. A purge gradient G3 is placed for zz-filtering whereas the final and optional 90°O C) stands for the so-called CLIP pulse to remove heteronuclear AP contributions. F2-heterodecoupled versions of all three HSQC schemes should be obtained by applying broadband heterodecoupling during the acquisition period. In such cases, the CLIP pulse In (A) and (B) is not required.

Several modifications have been proposed for the basic HNN-COSY experiment. For example, frequency separations between amino and aromatic 15N resonances are typically in the range 100-130 ppm and therefore much larger than between imino 15N donor and aromatic 15N acceptor resonances. As has been pointed out by Majumdar and coworkers [33], such 15N frequency separations are too large to be covered effectively by the non-selective 15N pulses of the homonuclear HNN-COSY. They therefore designed a pseudo-heteronuclear H(N)N-COSY experiment, where selective 15N pulses excite the amino and aromatic 15N resonances separately to yield excellent sensitivity [33]. An inconvenience of this experiment is that the resonances corresponding to the amino 15N nuclei are not detected, and a separate spin-echo difference experiment was used to quantify the h2/NN values. A slightly improved version of this pseudo-heteronuclear H(N)N-COSY [35] remedies this problem by the use of phase-coherent 15N pulses such that both amino and aromatic 15N resonances can be detected in a single experiment. 

Fig. 2. Pulse sequence for selective reverse INEPT using pulsed field gradients to select the coherence transfer echo. The 180° pulse pair in the middle of the 2r delay is not normally needed for t < 50 ms, and the second proton 180° pulse and first t2 delay maybe omitted if a linear phase gradient in the resultant spectrum can be tolerated. The second field gradient pulse has an area (7c/th) times that of the first.

Figure 3 presents an example of such a situation. The 2Q-HoMQC spectrum of apo-cytochrome c was acquired in 93% H2O at 480 ixM concentration on a Varian Unity/INOVA 600 MHz NMR instrument overnight, using a pulse sequence with gradient coherence selection and weak gradient spin-echo during excitation delays and the evolution period [29], respectively (I.P., not published). The spectral windows were 8 kHz both in F2 and F. ... 

Fig. 4. Modified X/Y IMPEACH-MBC pulse sequence used for 19F/15N shift correlation according to Ref. 27. The notation of 90° and 180° pulses is as before. The (d/2 — 180°(Y) — d/2) element represents a variable delay that is incremented concurrently with the decrementation of the accordion delay vd. Pulse phases are x, unless specified x = — x 2 = x, — x 3 = x, x, — x, — x = , — x, — x, x. The bipolar gradients Gs flanking the 180°(Y) pulse can be set to arbitrary power levels, and the relative strengths of the coherence selection gradients G and G2 are determined by G2/G1 =2 Yy/Tx-...

We saw in Chapter 8 how a selective 180° pulse can be placed between two gradients of the same sign and duration to give a pulsed field gradient spin echo (PFGSE) that not only selects the desired coherence but also destroys any other coherences. First, we use a hard 90° pulse to create coherence on all spins, and then the first gradient twists the coherence into a helix (Fig. 8.21). The selective 180° pulse reverses the direction of twist in... 

A very efficient suppression of parent resonances can be achieved using the T filter. This, however, requires a rather careful tuning of the relaxation delay T (see Figure 8). If the jump and return inversion pulse is employed, the pulse sequence can be regarded as a selective version of the BIRD experiment [57-59]. Obviously, multiple-frequency selective inversion pulses may be necessary in the case of complex proton spectra. Usually the /-BIRD HMQC experiment gives cleaner spectra as compared with equivalent heteronuclear singlequantum coherence (HSQC) experiments, presumably because of fewer 180° pulses which are frequently a source of various artefacts. 

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