Echo/antiecho selection

Fig. 9. Pulse sequences for 3D- and 2D- H, X, Y correlations. The same notation as in Fig. 1 is used, (a) HNCA-analogue 3D experiment. (b) HNCO-analogue 3D experiment. (c) gs-SELTRIP with a selective 180° gauss pulse the gradient strength (for the case H. C. P) is +/-2S, —/+25,22.7 for echo/antiecho selection. ...

Finally, we note an alternative approach to quadrature detection known as echo-antiecho selection [11], which is applicable only to pulsed field gradient selected 2D methods and which now finds widespread use. As this involves a quite different procedure, it will not be considered further here but will be introduced in Section 5.5.2 after field gradients have been described. 

Figure 6.27. The phase-sensitive HMBC experiment with echo-antiecho selection. The gradient ratios G1.G2 are 5 -3 and 3 -5 for odd and even experiments, respectively, when selecting carbon-13.

The gradient strength is G and — G for echo and antiecho selection, respectively. 

ST2-PT thus results in a 2D [15N, H]-correlation spectrum that contains only the most slowly relaxing component of the 2D 15N- H multiplet. The data are processed as described by Kay et al. [44] in an echo/antiecho manner. Water saturation is minimized by keeping the water magnetization along the z-axis during the entire experiment, which is achieved by the application of the water-selective 90° rf pulses indicated by curved shapes on the line H. It was reported that on some NMR instruments the phase cycle mentioned above does select the desired multiplet component. On these instruments, the replacements of S, with S, = y, x for the first FID and 9, =... 

We also have to think about the phase sensitive detection in both indirect dimensions. For example, if the phase is encoded in States mode in t and in echo-antiecho mode (using gradients) in t2, we have to acquire four FIDs for each combination of t and t2 delay values. States mode real or imaginary is selected by using an x or y phase for the pulse just before the t delay (lH SQC), and echo or antiecho mode is selected by using a negative gradient... 

Figure 5.69. Gradient-selected TOCSY. Sequence (a) is suitable for absolute-value presentations with a 1 1 gradient combination selecting the N-type spectrum. Sequence (b) provides phase-sensitive data sets via the echo-antiecho method for which separate P- and N-type data are collected through inversion of the first gradient.

Figure 6.10. A gradient-selected, phase-sensitive HSQC sequence using the echo-antiecho approach. The N- and P-type pathways are selected by the last gradient.

A similar logic to that above applies to gradient selection in the HSQC experiment, for which a variety of different approaches are also possible [7]. A suitable sequence employing the echo-antiecho approach is illustrated in Fig. 6.10, and requires only two gradients in proportion to the magnetogyric ratios of the X and H spins since each acts on single-quantum X and H magnetisation. Thus, for a correlation experiment, ratios of 4 1 and... 

The quadrature detection mode is given as a suffix to the main sequence name me magnitude calculated, TPPI time proportional phase increment, E/A Echo / Antiecho The term "selective" is reserved exclusively for sequences using selective pulses. If selectivity is achieved using other methods this is defined using a different term. 

Particular improvements to the HSQC experiment has been the implementation of phase sensitive echo/antiecho-detection in combination with gradient coherence selection. Further variants include the sensitivity-enhanced HSQC experiment [5.194] and experiment developed for long-range coupling detection and J-scaled experiments (for references see sections 5.2.5 and 5.7.3). 

Load the configuration file ch722b.cfg and run a gradient selected HSQC experiment with TPPI detection. Compare the result with the similar HSQC experiment under echo/antiecho (E/A) detection, i.e. with the same number of scans and same experimental parameters. Extract the rows containing the correlation peaks. 

Figure 6.32. The constant-time HMBC experiment based on the echo- antiecho/i selection scheme of Fig. 6.27 (AcT = ti(max))- The combined period (Aci-ti) is decremented as ti increases, thus defining a constant-time period for proton coupling evolution that is invariant with changes in h.

VcH(max)- Gradient selection follows the echo-antiecho protocol and the gradient ratios for the low-pass filter are G2 G3 G4 = 0.075Gi 0.175Gi 0.75Gi. 

Figure 9.37. The constant-time-HSQC-IDOSY sequence. The delays T are set to 1/2Jhx as required for the INEPT transfer and the constanttime period 2T remains fixed, its duration being dictated by the desired diffusion time A. The effective ti evolution time is varied by moving the two 180° refocusing pulses within the constant time period (pulses shown with arrows over) and coherence selection is made with the echo/antiecho (E/A) scheme. The diffusion encoding/decoding gradients are applied as bipolar pairs during the INEPT and reverse-INEPT transfer steps.

Figure 1 Schematic representation of the different steps involved in a standard 2D HSQC pulse sequence. Thin and thick vertical rectangles represent 90° and 180° hard pulses, respectively. The delay should be set to 1/(2 /(CH)), and S represents the duration of the PFG and its recovery delay. In this scheme, coherence selection is performed by the gradient pair G1/G2 using the echo-antiecho protocol.

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.

A second solution is offered by the whole shifted-echo method, whereby a selective n pulse is added after a time delay <5 at the end of the refocusing block to create an additional spin echo on the CT (Fig. 12b) [139, 164, 167]. In this case, the echo and antiecho signals are delayed by... 

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