Coherence transfer pathways

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

Fig. 5 The quadrupolar echo pulse sequence and coherence transfer pathways for I = 1. The pathway marked in red is eliminated by the EXORCYCLE but not by the conventional quadrupolar echo...

If the two symmetric coherence transfer pathways are designed to have the same efficiency and thus yield signals with the same amplitude (e.g., by using the popular z-filter method (Fig. 12a) [168]), the resulting signal can be written as... 

Fig. 18 (a, b) Pulse sequences and coherence transfer pathways for SQ-CP-3QMAS with z-filter and MQ-CP-3QMAS with z-filter... 

Fig. 19 Pulse sequences and coherence transfer pathways used in MQMAS- >-HETCOR experiments. In MQMAS-/-HETCOR, CP is replaced by /-based transfer...

Fig. 21 HMQC pulse sequences for (a) 14N-13C and (b) 14N- H correlations under rotor-synchronized MAS. In (b), dipolar recoupling is usually applied during time intervals Texc and Trec. (c) Coherence transfer pathways for the observation of SQ (solid lines) and DQ (dashed lines) in the 14N dimension...

Thus, the magnetization is transferred from the amide proton to the attached nitrogen and then simultaneously to the intra- and interresidual 13C spins and sequential 13C spin. The 13C chemical shift is labelled during /, and 13C frequency during t2. The desired coherence is transferred back to the amide proton in the identical but reverse coherence transfer pathway. The 15N chemical shift is frequency labelled during t3, and implemented into the 13C 15N back-INEPT step. The sensitivity of the HNCOmCA-TROSY experiment is excellent and nearly similar to HNCA-TROSY except for the inherent sensitivity loss by a factor of /2, arising from additional quadrature detection needed for 13C frequency discrimination in the fourth dimension. The excellent sensitivity is due to a very efficient coherence transfer pathway,... 

Coherence transfer pathways in purine (a) and pyrimidine (b) nucleosides are shown in bold. 

Selected entries from Methods in Enzymology [vol, page(s)] Acquisition of frequency-discriminated spectrum, 239, 162-166, 170 sensitivity, 239, 169-173 constant-time, 239, 23-26 doublequantum filtered, 239, 236 gradient pulse experiments, 239, 185-189 protein structural information, 239, 377-379 pulse sequence and coherence transfer pathway, 239, 148-149 paramagnetic metalloprotein, 239, 494-497 data recording, SWAT method, 239, 166-169, 172 line shapes, effects of gradient pulses, 239, 162-166 identification of protein amino acid resonances, 232, 100 cyclosporin A, 239, 240-241. 

Several such gradients G, all of equal duration, applied at different times in a particular pulse sequence would each contribute to the total dephasing of all coherence transfer pathways, except those for which O ... 

Figure 1 shows the pulse sequence of the C HSQC experiment supplemented by a spin-lock pulse to suppress the signals from C-bound protons. The experiment is readily described in terms of Cartesian product operators [9]. For a two spin system consisting of a proton spin H coupled to a C spin C, the relevant coherence transfer pathway is... 

The first spin-lock pulse in equation 4 apparently does not alter the magnetization. Its function is to ensure the desired coherence transfer pathway by defocusing any magnetization which is not aligned along the y axis. 

Excellent results are routinely obtained when applying PFGs to the transverse C coherence in a C HMQC experiment which is conducted at natural isotopic abundance [27]. Under those circumstances, however, half of the coherence transfer pathways are rejected by the PFGs. More complicated acquisition schemes and additional pulses in the sequence are required to restore the full sensitivity [26, 28, 29]. Spin-lock purge pulses may not purge as well as PFGs, but they are easier to use and don t interfere with a phase-sensitive recording. 

Comparison of the results of the one-dimensional gradient supported 31P/15N 1H -se-HSQC experiment with phase-cycled HSQC and HMQC experiments gave relative S/N-ratios of 0.75 0.92 1 which was, under consideration of the suppression of one of the two possible coherence transfer pathways by the field gradients and the longer duration of the pulse sequence, interpreted in terms of a very good performance.25 The main benefits of the PFG-es-HSQC sequence were seen, however, in the excellent level of artefact suppression which allowed one to observe correlations via very small couplings even in cases where the active isotopomer is present in low natural abundance and its lines are normally obscured by residual parent signals (Fig. 3). 

Fig. 8. Schematic representation of heteroatom-containing structural elements in polymers that are disposed for characterisation by 1H/X/Y triple resonance experiments where X = 13C and Y = 19F, 31P, 29Si, 119Sn, with possible coherence transfer pathways being indicated by single and double headed arrows.36 39 Selective observation of the correlations of the building blocks in (a)-(c) requires experiments involving out-and-back coherence transfer via Vc.h/ -A.x (a), Vc.h/ cx (b), or / . (c), whereas the simultaneous observation of all correlation signals originating from a chain of an isotope labelled sample (d) is feasible by means of a HCa(Y)-CC-TOCSY sequence.39...

Zhu, J. M., Smith, 1. C. P., Selection of Coherence Transfer Pathways hy pulsed-Field Gradients in NMR Spectroscopy, Concept.s in Magnetic Resonance An Educational Journal, 1995, 7 (No 4)... 

Because the sum is not equal to zero, we end up with twisted coherence and no signal in the receiver. We call this a gradient-selected experiment because the gradients are being used to specifically refocus coherence in the desired coherence transfer pathway (XH SQC -> 13C SQC) and to reject all others. In Chapter 10, we will develop the idea of coherence order in a more precise manner, and we will see that coherence order can be either positive or negative. 

Figure 10 (A) POST C7 pulse sequence and double-quantum coherence transfer pathway. (B)...

Figure 12 Pulse sequence and corresponding coherence transfer pathway diagram for (A) the z-filtered spin-echo (B) the refocused INADEQUATE and (C) the refocused INADEQUATE spin-echo (REINE) experiments.Thin and thick rectangles, respectively, represent n/1 and n pulses. Taken from Ref. [65].

For example, consider NOESY, which was discussed in Chapter 10 and appeared to be quite straightforward. In our current terminology, the following coherence transfer pathway leads to a cross peak ... 

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