Pulse sequences REDOR experiment

Figure 2. Pulse sequence for 13C-serve REDOR NMR. This sequence differs from the original REDOR pulse sequence (ref. 18) in that n pulses alternate between 13C and 15N r.f channels. On alternate scans of the REDOR experiment, the 15N iz pulses are either applied or omitted. This figure illustrates that the REDOR pulse sequence with four rotor periods of 13C-15N dipolar-coupling evolution (Nc = 4) NC can be increased (in increments of two) by adding rotor periods and pairs of 13C and 15N n pulses between the end of the cross-polarization preparation and the start of data acquisition.

Fig. 5 Radio frequency pulse sequences for measurements of Sj and Si in DSQ-REDOR experiments. The MAS period rR is 100 ps. XY represents a train of 15N n pulses with XY-16 phase patterns [98]. TPPM represents two-pulse phase modulation [99]. In these experiments, M = Nt 4, N2+ N3 = 48, and N2 is incremented from 0 to 48 to produce effective dephasing times from 0 to 9.6 ms. Signals arising from intraresidue 15N-13C DSQ coherence (Si) are selected by standard phase cycling. Signal decay due to the pulse imperfection of 15N pulses is estimated by S2. Decay due to the intermolecular 15N-I3C dipole-dipole couplings is calculated as Si(N2)/S2(N2). The phase cycling scheme can be found in the original figure and caption. (Figure and caption adapted from [45])...

FIGURE 20. The first two experiments in the REDOR pulse sequence, where the dephasing delay is increased by adding more rotor periods and Li jr-pulses symmetrically around the jr-pulse. Reproduced with permission of Blackwell Pubhshing from Reference 221... 

Figure 1 The archetypical REDOR pulse sequence. (A) rotor-synchronised S-spin-echo experiment defining the reference echo amplitude Sq the REDOR pulse sequence in (B) with the additional rotor-synchronised /-channel 7i-pulses provides the signal intensity S.

Figure 3 Pulse sequences for the different CT-REDOR versions for two rotor cycles (A) MAS spin-echo reference experiment (B) CT-VPP-REDOR with the pulse position fpp of the /-spin dephasing pulses stepped from 0 to Tp and (C) CT-VPD-REDOR with the pulse width fpp of the /-spin dephasing pulses stepped from 0 to In.

REDOR and RR methods have been used extensively to determine the three-dimensional structure of biomolecules, because the pulse sequence and the data analysis to yield the interatomic distance are simpler compared with the other methods. It seems worthwhile to describe the formalism of the REDOR experiment by a density operator to take into account the effect of a finite pulse length and by the three-spin system encountered on many occasions. 

Fig. 2.3. Pulse sequence and timing chart of REDOR experiment.

For most spectrometers, it is very difficult to be free from fluctuations of rf power during the acquisition of REDOR experiments. Therefore, it is very important for the rf power to be stabilized after waiting a certain time period. If not, the rr-pulse cannot stay as the exact rr-pulse for a long time. Consequently, the REDOR factor is greatly decreased to yield relatively longer interatomic distances if the rf power changes. Compensation of instability of such rf power by the pulse sequence is necessary, therefore, to be free from long-term fluctuation of amplifiers, xy-4 and xy-8 pulse sequences have been developed for this purpose and an xy-8 pulse is known to be the best sequence to compensate for the fluctuation of the rf power [20]. 

Rotational-echo double resonance (REDOR) experiments follow another strategy to achieve recoupling of weak heteronuclear dipolar coupling interactions under MAS conditions, quite different from the approach. REDOR experiments prevent the refocusing of weak heteronuclear dipolar couphng interactions under MAS by applying rotation-synchronized trains of 7T pulses. Figure 9 depicts some (of many possible) versions of REDOR sequences. The theory of REDOR has been treated extensively in the literature. 

The FDR (frequency-selective dipolar recoupling) experiment utilizes a pulse sequence (Fig. 11a) similar to the REDOR sequence. The major difference is that 7t/2 pulses are applied at the Larmor frequency of the nonobserved spin S2 instead of tt pulses as applied in REDOR. In the FDR experiment, dipolar dephasing of the Si spin depends on the chemical shielding of the S2 spin, as will be shown. Using AHT, Bennett et have calculated the evolution of the magnetization of the Si spin under the influence of the pulse sequence depicted in Fig. 11a as... 

Figure 24 Pulse sequences and coherence transfer pathways for combined AI H REDOR-trMQ/MAS NMR (left) and AI H REDOR-fj-MQ/MAS NMR (right) experiments [85,92,93],...

Only the longitudinal spin term is present in Eq. (14) and aU heteronuclear dipole-dipole coupling interactions commute in a multispin system making the analysis much more straightforward. One of the best known pulse sequences in solid-state NMR spectroscopy, the rotational-echo double-resonance NMR (REDOR) experiment [44], employs appropriately placed pulses to avoid averaging of the heteronuclear dipolar coupling interactions by MAS. REDOR has been used in numerous cases to extract precise dipolar couplings and the inventor. Prof Schaefer, was commemorated recently for his contributions to solid-state NMR spectroscopy [45]. 

The procedure that is useful for intermolecular distance determination in the solid materials is termed REDOR (rotational echo double resonance) experiment [25,26]. It allows recovering dipolar interaction under MAS. REDOR is a spin-echo double-resonance experiment. A typical REDOR pulse sequence is shown in Fig. 5B. The rotor-synchronized echo sequence is applied to the spin I system and the echoes are detected after the time 2t equal the even number n of rotation periods t, . For recoupling the dipolar interaction between the spins I and S, r-pulses are applied to the spin S system at every half rotation period Tj. The dipolar coupling is determined by measuring the REDOR fraction AS/S, which describes the decrease of the echo amplitude 5 as a function of the number of rotation periods n [25] ... 

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