Homonuclear DRAMA

We note that DRAMA has also formed the basis for refined variants such as the chemical-shift-compensated MELODRAMA [85], windowless DRAWS [29], and the DRAMA-XY4 [86] experiments, finding important applications for example for measurement of 13C-13C distances involving carbonyl spins. We should also note the existence of many other important pulsed homonuclear recoupling experiments such as the widely used Radio Frequency Driven Dipolar Recoupling (RFDR) [24] and Back-to-Back (BaBa) [87] experiments. 

Fig. 11.15 Diagram showing the relative proximity of the two ligands Glp and S3P, which form a stable ternary complex with the enzyme EPSP synthase. The distance constraints were obtained from both homonuclear and heteronuclear dipolar couplings obtained using the REDOR and DRAMA pulse sequence, together with a model showing a...

A number of recoupling techniques are utilized for specific purposes, but they all rely on synchronizing measurements, and, in most instances, pulse sequences, with the rotation period. For homonuclear recoupling (e.g., 13C-13C or 31P-31P), rotational resonance (R2) is based on magnetization exchange when the spinning rate wr is adjusted so that a multiple of it equals the chemical shift difference between the two dipolar coupled nuclei, and DRAMA (dipolar recovery at the... 

For the excitation of DQC under MAS, the interference with the sample rotation must be considered, as was the case for the CRAMPS experiment discussed in the previous section. In particular, if a sequence designed for a static MQ experiment is used without modification, the excitation (and reconversion) time is limited to ttr/2, since the rotor modulation causes the action of the pulse sequence in the second half of the rotor period to be the time reversal of that which occurred in the first half of the rotor period. Starting with the suggestion of Meier and Earl,89,90 who simply proposed the phase switching of the static sequences used by Baum et al.,47,48 every half rotor period to prevent the process of self-time-reversal, many different approaches have been presented that allow excitation (and reconversion) times of one or more rotor periods. Such pulse sequences that counteract the effect of MAS are referred to as recoupling methods,1112 examples that have been used in homonuclear DQ MAS NMR spectroscopy include BABA,91 C7,92 DRAMA,93 DRAWS,94 and HORROR.95 We note that Levitt and co-workers have recently introduced a very helpful classification system, based on symmetry principles, which covers such sequences.96,97... 

The first experiment to reintroduce homonuclear dipolar coupling under MAS conditions by applying a multiple-pulse sequence was DRAMA (dipolar recovery at the magic angle). The basic sequence of DRAMA consists of two 7t/2 pulses in each rotation period to spoil the averaging of the dipolar coupling interaction by MAS. In the complete DRAMA sequence (Fig. 25)... 

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