Multiple quantum coherence transfer

Kanamori K, Ross BD and Tropp J (1995) Selective, in vivo observation of [5- N]glutamine amide protons in rat brain by H- N heteronuclear multiple-quantum-coherence transfer NMR. Journal of Magnetic Resonance B 107 107-115. 

Many subspectral editing techniques alternative to DEPT, such as SEMUT (Subspectral Editing using a Multiple Quantum Trap) (Bildsoe et al., 1983) and SEMUT GL, have been developed that utilize the fact that the transfer of magnetization to unobservable multiple-quantum coherence for CH, CHj, and CH spin systems is dependent on the last flip angle 0. However, these experiments have not been widely used. 

J splittings cannot be directly resolved. In addition to the obvious advantage of providing a map of chemical bonds between the spins, /-based transfers do not require spin-locking and are not disturbed by molecular motions. The major drawback of polarization transfer through J coupling is that the delays involved in the pulse sequences, such as insensitive nuclei enhanced by polarization transfer (INEPT) [233] or heteronuclear multiple-quantum coherence (HMQC)... 

To understand the pulse sequence, we will try to get an overview of what is happening and then look at some simplified product operator analysis. Consider first the CH case in the DEPT-90 experiment. Ignoring the 180° pulses, the DEPT-90 sequence can be viewed as an INEPT sequence in which the coherence transfer is split up into two steps (Fig. 7.41) the two 90° pulses are no longer simultaneous and between them we have an intermediate state in coherence transfer multiple-quantum coherence (ZQC and DQC). 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

SateUite Transition MAS (SATRAS or STMAS), developed by Gan in 2000 [25], is an alternative approach to MQMAS for the acquisihon of high-resolution NMR spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS is that it is not dependent upon an efficient transfer of multiple-quantum coherences. like MQMAS, SATRAS is a 2D experiment performed under MAS conditions. The technique involves exciting the sateUite transitions in the spin manifold of quadmpolar nuclei using short radio frequency (rf) pulses. The second-order... 

The sensitivity gain of proton detected Heteronuclear Multiple-Quantum Coherences (HMQ.C) experiments, as compared with direct heteronuclear detected ones and with polarization transfer techniques like INEPT [22-24] using heteronuclear detection, can be calculated [27,28] for the Sn, Sn and Sn isotopes (Table 3). 

Using fast amplitude modulation pulses it is possible to redistribute the population of the spin energy levels. This is the Rotor Assisted Population Transfer (RAPT) method introduced by Yao et al. [57]. It has been shown that enhancement by a factor of 1.5-2 is achieved in a MAS experiment of spin-3/2 nuclei when RAPT is applied before the excitation pulse. It is also possible to combine RAPT with MQMAS in an experiment which uses single-quantum coherences for the excitation of multiple-quantum coherences. Madhu and Levitt [58] have shown that a combination of RAPT and RIACT-FAM gives the best performance for MQMAS experiments of spin-3/2 systems. 

The inverse detection heteronuclear multiple quantum coherence (HMQC) experiment is another approach to two-dimensional NMR techniques, which consists of a transfer of chemical shift and coupling information from relatively insensitive nuclei such as and some metals, to more sensitive nuclei such as H. The advantage of this method is a substantial increase in the sensitivity obtained, due to the greater natural abundance of H (Kingery et al., 2001). 

The transverse magnetisation we observe directly in an NMR experiment is known as single quantum coherence. Multiple quantum coherence, however, cannot be directly observed because it induces no signal in the detection coil. For multiple quantum coherence to be of use to us, it must be transferred back into signal quantum coherence by the action of rf pulses. The concept of coherence is developed further in Chapter 5. 

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