INEPT coherence transfer

SELECTIVE POPULATION TRANSFER (SPT) AS A WAY OF UNDERSTANDING INEPT COHERENCE TRANSFER... 

A different approach to destroying the 12C-bound artifact is to use the gradient in a simpler way—as a spoiler that just kills all of the magnetization in the x -y plane while our desired signal is stored briefly on the z axis. To do this we go back to our discussion of intermediate states in INEPT coherence transfer (Section 7.10) and recall that instead of using simultaneous 90° pulses on lH and 13C to effect coherence transfer, we can start with the H 90° pulse and then, after a short delay, complete the INEPT transfer with the 13C 90° pulse ... 

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H/), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

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,... 

As can be seen by inspecting Figure 23, the coherence transfer efficiency is overwhelmingly good thanks to the very efficient 15N— -13C INEPT step employed for transferring magnetization exclusively from 15N to the sequential 13C nucleus. In addition, the possibility of an accidental resonance... 

Fig. 2. Pulse sequence for selective reverse INEPT using pulsed field gradients to select the coherence transfer echo. The 180° pulse pair in the middle of the 2r delay is not normally needed for t < 50 ms, and the second proton 180° pulse and first t2 delay maybe omitted if a linear phase gradient in the resultant spectrum can be tolerated. The second field gradient pulse has an area (7c/th) times that of the first.

The idea of back transformation of a three-dimensional NMR experiment involving heteronuclear 3H/X/Y out-and-back coherence transfer can in principle be carried to the extreme by fixing the mixing time in both indirect domains. Even if one-dimensional experiments of this kind fall short of providing any information on heteronuclear chemical shifts, they may still serve to obtain isotope-filtered 3H NMR spectra. A potential application of this technique is the detection of appropriately labelled metabolites in metabolism studies, and a one dimensional variant of the double INEPT 111/X/Y sequence has in fact been applied to pharmacokinetics studies of doubly 13C, 15N labelled metabolites.46 Even if the pulse scheme relied exclusively on phase-cycling for coherence selection, a suppression of matrix signals by a factor of 104 proved feasible, and it is easily conceivable that the performance can still be improved by the application of pulsed field gradients. 

In the INEPT experiment, the final step of coherence transfer is the simultaneous 90° pulses on 13C and H ... 

We can consider the state — 2IZSZ as an intermediate state in coherence transfer, just as we did in the analysis of populations (SPT) above. Often in INEPT-based experiments, this intermediate state is used as a way of cleaning up other coherences that are not desired. A pulsed field gradient (PFG, Chapter 8) is a way of temporarily messing up the shims, and this will destroy any magnetization that is in the x-y plane. The intermediate state —2IZSZ is not affected, however, because there is no net magnetization in the x-y plane. After... 

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