Reverse INEPT transfer

Figure 9.37. The constant-time-HSQC-IDOSY sequence. The delays T are set to 1/2Jhx as required for the INEPT transfer and the constanttime period 2T remains fixed, its duration being dictated by the desired diffusion time A. The effective ti evolution time is varied by moving the two 180° refocusing pulses within the constant time period (pulses shown with arrows over) and coherence selection is made with the echo/antiecho (E/A) scheme. The diffusion encoding/decoding gradients are applied as bipolar pairs during the INEPT and reverse-INEPT transfer steps.

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,... 

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.

Figure 1. Pulse sequences used to monitor the heteronuclear NOE (bottom panel) and the spin lattice relaxation (top panel). The NOE experiment is a simple extension of the basic pulse sequence introduced by Kay et al. (1989) and utilizes continuous broadband H decoupling during the preparation period to generate the NOE. Two dimensional spectra with and without H decoupling (lightly shaded region) define the NOE. The T, relaxation experiment is a simple extension of the basic pulse sequence introduced by Sklenar et al. (1987). The NOE via H decoupling rather than coherent polarization transfer is used to polarize the carbons. For both the NOE and T, measurement, the proton pulse 0 (or the delay of the corresponding reverse INEPT) is set to the magic angle as described by Palmer et al. (1991). The constant time period, A, is set to minimize cos(n [27i J + 27t Jo,]). When x is set to l/2 Jc then 2A = - 1/2 J( ...

The classical approach to generating multiplicity-edited proton spectra was to use the INEPT or DEPT sequences in reverse to transfer initial carbon magnetisation onto the proton for detection. These suffered from low sensitivity and poor suppression of resonances, so were never popular. 

The basic INEPT sequence does not allow IR decoupling during data acquisition and the IR refocused INEPT sequence incorporating an reverse INEPT unit has been developed. Essentially the sequence has two functional parts The first INEPT unit creates antiphase coherence (I lyS ) and polarization transfer (lyS Iz y). The overall signal enhancement depending upon the difference in value between D(C, R) and the optimum coupling constant Jnominal (d2 = 1 / (4 Jnominal))- The second, reverse... 

HSQC starts with an INEPT step which transfers the proton polarization to carbon. Then the polarization is modulated by the carbon chemical shift during the fi evolution period. Finally, a reverse-INEPT step returns the polarization back to proton for observation. Pulsed field gradients can also be incorporated... 

The operation of the proton-enhanced sequences is best understood by reference to the original INEPT-INADEQUATE sequence shown in simplified form in Fig. 5.80 in which the individual component parts have been identified and may be followed in a stepwise manner. Thus, initial transfer from proton to carbon is via the INEPT sequence introduced in Chapter 4 followed by the generation of DQC as for the INADEQUATE above. This again evolves to encode c after which gradient selection begins. The DQ then returns to single quantum, and the C- C-coupling is refocused by the spin-echo. The final step is INEPT in reverse to transfer from carbon back onto proton, followed by... 

Most NMR spectroscopists have on some occasion examined the Periodic Table of NMR-active nuclei with 13C, I70, etc., and contemplated that at first glance essentially the whole Periodic Table is potentially open to study by NMR. Solution-state NMR studies of nuclei with small magnetic moments exist, but often when papers report data concerning such nuclei the information is obtained not by simple direct excitation but indirectly using polarization transfer techniques such as INEPT and DEPT, and/or by using reverse detection methods.1 As many of these indirect methods are not available in the... 

HSQC. The Heteronudear Single Quantum Correlation (HSQC) experiment is an alternative to HMQC that accomplishes a similar objective. The experiment generates, via an INEPT sequence, single quantum (or " N) coherence, which evolves and then is transferred back to the proton frequency by a second INEPT sequence, this time in reverse. The main difference from the HMQC result is that HSQC spectra do not contain H- H couplings in the C ( Jj) dimension. As a result, HSQC cross peaks tend to have improved resolution over analogous HMQC cross peaks. HSQC is preferred when there is considerable spectral overlap. 

Figure 6.6. The HSQC experiment and asscx iated coherence transfer pathway. The experiment uses the INEE T sequence to generate transverse X magnetisation which evolves and is then transferred back to the proton by an INEPT step in reverse. Notice that, in contrast to HMQC, only single-quantum X coherence evolves during ti (see text).

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

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