Refocused INEPT technique

An alternative pulse sequence that provides the same multiplicities as INEPT but with intensity ratios that follow the binomial theorem is DEPT (distortionless enhancement by polarization transfer).The pulse sequence, depicted in Fig. 12.3a, can be used, like refocused INEPT, for sensitivity enhancement but is usually employed as an editing technique. The three evolution periods T are chosen to approximate 1/2/, but the length of the pulse labeled 0 can be varied. As we show in the following, CH has maximum intensity at 0 = 90° CH2 has zero... 

Figure 5-24 Variation of signal intensities forCH, CHi> and CH3 as a function of 0 = ITJA2 in the refocused INEPT experiment. (Reproduced from T. D. W. Claridge. High-Resolution NMR Techniques in Organic Chemistry, Pergamon Press, Amsterdam, 1999, p. 138.)...

While the SPT method, as the name indicates, is a selective experiment, techniques derived from the principle of population transfer of a non-selective nature are also availahle. The first of these to appear was the INEPT experiment [38]. The pulse sequence schematics for INEPT and refocused INEPT are shown in Pig. 8.1. As with the SPT experiment described above, INEPT and its successor, the DEPT ex-p)eriment, both operate with enhanced sensitivity through the perturhation of the Boltzmann populations. This prototypical non-selective experiment is now relatively seldom used as more refined variants have been developed. The INEPT experiment (r = Fig. 8.1A) generates antiphase responses analogous to... 

Thus we cannot escape the depressing reality that 7 2 will get shorter and linewidth will get bigger as we increase the size of the protein studied. The reduced T2 is not only a problem for linewidth, but also causes loss of sensitivity as coherence decays during the defocusing and refocusing delays (1/(2J)) required for INEPT transfer in our 2D experiments. The only ray of hope comes in the form of a new technique called TROSY (transverse relaxation optimized spectroscopy), which takes advantage of the cancellation of dipole-dipole relaxation by CSA relaxation to get an effectively much longer 7 2 value we will briefly discuss TROSY at the end of this chapter. 

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