Chemical shift evolutions

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

The method relies on the measurement of cross-correlated relaxation rates in a constant time period such that the cross-correlated relaxation rate evolves during a fixed time r. In order to resolve the cross-correlated relaxation rate, however, the couplings need to evolve during an evolution time, e.g. tt. The first pulse sequence published for the measurement of the cross-correlated relaxation rate between the HNn and the Ca j,Ha i vector relied on an HN(CO)CA experiment, in which the Ca chemical shift evolution period was replaced by evolution of 15N,13C double and zero quantum coherences (Fig. 7.20). 

Obviously, this approach cannot be used for selecting the nonisotope-labeled components. In the following we will consider isotope filtering/editing techniques that do not use heteronuclear chemical shift evolution. 

After a time x =1 /y, the cosine term will be zero, and the sine term unity, so that in-phase 1H coherence is completely converted into heteronuclear antiphase coherence, 2 Iy Sz. For protons not bound to an Ix spin, nothing happens (neglecting chemical shift evolution, other couplings, relaxation etc.), and they will stay at Ix coherence ... 

An alternative to using selective pulses in selective ID TOCSY has been proposed [52]. The frequency selection is instead accomplished by using a homonuclear ( H) chemical shift selective filter (CSSF) [53, 54]. The chemical shift filter for frequency selection consists of a non-selective 90° pulse which is set at the frequency of the selected signal, and a systematic increment of the chemical shift evolution between this pulse and the... 

In practice, the suppression of the signals from C-bound protons is not complete. In part, this arises from imperfections of the 180°(if) pulse in the delay r. If the chemical shift evolution is not refocused, pure proton terms are generated which pass the spin-lock purge pulse. Therefore, the suppression of the signals from C-bound protons is improved by applying the Excorcycle [11] phase cycle to this 180°(if) pulse [10]. To keep the phase cycle short, only the first two steps of Excorcycle can be used. The selection of the correlations is further improved by phase cycling... 

MLEV17 sequence being one of the most used sequences [23]) in such a way as to continuously refocus the chemical shift evolution of the various signals in the xy plane. Analogously to ROESY experiments, the magnetization during the spin-lock (mixing) time disappears with T p (i.e. essentially Tj, see Section 3.4). It follows that coherence transfer in the xy plane, which is built up with a sin(7T J/jt) function, also decreases with time constant p p — p[p + p p)/2 ... 

THE HETERONUCLEAR SPIN ECHO CONTROLLING /-COUPLING EVOLUTION AND CHEMICAL SHIFT EVOLUTION... 

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