Chemical shift selective filter

Selective pulses and chemical-shift-selective filters... 

As pointed out in the Introduction, the essential part of a successful transformation of an nD experiment into its ID analog is the selective excitation of spins. There are two distinct tools available for this purpose selective pulses and chemical-shift-selective filters. 

Chemical-shift-selective filters in multiple polarization transfer experiments... 

As an alternative to selective pulses, chemical-shift-selective filters (CSSF) were successfully used in ID COSY, ID RELAY and ID NOESY experiments when signals partially overlapping, but different in their chemical... 

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... 

A simplification of the COSY H-n.m.r. spectra of oligosaccharides by use of a 2-ditnensional version of 3-dimensional HOHAHA COSY has been described. A new and economical (in spectrometer time) technique for assigning oligosaccharide n.m.r. specua, which employs selective excitation combined with a DANTE pulse train and chemical shift selective filtering (CSSF), was found suitable for problems of medium complexity, where each spin network contains at least one separate multiplet. It has been applied to hepta-O-acetyl-a-cellobiosyl ande as an example. ... 

Tip filters for the longitudinal relaxation time in the rotating frame by applying spin-lock fields or multisolid echoes combinations of relaxation-time filters and multipulse excitation chemical-shift selective excitation ... 

Filter elements have been developed, not just for coupling evolution, but also for chemical shift selection [5.215 - 5.221]. An early example was the jump-return method for solvent signal suppression. Check it 5.2.3.3, whereby the resonances at a given chemical shift and related multiples were suppressed. In common with the z-filter for zero-quantum suppression, the CSSF (Chemical 5hift iSelective Filter) element uses different free precession periods to give varying degrees of chemical shift evolution for each scan in a multiple scan experiment. 

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. 

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. 

A careful study by McDermott and co-workers of the high-K+ and low-K+ states of the KcsA channel has focused exclusively on the selectivity filter.101 They discovered that low K+ also induces the "non-con-ductive" or "collapsed" structure of the selectivity filter at neutral pH, but only if the sample remains well hydrated this state is lost if the bulk buffer is removed from the NMR rotor after spinning. Comparison of the measured chemical shifts with predictions by the SHIFTX and SPARTA programs identified the crystal structures that are most consistent with the selectivity filter conformations in the high-K+ and low-K+ proteolipo-some samples. Titrations of the chemical shift changes were used to measure site-specific affinities for K+. Based on the slow exchange rate between these conformations (<500 s 1), the authors suggest that the low-K+ conformation is relevant to channel inactivation rather than to conduction.101... 

More sophisticated experiments may involve binomial coefficients [69,73] for the number of t experiments and application of selective pulses [74,75] in order to minimize signal overlap. The advantage of chemical shift filters over corresponding 2D experiments is that the number of t experiments can be significantly reduced and considerably higher digital resolution can be obtained. 

There are many situations in organometallic NMR when the magnitude of the spin-spin coupling between rare spins is known and only the relative sign of the coupling has to be determined. In such cases the 2D correlation experiment can be reduced to a chemical shift filtered ID experiment. For example, a chemical shift filtered INEPT pulse sequence [71] can be used for this purpose [70,72] (see Figure 10). Alternatively, selective excitation can be used [70], or both the... 

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