Insensitive nucleus enhancement

Coherent transfer experiments can roughly be divided into two classes pulse-interrupted free-precession experiments and Hartmann-Hahn-type experiments (Ernst et al., 1987). Examples of homo- and heteronuclear pulse-interrupted free-precession coherence transfer are COSY (correlation spectroscopy Aue et al., 1976), RELAY (relayed correlation spectroscopy Wagner, 1983), and INEPT (insensitive nucleus enhancement by polarization transfer) transfer steps (Morris and Freeman, 1979 Burum... 

Heteronuclear multiple-quantum correlation Experiment for tailored correlation spectroscopy of H and H resonances in peptides and proteins Homonuclear Hartmann-Hahn spectroscopy Heteronuclear quadruple-quantum coherence Heteronuelear triple-quantum coherence Heteronuclear single-quantum coherence TOCSY sequences developed at the Indian Institute of Chemical Technology Insensitive nucleus enhancement by polarization transfer... 

INEPT (Insensitive Nucleus Enhancement by Polarization Transfer)... 

INEPT Insensitive nucleus enhancement by polarization transfer... 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Some important nuclei, including C and N, have low natural abundances and sensitivities. Pulse sequences have been devised to improve the observability of these nuclei when they are coupled to another nucleus of high receptivity, usually a proton. Pulses are applied in such a way that the favorable population of the sensitive nucleus S is transferred to the insensitive nucleus I. A common sequence developed by Freeman for this purpose is called INEPT, for Insensitive Nuclei Enhanced by Polariz.ation Transfer, as follows ... 

The INEPT experiment [26] (Insensitive Nuclei Enhanced by Polarisation Transfer) was one of the forerunners of many of the pulse NMR experiments developed over subsequent years and still constitutes a feature of some of the most widely used multidimensional experiments in modem pulse NMR. Its purpose is to enable non-selective polarisation transfer between spins, and its operation may be readily understood with reference to the vector model. Most often it is the proton that is used as the source nucleus and these discussion will relate to XH spin systems throughout, although it should be remembered that any high-y spin- /2 nucleus constitutes a suitable source. 

A second example of coherence selection using phase cycling is the suppression of the quaternary carbon signals in a polarization transfer spectrum. DEPT and INEPT type pulse sequences use a polarization transfer step to enhance the signal of an NMR insensitive nucleus such as which exhibits scalar coupling to a NMR sensitive nucleus such as IR. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

We now want to consider observing the high resolution NMR spectrum of a dilute and/or insensitive nucleus (usually 13C) in a solid with enhanced sensitivity by taking advantage of the dipolar reservoir of an abundant nucleus in the same system. These experiments are an outgrowth of two lines of development. One of these is the attempt to suppress dipolar coupling in a solid sample. The second resulted from the experiments of Hartmann and Hahn (1962) in which the detection of a dilute or insensitive spin was made possible through... 

Ag and Ag both have 7=1/2 but ° Ag is more suitable for NMR measurements . The ° Ag NMR spectra, using the INEPT (insensitive nuclei enhanced by polarization transfer) sequence [proton polarization transfer sequence between two / coupled spins /( Ag) and s( H), 7( ° ° Ag- H-imine) = 9.3 Hz (5 8.87) and 6.3 Hz (S 8.69)], have been applied to gain insight into the electronic environment and the structural and dynamic behaviour of Ag complexes in solutions . The chemical shift of the ° Ag nucleus is highly sensitive towards changes in its environment . The... 

There are other two-dimensional techniques, more sensitive than HETCOR, that make use of polarization transfer (Section 4.12.2). Even greater enhancement can be obtained if the magnetization is generated at the insensitive nucleus and then transferred back to the sensitive nucleus for detection. Procedures making use of this principle are called inverse techniques and lead to a great reduction of sample concentration or measurement time. Typical experiments involve recording spectra for insensitive nuclei such as C, Si and N, which are recorded in inverse, proton-detected procedures. The information given by such experiments is the same as that from the HETCOR experiments, but the experiments are much more sensitive and are quicker to perform. 

The Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) experiment is useful to discuss for two reasons. First is its utility in enhancing the S/N level in X-nucleus-detected spectra such as NMR. Second is because it is the basis for building many 2D- and 3D-NMR experiments described below. 

At the calyx of Held in the medial nucleus of the trapezoid body, activation of presynaptic glycine receptors was also shown to facilitate spontaneous glutamate release and to enhance amplitudes of evoked excitatory postsynaptic currents (Turecek and Trussell 2001). The action on spontaneous postsynaptic currents were insensitive towards a blockade of Na+ channels by tetrodotoxin, but abolished by the Ca2+ channel blocker Cd2+. This indicated that glycine acted by causing a depolarization, which was also corroborated by direct voltage measurements. 

Many MFEs and MIEs have been observed in reactions of light atom-centered radicals such as C- and 0-centered ones. On the other hand, MFEs and MIEs have been believed to decrease drastically with increasing atomic number of the nucleus where one of the unpaired electrons in a RP is mainly localized. This is due to the magnetic-insensitive spin-orbit (SO) interaction of heavy atoms, which enhances the spin conversion of RPs. As shown in Chapter 7, however, many MFEs have been observed in the reactions of heavy atom-centered radicals such as Si-, S-, Ge-, and Sn-radicals if some suitable reaction systems are designed for such radicals. Using such reaction system which show MFEs, we will be able to find MIEs of such heavy isotopes. 

Determination of magnetically equivalent sensitive nuclei which are scalar coupled to an insensitive, non-abundant nucleus like Dc or 15N, extended spin echo sequences for signal enhancement by nuclear Overhauser effect (APT, SEMUT) or polarization transfer (DEPT, INEPT, etc). 

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