Non-Selective Polarization Transfer

Pulse sequences for non-selective polarization transfer, not only useful for signal enhancement but also for multiplicity selection, are referred to as INEPT [54], abbreviated from Insensitive Nuclei Enhanced by Polarization Transfer . An improved method denoted as Distortionless Enhancement by Polarization Transfer or DEPT [55] permits the cleanest multiplicity selection known so far, with full enhancement and low sensitivity to individual CH coupling constants. In addition, fully enhanced and undistorted coupled spectra can be recorded. Finally, subspectra for CH, CH2 and CH3 groups can be generated. 

DEPT spectra can be detected with and without proton decoupling. Correspondingly, decoupled and coupled spectra with multiplicity selection are observable. Fig. 2.46 illustrates such experiments with ( — (-menthol, also providing clear analysis of all CH coupling constants of the molecule. 

The insensitivity of the DEPT sequence to different JCH coupling constants, as illustrated in Fig. 2.46, makes it useful for editing 13C NMR spectra. To edit a carbon-13 spectrum, three DEPT experiments for the polarization transfer angles 

4 Measurement of Carbon-Carbon Coupling Constants Without 13C Enrichment INADEQUATE 

If all carbon-carbon bonds of an organic compound are known, the carbon skeleton is defined, and an essential part of structure elucidation is achieved. One method of determining carbon-carbon bonds involves the measurement of 13C — 13C coupling constants, since identical couplings of two carbon atoms indicate the presence of bonding between these carbons. 

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Modern techniques.. /-Modulated spin-echo ( APT ), non-selective polarization transfer ( DEPT ) experiments and two-dimensional carbon-proton, proton-proton ( COSY ) and carbon-carbon ( 2D-INADEQUATE ) shift correlations are in use (see the following section) and will be used in order to clarify doubtful signal assignments (see Sections 2.9 and 2.10). 

B. Non-selective Polarization Transfers (INEPT, DEPT, PENDANT,... 

The ID TOCSY module has been used in many pseudo-3D experiments (or alternatively referred to as ID analogues of 3D experiments in the literature) such as ID TOCSY-NOESY or ID TOCSY-ROESY experiments. The TOCSY part of these experiments are similar to that of a regular ID TOCSY where a selective excitation of a desired signal is followed by a MLEV17-type isotropic mixing. The second polarization transfer (NOESY or ROESY) step can either be non-selective [29, 59-61] or selective [62-65]. 

Methotrexate acts by inhibition of dihydrofolate reductase, the enzyme requisite for the reduction of dihydrofolic acid (3) to 5,6,7,8-tetrahydrofolic acid (4). In turn, (4) is a precursor to a series of enzyme cofactors (5-7) essential for the transfer of one carbon unit necessary for the biosynthesis of purines and pyrimidines and hence, ultimately, DNA. As an inhibitor of dihydrofolate reductase, methotrexate kills cells during the S phase of the cell cycle, when the cells are in the log phase of growth. Unfortunately, this cytotoxicity is non-selective, and rapidly proliferating normal cells, e.g., gastrointestinal epithelium cells and bone marrow, are dramatically affected as well. In addition, recent use of high dose methotrexate therapy with leucovorin rescue has led to additional clinical problems arising from a dose-related nephrotoxic metabolite, 7-hydroxy methotrexate (8). Finally, the very polar nature of methotrexate renders it virtually impenetrable to the blood-brain barrier, which can necessitate direct intrathecal injection in order to achieve therapeutic doses for the treatment of CNS tumours. 

The selectivity of INEPT can be achieved in the most straightforward way, as suggested by Bax and coworkers301 302, by replacing all the hard non-selective proton pulses of INEPT with selective ( soft ) pulses (this pulse sequence is sometimes denoted as SPINEPT). To retain full sensitivity, the selective pulses must cover the selected multiplet and its 29Si satellites, i.e. the excitation band should be approximately equal to the width of the multiplet plus the value of /(29Si—XH) to be used for polarization transfer. For the choice of suitable selective pulses, see Reference 135. 

Isotropic mixing [29] known also as Hartmann-Hahn polarization transfer [30,31] is a unique and very efficient method of coherence transfer between spins. Non-selective isotropic mixing is widely used in different types of correlation experiments. Selective Hartmann-Hahn transfer has been introduced quite recently [32-37] and provides the means for multiplet-selective [33-37] or band-selective [32] correlation experiments. 

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. 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

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