15N magnetization

Fig. 8.6 Schematic representation of the modified experiments HNCO (a) and (HA)CA(CO)NH (b) to measure residual dipolar couplings. The white bars represent the extra pulses that are applied in an interleaved manner to collect the in-phase 15N magnetization. The anti-phase 15N...

The effect could be considerable for solvent-exposed parts of the backbone and could render the NOE values inaccurate. These systematic errors could be minimized by using water flip-back pulses in order to avoid saturation of H20 magnetization [11]. The NOE data are generally more susceptible to errors than Ri and R2 because (i) the NOE experiments start with the equilibrium 15N magnetization that is 10 fold lower than that of (XH) in the Hi and R2 experiments, hence relatively low sensitivity, and (ii) the NOE values are derived from only two sets of measurements, whereas R1 and R2 data are obtained from fitting multiple sets of data the latter is expected to result in a more efficient averaging of experimental errors. 

The 15N magnetic resonance studies of the pyrimidine bases or their derivatives are scarce. Roberts et al.BB have measured the 1H and 15N magnetic resonance spectra of a number of pyrimidines including uracil and 1-methylcytosine. The most important result of this study was the elucidation of the dominant tautomeric structures of uracil and proto-nated 1-methylcytosine as the diketo, 32, and keto-amine form, 7, respectively (cf. Sections II and IV and the spectrum of 1-methylcytosine hydrochloride labeled only in the amino group62). In the case of uracil,85 the two 15N-bonded protons gave two doublets centered at 10.78 and 10.96 ppm (measured downfield from internal tetramethyl-... 

Cross-polarization is a double-resonance experiment in which the energy levels of the H and 15N spins are matched to the Hartman-Hahn condition in the rotating frame [47], Under this condition the energy between the two coupled spin systems may be exchanged and the 15N magnetization (S) develops as a function of time at the expense of the proton magnetization according to the relation ... 

The most important magnetic property by far is the chemical shift of NMR spectroscopy. While proton H) and 13C shieldings hold a prominent place in organic chemistry, other magnetic nuclei such as 15N, 29Si, or 31P but also heavier nuclei such as transition-metals are increasingly important in many areas of chemistry. Obviously, all these nuclei are equally... 

DQ coherence between C (,-) and C m and then let the DQ coherence evolve under the influence of the heteronuclear I3C-15N dipole-dipole interaction [181, 182]. The virtue of this design is that it can be easily combined with other resolution enhancement technique such as INADEQUATE [183]. Alternatively, the magnetization of C (,) dephased under the 13C-15N dipolar coupling can be transferred to C (j) for another period of 13C-15N dipolar dephasing [183]. This idea can be combined with the NCOCA experiment so that the superior resolution provided by the C (,-)-N(j+i) correlation could be exploited. The overall efficiency, however, is relatively low due to the use of two polarization-transfer steps, viz. 15N —> 13C and 13C —> 13C [183]. In comparison with the techniques, the advan-... 

Nuclear magnetic resonance spectroscopy for /V-acyloxy-/V-alkoxyamides 13C NMR spectroscopy, 56-58 15N NMR spectroscopy, 58-59 dynamic 1H NMR spectroscopy, 59 Nucleophilic substitution (SN2) reactions, /V-acyloxy-/V-alkoxyamidcs, 70-90 alcoholysis reactions, 89-90 with aromatic amines, HERON reactions, 70-74... 

Likewise, the original proton polarization can be transferred to other magnetically active heteronuclei, notably to 13C, 15N, 19F, 29Si, 31P or appropriate isotopes of various transition metals. This is especially attractive because of the frequently low sensitivity of many heteronuclei, in particular of those with low magnetic moments [8]. 

Likewise, the initial proton polarization may be transferred to other magnetically active heteronuclei, most attractively to those associated with a low y-value of their nucleus (i.e., to 15N, 29Si), and similarly difficult ones, using heteroatom PHIP at low magnetic fields [8, 45]. 

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