Polarization transfer efficiency

However, typical polarization transfer efficiency reported for 13C-13C TOBSY measurements of polypeptides is significantly less than the theoretical maximum. One of the major reasons is due to the requirement of high power 1H decoupling during the polarization transfer, which is usually set to be three times larger than the recoupling field [37, 77]. 

The potential signal enhancement available from these double-resonance techniques depends on the available polarization ( H or 14N), the detection frequency, and the polarization transfer efficiency. The transfer efficiency depends not only on the coupling between the isotopes and the time spent with the frequencies matched but also on the number of 14N transitions matched and the order in which the matching conditions occur [91]. Irradiating multiple transitions, as described in Section 4.2.1., will also affect the choice of 14N transitions to polarize [94,95],... 

Calculation of the frequency at Oppm for Hg from the measured frequency of 0 ppm for protons in the sample is a prudent alternative with modem spectrometers. For solid-state NMR, hexakis(dimethylsulphoxide)mercury(II) trifluo-romethanesulphonate, 5( Hg) = -2313 ppm, is a suitable secondary standard. Its high symmetry Octahedral coordination environment results in small chemical shift anisotropy and short H-Hg separations lead to high H Hg polarization transfer efficiency. 

Fig. 15. Polarization-transfer efficiency in a spin system consisting of three spins 1/2 under ideal isotropic-mixing conditions (effective ///-TOCSY coupling topology). (A) Damped transfer functions - t U12I) as a function of the relative coupling constant...

With this approach, any broadband Hartmann-Hahn mixing sequence can be converted into a sequence that is selective for chemical shift differences. In complete analogy to the selectivity of binomial (Plateau and Gueron, 1982 Sklenaf and Starcuk, 1982 Hore, 1983) or DANTE-type sequences (Bodenhausen et al., 1976 Morris and Freeman, 1978), the selectivity of the corresponding zero-quantum sequences depends on the duration and number of delays A. In Fig. 28B, the two-dimensional offset dependence of the polarization-transfer efficiency is shown for the zero-quantum analog of a 1-2-1 sequence. 

It is common to use value l/(3/opt) for the delay 5 to have all the signals positive. If we look what the polarization transfer efficiency is for the whole refocused INEPT period with A = 1/(2/opt) arid 8 = l/(3/opt)/ we can formulate... 

It is now possible to follow what is the polarization transfer efficiency from proton to carbon for each CH group, and also how the mismatch between true Jch and the /opt used for polarization transfer delay calculation affects the signal intensity (Figure 8). When the /chS of the sample are known, suitable coefficients can be formulated to correct the cross peak volumes. 

Polivka, T., M. Pellnor, E. Melo, T. Pascher, V. Sundstrom, A. Osuka, and K. R. Naqvi. 2007. Polarity-tuned energy transfer efficiency in artificial light-harvesting antennae containing carbonyl carotenoids peridi-nin and fucoxanthin. J. Phys. Chem. C 110 467 -76. 

Growth of the degree of fluorescence polarization (the Weber s effect) and a decrease of energy transfer efficiency while shifting the excitation wavelength to the red edge. 

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-... 

Furthermore, the qualitative influence of substituents on the symmetry and electronic structure of the substrate and its hydrogenation product on the efficiency of the transfer of polarization to the 13C-nuclei have been discussed, as well as the feasibility of a polarization transfer to other heteronuclei. Evidence in the form of a shift of the aromatic 13C resonances has been found for an initial attachment of hydrogenation products containing aromatic segments to the metal center of the cationic hydrogenation catalyst - probably in the form of a re-complex. 

By contrast, in the high field of a superconducting magnet, polarization transfer to heteronuciei is less efficient, because now the difference in resonance frequency of XH and 13C is significant and exceeds the magnitude of the coupling constants between the carbons and the protons. In this latter case, polarization transfer can be achieved most effectively by appropriate pulse sequences (e.g., via cross-polarization). 

Hartmann-Hahn cross polarization between two low-y nuclei has been successfully used to record chemical-shift correlation spectra between 13C and 15 N nuclei. Cross polarization between two low-y nuclei suffers from a high sensitivity to the exact matching condition at one of the side bands of the Hartmann-Hahn condition [101]. Adiabatic methods (APHH-CP) can eliminate most of this sensitivity and lead to high transfer efficiencies [34, 62, 90]. 

By reversing the order of polarization transfer steps, NOE contacts of signals hidden under the bulk of other resonances can be established in a ID TOCSY-NOESY [39] experiment (fig. 1(d)). The necessary requirement is that an efficient TOCSY transfer can be made to a particular proton from... 

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