Coherent magnetization transfer

Here the generic term Hartmann-Hahn experiment is used for polarization- or coherence-transfer experiments that are based on the Hartmann-Hahn principle (see Section II), that is, on matched effective fields that are created by a rf irradiation scheme. These experiments may be classified according to the following practical and theoretical aspects (see Fig. 6) that are related to properties of samples, spin systems, coherent magnetization transfer, effective Hamiltonians, multiple-pulse sequences, and incoherent magnetization transfer ... 

Fig. 7.2.31 Filters for homonuclear coherent magnetization transfer. All filters start from and end with longitudinal magnetization, (a) Selective excitation and reconversion of coherences with a nonselective mixing period, (b) Realization of a nonselective mixing period in a z filter via longitudinal magnetization, (c) Nonselective excitation and reconversion of coherences with a selective mixing period, (d) Realization of a selective mixing period by a multi-quantum filter, (e) Selective exchange of transverse magnetization within the multiplets of coupled homonuclear spin pairs by a homonuclear version of the INEPT method.

Thus, the magnetization is transferred from the amide proton to the attached nitrogen and then simultaneously to the intra- and interresidual 13C spins and sequential 13C spin. The 13C chemical shift is labelled during /, and 13C frequency during t2. The desired coherence is transferred back to the amide proton in the identical but reverse coherence transfer pathway. The 15N chemical shift is frequency labelled during t3, and implemented into the 13C 15N back-INEPT step. The sensitivity of the HNCOmCA-TROSY experiment is excellent and nearly similar to HNCA-TROSY except for the inherent sensitivity loss by a factor of /2, arising from additional quadrature detection needed for 13C frequency discrimination in the fourth dimension. The excellent sensitivity is due to a very efficient coherence transfer pathway,... 

Magnetization is initially transferred from 1Hn(i) to 15N(/) spin. Unlike in the HNCA-TROSY scheme, the desired coherence is transferred from the 15N(i) spin to the 13C7(/ — 1) spin of the preceding residue. To this end, nearly uniform fNc( 15 Hz) scalar coupling is used. As 2/NC is negligibly small, the coherence is transferred exclusively to the 13C (/ — 1) nucleus. Finally, the 13C —13C INEPT is used to transfer magnetization from 13C (i— 1) to... 

J. Martin, B. W. Shore, and K. Bergmann. Coherent population transfer in multilevel systems with magnetic sublevels. 111. Experimental results. Phys. Rev. A, 54(2) 1556-1569(1996). 

In the two examples above on the right, observable magnetization (antiphase coherence) is transferred by the 90° 1H pulse from Hb to Ha (top) and from Ha to Hb (bottom). This is a key process in all advanced NMR experiments that depend on / couplings. The role of the two operators is reversed as the operator in the x-y plane (the observable net magnetization) rotates to the z axis and the operator on the z axis (the multiplier that represents microscopic z magnetization) rotates to the x-y plane. After the rotations, we reverse the order of the two operators because we always write the observable operator first in the product. 

The MQC intermediate state in coherence ( INEPT ) transfer can also be used to clean up the spectrum. In this case, we can apply a double-quantum filter (using either gradients or a phase cycle) to kill all coherences at the intermediate step that are not DQC. We will see the usefulness of this technique in the DQF (double-quantum filtered) COSY experiment (Chapter 10). As with the spoiler gradient applied to the 2IZSZ intermediate state, a doublequantum filter destroys any unwanted magnetization, leaving only DQC that can then be carried on to observable antiphase magnetization in the second step of INEPT transfer. 

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