Heteronuclear magnetization transfer

Once a heteronuclear correlation is established, the water resonance can be saturated since it is not involved in the correlation.TTie heteronuclear magnetization transfer sequence can be designed so that at some point the heteronuclear correlation is oriented along z and the water magnetization is transverse. A z-gradient pulse can then be used to purge the water magnetiza-... 

The most commonly used method for heteronuclear magnetization transfer is the CP. Interestingly, it has been shown by Samoson and co-workers that ultrafast MAS systems were also able to vary rapidly the MAS rate and this can be used for CP transfers [160]. Two types of transfers can be obtained by CP Zero-quantum CP occurs when — vf =pvp and positive peaks are... 

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

Note that we do not start directly with 13 C magnetization because we want to take advantage of the larger (by a factor of 4) equilibrium population difference of lH compared to13 C, as well as the shorter T (faster relaxation) of 1H, which will permit shorter relaxation delays. We now know a lot of tricks, and the main one we need here is the heteronuclear INEPT transfer ... 

A number of theoretical transfer functions have been reported for specific experiments. However, analytical expressions were derived only for the simplest Hartmann-Hahn experiments. For heteronuclear Hartmann-Hahn transfer based on two CW spin-lock fields on resonance, Maudsley et al. (1977) derived magnetization-transfer functions for two coupled spins 1/2 for matched and mismatched rf fields [see Eq. (30)]. In IS, I2S, and I S systems, all coherence transfer functions were derived for on-resonance irradiation including mismatched rf fields. More general magnetization-transfer functions for off-resonance irradiation and Hartmann-Hahn mismatch were derived for Ij S systems with N < 6 (Muller and Ernst, 1979 Chingas et al., 1981 Levitt et al., 1986). Analytical expressions of heteronuclear Hartmann-Hahn transfer functions under the average Hamiltonian, created by the WALTZ-16, DIPSI-2, and MLEV-16 sequences (see Section XI), have been presented by Ernst et al. (1991) for on-resonant irradiation with matched rf fields. Numerical simulations of heteronuclear polarization-transfer functions for the WALTZ-16 and WALTZ-17 sequence have also been reported for various frequency offsets (Ernst et al., 1991). 

Heteronuclear Hartmann-Hahn sequences also effect homonuclear Hartmann-Hahn transfer, resulting in (heteronuclear and homonuclear) total correlation spectroscopy (TOCSY Bearden and Brown, 1989 Zuiderweg, 1990 Brown and Sanctuary, 1991 Ernst et al., 1991). Simultaneous heteronuclear and homonuclear magnetization transfer can be beneficial in relayed transfer experiments (Gibbs and Morris, 1992 Tokles et al., 1992 Majumdar et al., 1993). However, as pointed out by Ernst et al. 

As in the case of homonuclear correlations the heteronuclear Li,X TOCSY experiment, pulse sequence (viii), forms an alternative to the HETCOR and HMQC experiments introduced above. It has been tested for H, Li as well as H, Li shift correlations [147]. The MLEV16 decoupling sequence [148] was used for magnetization transfer, and Li as well as H detection was successful. An advantage of these experiments compared with correlation experiments based on the sequences (iii), (iv) and (v) must be seen in the fact that pure absorption spectra are produced. Signal elimination as a consequence of small coupling constants, as observed for antiphase crosspeaks, is thus prevented. On the other hand, adjusting the two decoupler fields to match the Hartmann-Hahn condition Y/Bi(7) = y,Bi(S) is not trivial and needs considerable experimental experience. 

M. Bjerring, J. T. Rasmussen, R. S. Krogshave and N. C. Nielsen, Heteronuclear coherence transfer in solid-state nuclear magnetic resonance using a 7-encoded transferred echo experiment. J. Chem. Phys., 2003, 119, 8916-8926. 

The number of pulse sequences incorporating TROSY type magnetization transfer has steadily grown over the last year. The discovers of TROSY describe a N- H/ C- H-TROSY experiment for the simultaneous acquisition of the heteronuclear chemical shift correlations of backbone amide groups, side-chain N- H2 groups and aromatic groups. ... 

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