PISEMA pulse sequence

In order to increase the sensitivity and resolution, and to reduce the experimental time, of PISEMAMAS technique, it is essential to perform the experiment with fast spinning speeds. While the PISEMA pulse sequence has been successfully used under slow spinning conditions, unfortunately, it cannot be used at high spinning speeds as the heteronuclear dipolar couplings are suppressed. However, changing the S -spin-lock rf field strength (in SEMA) to satisfy the Hartmann-Hahn condition, B s[= " ioi with... 

Fig. 7. A PISEMA pulse sequence for studies under a fast magic angle spinning condition. The modified SEMA pulse sequence in the tx period recovers I S dipolar couplings under MAS. Unlike other SLF pulse sequences, this pulse sequence does not require the synchronization of the tx period with the spinning speed of the rotor.

Fig. 17. A proton-detected one-dimensional PISEMA pulse sequence to measure...

Figure 2.23 The original PISEMA pulse sequence. Adopted from Ref [138], Copyright 2005 Elsevier.

A low RF power PISEMA pulse sequence, PITANSEMA, has been described for the measurement of heteronuclear dipolar couplings from solids." The method employs a time averaged nutation concept to significantly reduce the RF power required to spin-lock low frequency nuclear spins in PISEMA experiments. The efficacy of the 2D method is demonstrated on a single crystal of n-acetyl-L- N-valyl-L- N-leucine dipeptide to measure dipolar... 

Fig. 4. Pulse sequences for 2D PISEMA experiments on static or slow-spinning samples. It has been shown that the use of ramped rf pulses for S-spin-lock during the ti period makes the pulse more tolerant to experimental errors. The original PISEMA sequence (A) does not suppress the effects of phase transients. Incorporation of the modified SEMA sequences, (B) and (C), in the ti period of the PISEMA sequence (A) have been shown to suppress the effects of phase transients.

As discussed later, the application of the SEMA sequence is not limited to the 2D PISEMA experiment, but it can be used to design a variety of sophisticated pulse sequences. For example, the selective transfer of magnetization between strongly coupled heteronuclei by SEMA is the key feature of the sequence that can be utilized in experiments such as HETCOR, H-detected experiments," and experiments to selectively measure relaxation parameters of a proton bonded to S nuclei. 

Fig. 5. (A) Vector picture describing the relationship between the rf field strength and offset frequency in off-resonance experiments such as LGCP and PISEMA. (B) A simple pulse sequence (called LGCP or FFLGCP) to set-up the Lee-Goldburg condition. As explained in the text, this sequence can be used to determine the S-spin-lock field strength that matches ileff./for the effective spin exchange between / and S spins.

Fig. 18. Proton-detected ID PISEMA spectra of a powder sample of A-acetyl- A-D,L-valine obtained using the pulse sequence shown in Fig. 17 under static condition. The experiments were performed with a H Lee-Goldburg spin-lock RF power of...

The ability to retain anisotropic spin interactions and the power to selectively suppress unwanted interactions using pulse sequences such as PISEMA in aligned solids have played a significant role in the applications of solid-state NMR spectroscopy. In addition, the excellent control over the order parameter (ranging from isotropic to rigid solids) of a system and creation of model biological systems obviously invited, simplified, created, and amplified the applications of solid-state NMR techniques. Although solid-state NMR... 

A new pulse sequence has been described for the recoupling of heteronuclear dipolar interactions under MAS. " The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two RF fields. The technique was used for measurements of dipolar couplings in... 

S. V. Dvinskikh, H. Zimmermann, A. Maliniak, and D. Sandstrom, Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences, J. Magn. Reson 164 165-170 (2003). 

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