Compound Radiofrequency-Driven Recoupling Pulse Sequences for Efficient Magnetization Transfer by Homonuclear Dipolar Interaction under Magic-Angle Spinning Conditions
✍ Scribed by Toshimichi Fujiwara; Purnima Khandelwal; Hideo Akutsu
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 140 KB
- Volume
- 145
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
The maximum of the transferred magnetization in rotating powdered solids under the radiofrequency-driven recoupling (RFDR) pulse sequence is enhanced by reducing the orientation dependence of the effective recoupled homonuclear dipolar interaction. The compound RFDR (CRFDR) pulse sequence for this enhancement consists of RFDR pulse units ( i --R -i ) with different i , where R is the sample rotation period, i and i ؍( R ؊ i ) are delays, and is a 180°pulse. The delay i modifies the zero-quantum spin operators and the sample rotation-angle dependence of the recoupled dipolar Hamiltonian. The CRFDR pulse sequences were optimized for mixing by varying i . Numer- ical simulation for the two-spin system only with a dipolar interaction and isotropic chemical shifts indicates that the transfer efficiency of CRFDR averaged over the powder is about 70%, which is 30% higher than the efficiency of the RFDR pulse over a broad range of about 1/ R in resonance frequency difference. The CRFDR sequences need about 60% longer mixing times to maximize the transferred magnetizaion in comparison with the original RFDR sequence. Chemical shift anisotropy, the other dipolar interactions, and relaxation generally reduce the enhancement by CRFDR. Experiments for fully 13 C-labeled alanine, however, show that the maximum of the magnetization transferred with CRFDR from the carboxyl to ␣ carbon is about 15% greater than that with RFDR.