期刊
JOURNAL OF MAGNETIC RESONANCE
卷 224, 期 -, 页码 13-21出版社
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jmr.2012.08.013
关键词
Dynamic Nuclear Polarization; Magic Angle Spinning; Cross-effect; Solid-effect; Biradical; Totapol; Spin relaxation; Liouville equation; Simulations
资金
- German-Israeli Project Cooperation of the DFG through Ministry of Education and Research (BMBF) of the Federal republic of Germany
Magic Angle Spinning (MAS) Dynamic Nuclear Polarization (DNP) has proven to be a very powerful way to improve the signal to noise ratio of NMR experiments on solids. The experiments have in general been interpreted considering the Solid-Effect (SE) and Cross-Effect (CE) DNP mechanisms while ignoring the influence of sample spinning. In this paper, we show experimental data of MAS-DNP enhancements of H-1 and C-13 in proline and SH3 protein in glass forming water/glycerol solvent containing TOTAPOL. We also introduce a theoretical model that aims at explaining how the nuclear polarization is built in MAS-DNP experiments. By using Liouville space based simulations to include relaxation on two simple spin models, (electron-nucleus) and (electron-electron-nucleus), we explain how the basic MAS-SE-DNP and MAS-CE-DNP processes work. The importance of fast energy passages and short level anti-crossing is emphasized and the differences between static DNP and MAS-DNP is explained. During a single rotor cycle the enhancement in the (electron-electron-nucleus) system arises from MAS-CE-DNP involving at least three kinds of two-level fast passages: an electron-electron dipolar anti-crossing, a single quantum electron MW encounter and an anti-crossing at the CE condition inducing nuclear polarization in- or decrements. Numerical, powder-averaged, simulations were performed in order to check the influence of the experimental parameters on the enhancement efficiencies. In particular we show that the spinning frequency dependence of the theoretical MAS-CE-DNP enhancement compares favorably with the experimental and 13C MAS-DNP enhancements of proline and SH3. (C) 2012 Elsevier Inc. All rights reserved.
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