by the Switching Angle Sample Spinning (SASS) technique
[ Japanese | English ]
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Measurement of Chemical Shift Anisotropy (CSA) by the Switching Angle Sample Spinning (SASS) technique [ Japanese | English ] |
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[Background and purpose] Magic Angle Spinning (MAS) is a standard technique in solid-state NMR, which enables high-resolution solid-state NMR measurements through macroscopic rapid sample rotation around an axis tilted by the so-called magic angle with respect to the static field. The individual peak position in a resonance line measured under MAS represents an isotropic chemical shift of the corresponding ensemble of nuclei. There have been studies to extract information as to secondary structure of polymers through measurement of isotropic chemical shift. Although MAS enables us to readily obtain isotropic chemical shifts, it does so at the cost of losing information with regard to anisotropy of chemical shift tensors, which must contain fruitful chemical information. This fact have motivated the NMR community to develop techniques that enable measurement of both anisotropic and isotropic chemical shifts. They include (1) rotor-synchronized 180 deg. pulse[1], (2) Magic Angle Turning[2], and (3) Switching Angle Sample Spinning (SASS)[3-4]. We aim at getting secondary structual information of polymers through measurement of chemical shift anisotropy by the SASS mathod. The reasons for choosing the SASS technique are (a) its robustness against rf pulse imperfections, (b) relatively large scaling factor, and (c) no restriction of f1 spectral width. It follows that SASS leads to high-quality measurement of chemical shift anisotropy! [Strategy] Figure 1 shows a SASS pulse sequence for measuring CSA of carbon-13 nuclei. The point is to let the carbon-13 magnetization evolve under sample spinning at off-magic angle and under proton decoupling, and then to make measurement at the magic angle.  [SASS probe development]    Electrically, the sample coil embedded inside the spinning module is doubly tuned and impedance-matched at 400 MHz and 100 MHz, enabling proton-carbon 13 double resonance experiments in a magnetic field of 9.4 T. [SASS Spectra] In Figure 5, carbon-13 NMR spectra are shown obtained by the SASS technique in a polycrystalline sample of dimedone. Anitotropic lineshapes from the individual carbon-13 groups have successfully been separated.  The next example is on chemical shift anisotropy in PBLA (poly-beta-benzyl-L-Aspartate, see Figure 6). PBLA is known to undergo irreversible transition upon heating. There has been an attempt to trace the process of structual transition in PBLA through measurement of isotropic chemical shift. In order to obtain further rich structual information, we decided to explore chemical shift anisotropy in PBLA by the SASS technique.  Figure 7(a) is an isotropic carbon-13 spectrum in PBLA, where two carboxyl carbon groups as indicated by the circle are barely distinguishable. We could obtain chemical shift anisotropies of such close resonance peaks, as demonstrated in the 2D SASS spectrum (Figure 7(b)) and its slice 1D spectra (Figure 7(c)). Experiments in heat-processed PBLA are currently in progress.  SASS is also useful for magnetization exchange experiments, which give us structual information in another respect. At the off-magic angle, broadening of resonance lines can cause spectral overlap, which allows the flip-flop transition between the relevant spins. Since the transition probability of the flip-flop process depends on the strength of the dipolar interaction or the distance between them, the amount of the exchanged magnetizations for a given time interval tells us distance information. The SASS exchange sequence and its demonstration are shown in Figure 8.  [References] [1] R. Tycko, G. Dabbagh, P.A. Mirau, J. Magn. Reson. 85 (1989), 265-274. [2] J.Z. Hu, R.A. Wind, J. Magn. Reson. 163 (2003) 149-162. [3] T. Mizuno, K. Takegoshi, T. Terao, J. Magn. Reson. 171 (2004) 15-19. [4] T. Terao, H. Miura, A. Saika, J. Chem. Phys. 85 (1986) 3816-3826. [People in contribution]
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