Accurate determination of C-H and N-H distances for unlabeled molecules by ultrafast solid-state NMR spectroscopy

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NM160005E

Cross-Polarization with Variable Contact-time (VC-CP) experiment has been used to measure dipolar interactions corresponding to C-H and N-H distances in solid samples [1]. If the VC-CP experiment using direct 13C or 15N detection (Fig. 1a) is performed at Magic Angle Spinning (MAS) at spin rates exceeding 60 kHz, it allows accurate measurement of dipolar distances [2,3]. However, if the pulse sequence is extended in such a way that 1H signal is detected (Fig. 1b), a S/N gain of approx. 2.5 or a time gain of approx. 6 can be obtained [4]. This is demonstrated on U-[13C, 15N] L-alanine in Fig. 2.  Spectra shown in Fig. 2a and Fig. 2b were recorded with 13C detection, while spectra in Fig. 2c and Fig. 2d were recorded with 1H detection [4].


Fig.1 CP-VC pulse sequences with (a) 13C/15N detection, or (b) 1H detection.

Fig.2 Slices taken in 2D CP-VC spectra of U-[13C, 15N] L-alanine recorded with 13C (a, b) or 1H (c, d) detection, and (a, c) 70 kHz or (b, d) 100 kHz MAS. Only the peak at 3.6 ppm (1H) is shown.

The sensitivity of 1H detected VC-CP experiment at ultrafast MAS is high enough so that samples at natural 13C and 15N could be measured. This is clearly evidenced on 13C, 15N-natural abundance L-histidine∙HCl∙H2O in Fig. 3 (1H-13C) and Fig. 4 (1H-15N). The spectra also demonstrate another advantage of ultrafast MAS which is resolved 1H resonances of small organic molecules [4].


Fig.3 2D CP-VC H-C-H spectrum of 13C-natural abundance L-histidine∙HCl∙H2O recorded at 70 kHz MAS. The 1H MAS spectrum is shown on top.

Fig.4 2D CP-VC H-N-H spectrum of 15N-natural abundance L-histidine∙HCl∙H2O recorded at 70 kHz MAS. The 1H MAS spectrum is shown on top.

References

[1] P. Paluch, T. Pawlak, J.P. Amoureux, M.J. Potrzebowski, J. Magn. Reson. 233 (2013) 56–63.
[2] P. Paluch, J. Trébosc, Y. Nishiyama, M.J. Potrzebowski, M. Malon, J.P. Amoureux, J. Magn. Reson. 252 (2015) 67-77.
[3] Application Note NM140017 (http://www.j-resonance.com/en/application/?appid=NM-140017) .
[4] Y. Nishiyama, M. Malon, M.J. Potrzebowski, P. Paluch, J.P. Amoureux, Solid State Nucl. Magn. Reson. 73 (2016) 15-21.

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