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

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.