Novel Three-Dimensional Correlation Experiments to Reveal Local Proton Networks under Fast Magic-Angle Spinning at Natural ¹³C Abundance　

¹H double quantum (DQ)/¹H single quantum (SQ) correlation solid-state NMR spectroscopy is widely used to obtain internuclear ¹H-¹H proximities, especially at fast magic angle spinning (MAS) rate > 60 kHz. However, ¹H signals are not well-resolved due to intense ¹H-¹H homonuclear dipolar interactions even at the attainable maximum MAS frequencies of ~100 kHz to date. We have recently introduced novel three-dimensional (3D) experiments to resolve the ¹H DQ/¹H SQ correlation peaks using the additional ¹³C dimension. Although the low natural abundance of ¹³C (1.1%) significantly reduces the sensitivities, the ¹H indirect measurements alleviate this issue and make this experiment possible. The two different implementations of ¹³C/¹H DQ/¹H SQ correlations and ¹H DQ/¹³C/¹H SQ correlations have been discussed and demonstrated using L-histidine.HCl.H₂O at natural abundance to reveal the local ¹H-¹H networks nearby each ¹³C. In addition, the complete ¹H resonance assignments have been achieved from a single 3D ¹³C/¹H DQ/¹H SQ experiment. We have also demonstrated the applicability of our proposed method on a biologically relevant molecule capsaicin.

The 2D ¹H DQ/¹H SQ spectrum of capsaicin (left) is too complex due to signal overlaps. On the other hand, the ¹³C-filtered spectrum (right) yields well-resolved peaks and thus signal assignment is possible. Both spectra were recorded at 70 kHz MAS. Please refer to the original paper for more detail.

Reference

• M. Malon, M. K. Pandey and Y. Nishiyama, J. Phys. Chem. B, 2017, 121, 8123-8131.

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