Matsuki, Yoh, Tomoaki Sugishita, and Toshimichi Fujiwara. “Surface-Only Spectroscopy for Diffusion-Limited Systems Using Ultra-Low-Temperature DNP MAS NMR at 16.4 T.” The Journal of Physical Chemistry C, August 12, 2020, acs.jpcc.0c04873.
The conventional dynamic nuclear polarization (DNP) technique at T ∼ 100 K can enhance sensitivity of magic-angle spinning (MAS) NMR over 100-fold for standard samples containing urea/proline under high-ﬁeld conditions, B0 = 9.4−16.4 T. In the scene of real applications, however, the achievable enhancement is often much lower than for urea/proline due to faster 1H relaxation (T1H) promoted by molecular segmental ﬂuctuations and methyl group rotations active even at low temperatures, hindering eﬃcient polarization diﬀusion within the system. Here, we show at 16.4 T that ultra-low temperature (T ≪ 100 K) provides a general way to improve the DNP eﬃciency for such diﬀusion-limited systems as we demonstrate on a microcrystalline sample of the tripeptide N-f-MLF-OH. In a further step, the hyperpolarization localized at the crystal surface enabled “surface-only” spectroscopy eliminating background signals from the crystal core. The surface-only data, rather than the currently popular surface-enhanced data, would prove to be useful in many applications in biological and materials sciences.