Low-power cross polarization in fast magic-angle spinning NMR experiments

We describe a low-power approach for heteronuclear cross-polarization (CP) at high magic-angle spinning (MAS) frequencies. It is based on second-order CP at the n = 0 Hartmann-Hahn condition. The mechanism for the polarization transfer in the low-power CP experiment relies on second-order cross terms between homonuclear and heteronuclear dipolar couplings. At a MAS frequency of 65 kHz, rf-field amplitudes below 10 kHz are sufficient to efficiently transfer polarization from H-1 to C-13. The low-rf field requirements of this approach make it well suited for the investigation of proteins and other temperature-sensitive samples without the risk of sample heating and degradation. (C) 2008 Elsevier B. V. All rights reserved.