Efficient and Robust Heteronuclear Cross-Polarization for High-Speed-Spinning Biological Solid-State NMR Spectroscopy

We present a new and highly efficient approach for heteronuclear coherence transfer in solid-state NMR spectroscopy under high-speed spinning conditions. The so-called (CP)-C-RESPIRATION experiment exploits phase-alternated recoupling on only one of the two rf channels intertwined in a synchronized train of short rf pulses on both channels. The method provides significantly higher efficiencies than state-of-the art techniques including ramped and adiabatic cross-polarization experiments with long durations of intense rf irradiation. At the same time, it is easier to setup experimentally and significantly more robust toward imperfections such as rf inhomogeneity, misadjustments, and sample-induced variations in the rf tuning. The method is described analytically, numerically, and experimentally for biological solids. We demonstrate sensitivity gains of factors of 1.3 and 1.8 for typical H-1 -> N-15 and N-15 -> C-13 transfers and a combined gain of a factor of 2-4 for a typical NCA experiment for biological solid-state NMR