High-efficiency low-power C-N cross polarization in MAS NMR.

Biomolecular solid-state magic angle spinning (MAS) NMR spectroscopy frequently relies on selective C-N magnetization transfers, for various kinds of correlation experiments. Introduced in 1998, spectrally induced filtering in combination with cross polarization (SPECIFIC-CP) is a selective heteronuclear magnetization transfer experiment widely used for biological applications. At MAS frequencies below 20 kHz, commonly used for C-detected MAS NMR experiments, SPECIFIC-CP transfer between amide N and C atoms (NCA) is typically performed with radiofrequency (rf) fields set higher than the MAS frequency for both C and N channels, and high-power H decoupling rf field is simultaneously applied. Here, we experimentally explore a broad range of NCA zero-quantum (ZQ) SPECIFIC-CP matching conditions at the MAS frequency of 14 kHz and compare the best high- and low-power matching conditions with respect to selectivity, robustness, and sensitivity at lower H decoupling rf fields. We show that low-power NCA SPECIFIC-CP matching condition gives rise to 20% sensitivity enhancement compared to high-power conditions, in 2D NCA spectra of microcrystalline assemblies of HIV-1 CA-SP1 protein with inositol hexakis-phosphate (IP6).