Magic angle Łee\-\Goldburg\ frequency offset irradiation improves the efficiency and selectivity of \SPECIFIC\-\CP\ in triple-resonance \MAS\ solid-state \NMR\
J. Magn. Reson., (September 2014)DOI: 10.1016/j.jmr.2014.06.012
Abstract
The efficiency and selectivity of SPECIFIC-CP, a widely used method for selective double cross-polarization in triple-resonance magic angle spinning solid-state NMR, is improved by performing the tangential-shaped (13)C irradiation at an offset frequency that meets the Lee-Goldburg condition (LG-SPECIFIC-CP). This is demonstrated on polycrystalline samples of uniformly (13)C, (15)N labeled N-acetyl-leucine and N-formyl-Met-Leu-Phe-OH (MLF) at 700MHz and 900MHz (1)H resonance frequencies, respectively. For the single (13)C$\alpha$ of N-acetyl-leucine, relative to conventional broad band cross-polarization, the SPECIFIC-CP signal has 47\% of the intensity. Notably, the LG-SPECIFIC-CP signal has 72\% of the intensity, essentially the theoretical maximum. There were no other changes in the experimental parameters. The three (13)C$\alpha$ signals in MLF show some variation in intensities, reflecting the relatively narrow bandwidth of a frequency-offset procedure, and pointing to future developments for this class of experiment.