Magic angle Lee-Goldburg frequency offset irradiation improves the efficiency and selectivity of SPECIFIC-CP in triple-resonance MAS solid-state NMR

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 700 MHz and 900 MHz 1 H resonance frequencies, respectively. For the single 13 Cα 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 Cα 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.