Optimisation of H PMLG homonuclear decoupling at 60 kHz MAS to enable N-H through-bond heteronuclear correlation solid-state NMR spectroscopy.

The Lee-Goldburg condition for homonuclear decoupling in H magic-angle spinning (MAS) solid-state NMR sets the angle , corresponding to arctan of the ratio of the rf nutation frequency, , to the rf offset, to be the magic angle, , equal to tan(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a H spectrum of N-glycine with at = 30° for a of ∼100 kHz at a H Larmor frequency, , of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor () of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower of 51 kHz, , a case where the nutation frequency is less than the MAS frequency, with = 18°, = 0.92. The ratio of the rotor period to the decoupling cycle time, = , is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a of ∼100 kHz also gives good performance in a H spin-echo experiment, enabling implementation in a H-detected N-H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial N transverse magnetisation as generated by H-N CP is transferred back to H using a refocused INEPT pulse sequence employing windowed H decoupling. Such an approach ensures the observation of through-bond N-H connectivities. For N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs N to H CP contact time), there is selectivity for the directly bonded NH moiety, while intensity is observed for the CHH resonances in the double CP experiment. Two-dimensional N-H correlation MAS NMR spectra are presented for the dipeptide β-AspAla and the pharmaceutical cimetidine at 60 kHz MAS, both at natural isotopic abundance. For the dipeptide β-AspAla, different build-up dependence on the first spin-echo duration is observed for the NH and NH moieties demonstrating that the experiment could be used to distinguish resonances for different NH groups.