Optimisation of 1 H PMLG homonuclear decoupling at 60 kHz MAS to enable 15 N- 1 H through-bond heteronuclear correlation solid-state NMR spectroscopy.

The Lee-Goldburg condition for homonuclear decoupling in 1 H magic-angle spinning (MAS) solid-state NMR sets the angle θ , corresponding to arctan of the ratio of the rf nutation frequency, ν 1 , to the rf offset, to be the magic angle, θ m , equal to tan -1 (√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a 1 H spectrum of 15 N-glycine with at θ = 30° for a ν 1 of ∼100 kHz at a 1 H Larmor frequency, ν 0 , of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor ( λ CS ) of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower ν 1 of 51 kHz, i.e. , a case where the nutation frequency is less than the MAS frequency, with θ = 18°, λ CS = 0.92. The ratio of the rotor period to the decoupling cycle time, Ψ = τ r /τ c , is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a ν 1 of ∼100 kHz also gives good performance in a 1 H spin-echo experiment, enabling implementation in a 1 H-detected 15 N- 1 H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial 15 N transverse magnetisation as generated by 1 H- 15 N CP is transferred back to 1 H using a refocused INEPT pulse sequence employing windowed 1 H decoupling. Such an approach ensures the observation of through-bond N-H connectivities. For 15 N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs 15 N to 1 H CP contact time), there is selectivity for the directly bonded NH 3 + moiety, while intensity is observed for the CH 2 1 H resonances in the double CP experiment. Two-dimensional 15 N- 1 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 3 + moieties demonstrating that the experiment could be used to distinguish resonances for different NH x groups.