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 [Image: see text] 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 [Image: see text] 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 [Image: see text] (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.