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

The Lee–Goldburg condition for homonuclear decoupling in 1H magic-angle spinning (MAS) solid-state NMR sets the angle θ, corresponding to arctan of the ratio of the rf nutation frequency, ν1, to the rf offset, to be the magic angle, θm, equal to tan−1(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a 1H spectrum of 15N-glycine with at θ = 30° for a ν1 of ∼100 kHz at a 1H Larmor frequency, ν0, of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor (λCS) of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower ν1 of 51 kHz, i.e., a case where the nutation frequency is less than the MAS frequency, with θ = 18°, λCS = 0.92. The ratio of the rotor period to the decoupling cycle time, Ψ = τr/τc, is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a ν1 of ∼100 kHz also gives good performance in a 1H spin-echo experiment, enabling implementation in a 1H-detected 15N–1H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial 15N transverse magnetisation as generated by 1H–15N CP is transferred back to 1H using a refocused INEPT pulse sequence employing windowed 1H decoupling. Such an approach ensures the observation of through-bond N–H connectivities. For 15N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs 15N to 1H CP contact time), there is selectivity for the directly bonded NH3+ moiety, while intensity is observed for the CH21H resonances in the double CP experiment. Two-dimensional 15N–1H 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 NH3+ moieties demonstrating that the experiment could be used to distinguish resonances for different NHx groups.

Following 1, in the second echo period (2), the antiphase 1 H coherence is converted into in-phase ˆx I that is then detected during acquisition (t2): in 2 sin 2 sin 2 . Figure S1. A stacked representation of a two-variable optimization (see Fig. 3a) of both LG_expt (in steps of 0.25 s) and 1 in a 1D 1 H-CRAMPS (0 = 500 MHz) MAS (r = 60 kHz) NMR experiment of 15 N-glycine, in which windowed 5 xx mm PMLG was applied with tilt = 0.54 s and a 1 H transmitter offset of 0.6 kHz, corresponding to the data shown in Figure 3a of the main text. 8 co-added transients were collected for each optimization point.

S2. Optimisation of PMLG 1 H homonuclear decoupling on 15 N-glycine
On the right, slices from the optimization are shown with the associated LG_expt and 1. The relative intensity of the NH3 + peak with respect to the best 1 H homonuclear decoupling performance at 2LG_expt = 6.25 s and 1 = 110 kHz is stated.
S3 Figure S2. Zoom of the region between LG_expt = 5.5 s -7.5 s for the two-variable optimization of LG_expt (in steps of 0.25 s) and 1 in a 1D 1 H-CRAMPS (0 = 500 MHz) MAS (r = 60 kHz) NMR spectrum of the 15 N-glycine a) CH2 and b) NH3 + peak intensity, corresponding to the data shown in Figure 3a of the main text. Windowed 5 xx mm PMLG was applied with w = 7.20 μs, tilt = 0.54 s and a 1 H transmitter offset of 0.6 kHz. 8 co-added transients were collected for each optimization point for a recycle delay of 3 s.

S3. Optimisation of tilt pulses via the NH3 + signal intensity in a 1D CRAMPS experiment of 15 N-glycine
The duration of the tilt pulses, tilt, was optimised in a two-variable optimization with LG_expt, The 1 H CRAMPS spectrum on the right in Figure S3c was acquired with the same nutation frequency and offset, but with no tilt pulses and 2LG_expt was chosen to be 7 µs such that the cycle time and hence  are the same. The intensity of the NH3 + peak obtained with windowed 5 xx mm PMLG at LG_expt = 6.20 µs and tilt = 0.54 µs is within 5% of that obtained without tilt pulses. Note, however, that the peak widths for 5 xx mm PMLG without tilt pulses are 235 Hz for the NH3 + peak, and 224 Hz and 231 Hz for the CH2 peaks. After scaling (CS = 0.80), the FWHM become 294 Hz, 280 Hz and 289 Hz, respectively, which is ~15 Hz larger than those stated in Table 3 for windowed 5 xx mm PMLG with LG_expt = 6.20 µs and tilt = 0.54 µs. PMLG , respectively. The States-TPPI method was employed to achieve sign discrimination in the indirect dimension.

S5. Cimetidine
Here, the normalized intensity is related to the respective maximum intensity for each peak , i.e. the maximum intensity is equal to 1 for all the resonances. However, note that the NH15 proton signal intensity is ~30 % of that of NH3. kHz. Fits to an exponential decay function are shown, with the spin-echo dephasing times, T2', as listed in Table   S1. 8 transients were co-added for a recycle delay of 5 s.