Histidine, glutamine, MLF, and M2TM spectra were measured on a Bruker 400 MHz spectrometer, while glucose and plant cell wall spectra were measured on a Bruker 600 MHz NMR spectrometer. Typical radiofrequency field strengths were ~70 kHz for 1H decoupling and 50 kHz for 13C pulses. All 13C chemical shifts were externally referenced to the adamantane CH2 peak at 38.48 ppm on the TMS scale.
The pulse sequence for the relaxation-compensated PDSD experiment (Fig. 1 ) contains a z-filter before the evolution period, and the sum of the z-filter (tz) and mixing time (tm) is kept constant to compensate for T1 relaxation. Glutamine, histidine and MLF spectra were measured at room temperature using constant z-periods of 0.505 s, 1.005 s, and 1.505 s, respectively, and the MAS frequencies ranged from 7 to 10 kHz (Table S1 ). The M2TM spectra were measured with a constant z-period of 1.505 s at 273 K under 9 kHz MAS. The glucose and plant cell wall samples were measured using a constant z-period of 1.005 s under 8 kHz MAS. The temperature was 273 K for glucose and 253 K for the plant cell wall.
The difference spectra were obtained by subtracting a short mixing-time spectrum from a long mixing-time spectrum, with an adjustable scaling factor for the former. For the histidine difference spectrum, no scaling was applied. For glucose, the difference spectrum between 1.0 s and 0.2 s involved scaling the latter by 0.95 to give null intensities, while the difference spectrum between 200 ms and 20 ms involved scaling the 20 ms spectrum by 0.78 to remove the one-bond cross peaks. For MLF, a difference spectrum between 300 ms and 30 ms used a scaling factor of 0.35 to remove one-bond cross peaks. For influenza M2TM, a scaling factor of 0.70 was applied to the 100 ms spectrum before subtraction from the 1.5 s spectrum. For the plant cell wall sample, the relaxation-compensated difference between the 1.0 s and 0.2 s spectra used a scaling factor of 0.78 for the latter, while a regular PDSD difference spectrum used a scaling factor of 0.69 for the 0.2 s spectrum.
The pulse sequence for the relaxation-compensated PDSD experiment (
The difference spectra were obtained by subtracting a short mixing-time spectrum from a long mixing-time spectrum, with an adjustable scaling factor for the former. For the histidine difference spectrum, no scaling was applied. For glucose, the difference spectrum between 1.0 s and 0.2 s involved scaling the latter by 0.95 to give null intensities, while the difference spectrum between 200 ms and 20 ms involved scaling the 20 ms spectrum by 0.78 to remove the one-bond cross peaks. For MLF, a difference spectrum between 300 ms and 30 ms used a scaling factor of 0.35 to remove one-bond cross peaks. For influenza M2TM, a scaling factor of 0.70 was applied to the 100 ms spectrum before subtraction from the 1.5 s spectrum. For the plant cell wall sample, the relaxation-compensated difference between the 1.0 s and 0.2 s spectra used a scaling factor of 0.78 for the latter, while a regular PDSD difference spectrum used a scaling factor of 0.69 for the 0.2 s spectrum.
