Figure 11.

Experimental FSLG decoupled proton spectra of natural-abundance L-histidine recorded at a proton resonance frequency of 500  $\mathrm{MHz}$ , using a 1.9  $\mathrm{mm}$ Bruker MAS probe. All spectra were recorded using a 2  $\mathrm{ms}$ I-BURP-2 pulse with a modulation frequency of 100  $\mathrm{kHz}$ (corresponding to the nominal rf field amplitude as determined using a nutation spectrum) for the $B_{\mathrm{1}}$  selection . The effective rf field strength along the magic angle during the FSLG decoupling was set to 125 (a, b), 185.2 (a), and 250  $\mathrm{kHz}$  (b) at MAS frequencies of 14 and 28  $\mathrm{kHz}$ . The arrows indicate the positions of the carrier frequency. No significant improvements are observed for stronger $B_{\mathrm{1}}$ fields and higher spinning frequencies, indicating that the residual linewidth is not decoupling limited.