(A) Protein concentration: 2 mg/ml, leucine concentration: 1 mM. The spectrum is dominated by the signal arising from the free unbound leucine, at ~42 ppm. Spectrum was recorded on a 400 MHz Bruker shielded wide bore magnet equipped with a 3.2 mm MAS HCN operating at ~100 K. Spinning rate: 8000 kHz. CP contact time: 1000 us, recycling delay: 3 s. 24 k scans were required to record presented spectrum. (B) 15N CP/MAS spectra from dry L-leucine powder (top panel) and from lyophilized leuT samples (bottom panel). Free 15N L-Leucine has a distinct chemical shift at 118 ppm. For the LeuT WT NaCl preparation (red) several additional peaks appear around the dominating free state peak. Though these peaks vary slightly in intensity when compared to LeuT WT KCl (dark). These peaks do not originate from structural leucine. Spectra were recorded on a 700 MHz Bruker shielded wide bore magnet equipped with a 4 mm MAS HCN probe operating at 298 K. Spinning rate: 12500 KHz, CP contact time: 1500 us, recycling delay: 2.5 s. 65 K scans were required to record presented spectra.