Solid-state NMR spectra of TpoR TM-JM peptides. Only the carbonyl region of the ¹³C spectrum is shown. The spectra of lipid alone (dashed line) exhibit only the resonance of the lipid acyl chain carbonyl due to the natural abundance of ¹³C. The specific ¹³C labels at the backbone carbonyl of L501, L503, L505, and L506 are observed at approximately 175 ppm (solid line), characteristic of helical secondary structure extending from L501 to F510.