Labelled peptides were packed into kevlar inserts (Bruker) which in turn fit into standard zirconia 4 mm rotors (Bruker).
13C and 15N cross polarization (13C CP) experiments: The standard CP sequence in the Bruker pulse program library was used with 1H 90° pulse length 2.5 μs, contact time 2.5 ms; during the contact pulse a ramped pulse was applied on 1H with average spin lock field of 70 kHz. During acquisition, SPINAL64 decoupling at field strength 100 kHz was applied on 1H. Chemical shifts were referenced to external glycine (α polymorph), using the methylene signal at 43.1 ppm relative to TMS (13C).
Variable temperature experiment: using a Bruker Variable Temperature Unit, the temperature of the sample was varied from 242 K to 342 K in steps of 5 K. At each temperature, the sample was allowed to equilibrate for up to 10 minutes before a standard 13C CP-MAS spectrum was recorded.