Sites in α270 where Bpa residues were introduced for photo-crosslinking with ε to detect proximity to residues for which no structural information was obtained by the crystal structure in Figure 2. The location of the εCTS determined by the crystal structure is shown in yellow, with the nine-residue linker peptide in green. The Cβ atoms of residues at sites leading to efficient, less efficient or no crosslinking are highlighted in red, magenta and cyan, respectively.