The cross-linking profile was similar for the three complexes at F160, however, cross-linking of the mutants was concentrated on P125 and F126, whereas, when phenylalanine was exchanged for leucine, cross-linking to F126 was abolished. Notably, cross-linking to the neighbouring amino acids 124-127 was also mostly lost (Fig. ), confirming the primary role of the aromatic side chain in mediating the cross-linking reactions with flanking amino acids at positions 124, 125 and 127.