While the PBC rearrangement is a relatively small structural change, this shift is sufficient to open up the space between the hinge and the PBC, moving L273 out and up, and allowing F300 of the hinge to take its place. The hinge "melts" at the C-terminus, swinging the lid region towards the cAMP binding site. The change in the C-terminus of the hinge conformation is shown through the root-mean-square-fluctuations (RMSF) of each residue in Figure . Residues 303-309 show increasingly higher fluctuations in the cAMP system. side-chain is too sterically hindered to move upwards, and instead remains in the space between the hinge and the PBC, shown in Figure . This allows the tryptophan to π-stack with F300, which in turn further stabilizes the helical conformation of the hinge. This causes the EPAC1 L273W -cAMP complex to favour the intermediate state. cAMP still acts on the PBC, pulling it into the "in" conformation (as well as the large W273 pushing it out), but the change in interaction between W273 and F300 means that the "out" hinge conformation is even further stabilized. In Figure , the L273W hinge fluctuations are lower than even in apo EPAC1. This increased stabilization of the hinge also allows for stronger ionic latch interactions at the RR/CR interface (Supplementary Figure ). The "inactive" microstates highly populated by this system are observed at higher PBC RMSD values (Figure ) because the larger W273 residue side chain separates further the PBC and hinge regions.