In the hexamer step, the more negative ∆G parallel conformation, it is unlikely that this 'receptor' complex would want to re-arrange in the bulk to form a more coiled-coil-like tetramer structure, given that the antiparallel tetramer state is accessed at high pH only. With regards to the hexamer state, while at first glance a negative ∆G RM SD bound + ∆G RM SD unbound value seems counter-intuitive given the hexamer at low pH is the experimentally observed structure, it should be noted that the E and F helices show C-terminal fraying, with residues missing from the structure; 29 as a consequence, the hexamer likely has increased conformational plasticity and has the potential to re-arrange its helices in the bound state to maximise hydrogen bond formation at the core. This may be less likely at high pH, due to repulsions from the negatively charged Glu residues at the core, and as such the ∆G RM SD bound + ∆G RM SD unbound value for the hexamer step at high pH is positive. At high pH, the ∆G RM SD bound + ∆G RM SD unbound is negative for the tetramer, trimer and dimer steps; the fact that it is negative for the tetramer step is consistent with the fact that the 'open' parallel conformation must be unstable and would like to re-arrange to form the experimentally observed antiparallel tetramer.