bind of the intermediate states in the parallel hexamer simulations again reveals which dissociation event contributes the most to the total binding free energy (Figure ). An interesting trend emerges for the pH-switchable system: at low pH, the ∆G • bind of the hexamer and pentamer dissociation steps are negative, whereas the ∆G Turning to the antiparallel tetramer simulations, the ∆G • bind contributions (Figure ) reveal that at low pH each helix separation step contributes to a similar small extent to the ∆G ). This implies that interactions of similar type and magnitude are disrupted in each helix dissociation step. The intermolecular interactions primarily affected by changes in the pH and the subsequent changes in the protonation states of titratable residues are salt bridges and hydrogen bonds. The CC-Hex*-L24E antiparallel tetramer however does not have salt bridges or hydrogen bonds in its structure. It has two distinct interface types, which are termed 'wide' and 'narrow'. However, these interfaces are flanked by pairs of the same residue from neighbouring helices, either Lys or Glu. As a consequence, the tetrameric CC-Hex*-L24E has alternating seams of positive and negative charge (or lack of charge when the tetramer is modelled at low pH), where no salt bridges can be formed (Figure ). This explains why the ∆G ). The ∆G • sep of the hexamer, tetramer and dimer dissociation steps are different, whereas the those of the pentamer and trimer steps are equal, within error.