The heterodimeric AB zipper. (a) Mean energy as a function of pRMSD over both chains and the sum of individual bRMSDs. The direction of the energy gradients implies that a system with two folded monomers is energetically favorable compared to unfolded monomers. The proper dimeric form is the area closest to the origin, and has a lower energy. (b) Mean energy of all states in which both chains have bRMSD < 5 Å, shown as a function of the dimer RMSD measure pRMSD.