Figure 1.

Scheme of a double-mutant cycle for a 2D lattice model protein. Two residues, i and j, are mutated (the mutations are designated by B on a dark background) either singly or in combination. ΔG(i,j→B,j) and ΔG(i,B→B,B) are the respective free energy changes upon mutation of residue i when residue j is present and when it has also been mutated. If these free energy changes are equal to each other then residues i and j are not coupled. Otherwise, residues i and j are energetically coupled. The same is true for the difference between the free energy changes ΔG(i,j→i,B) and ΔG(B,j→B,B). In this scheme, residues i and j form a direct contact in the native structure of the wild-type sequence. The double-mutant cycle method can be applied, however, also for residues that are distant in space in the native structure as carried out in the paper.