Residue contact matrix showing the frequency of the contact between two residues at the interface. a: Non-obligatory non-polar interactions, b: obligatory non-polar interactions c: non-obligatory polar interactions d: obligatory polar interactions. The residues contributed by the obligatory or non-obligatory chain of the complex is represented in rows. The residues from the partner chain (for which the assignment of obligatory or non-obligatory is ambiguous) is shown in columns. Colour gradation: white- No interaction; Cyan to black- increasing gradation of interaction with normalised frequency varying between 0–14 in discrete steps.