Interface definition and extraction of interfaces from the Protein Data Bank:
interface can be defined as the set of amino acids which represents a region that links
two polypeptide chains in a protein structure by non-covalent interactions.
Residues interacting with each other across the binding region form the interface between two chains.
Interface residues are selected according to the closeness of two residues, one from each chain.
We defined two types of residues in two chain interfaces; interacting and nearby residues.
Interacting residues are in direct contact with each other (each from one chain); nearby residues
are in the vicinity of the interacting residues in the same chain (Tsai, 1996)