 This paper investigated the role of electrostatic forces in protein-protein interactions. It was found that electrostatic forces play a significant role in the formation of transient protein complexes, but have little effect on more stable complexes. The authors developed a new method to measure electrostatic complementarity, which can distinguish between transient and permanent protein complexes. This method is based on expanding the electrostatic potential on a basis of 2D orthogonal polynomials, allowing for a better understanding of the role of electrostatic forces in protein-protein interactions. This article was authored by Greta Grassman, Lorenzo Di Rienzo, Giorgio Gosti, and others.