Coulomb energies of protein–protein complexes with monopole-free charge distributions

From an analysis of Coulomb energy distributions of a large set of protein–protein complexes we show that the positive tail in the energy distribution disappears when the monopole–monopole term, the only energy term independent of inter-subunit orientations, is removed. This indicates that unfavorable Coulomb energies associated with subunit orientations are excluded in protein–protein complexes. The overall result remained unchanged when solvent effects were included. Our results have important bearing on the restriction of subunit orientations in protein–protein complexes and complement a recent work [K. Brock, K. Talley, K. Coley, P. Kundrotas, E. Alexov, Optimization of electrostatic interactions in protein–protein complexes, Biophys. J. 93 (2007) 3340–3352.] which showed that Coulomb energy of interaction in protein–protein complexes is sequence-optimized.