Histidine tautomers were checked for all the structures using the H++ webserver 39 at the pH of the respective crystallization experiments. In PDB: 1OTS, side chains E113 (Chains A and B) and D417 (Chain A) were manually protonated and neutrally charged which is supported by experiment. The UBA domain/Ubiquitin and Barnase/Barstar systems were simulated using the DES-Amber-SF0.9 force field which is optimized for soluble protein-protein complexes.