All models were checked using the Protein Preparation Wizard module to ensure no polypeptide chain atoms were missing, and to assign the relevant protonation states for all titratable residues. Protonation and tautomeric states were assigned using PropKa at pH 5.0 . To neutralize the overall charge of each model, chlorine ions were added to the surface and subject to energy minimization. Additional disulfide bond crosslinking was performed as described below. A mirror image of the surface is added with respect to the Z-axis to handle periodic boundary conditions. To create a larger surface, the system was periodically replicated in the +/-X and +/-Y directions.