MD simulations
Partial charges for MHC atoms and NO3-ions were obtained from density functional theory (DFT) calculations using Gaussian 09 program. The Becke three parameters hybrid exchange-correlation functional (B3LYP)15 and the standard Gaussian-type basis set 6-311 G* were employed. All other force field parameters for atoms were extracted from PLOS-AA force field16. The initial configuration was solvated into a cubic water box with 63 Å on each side, containing 8,000 water molecules. The simple point charge (SPC) water model as explicit solvent was used for solvation17. Then 15 Cl- were added to balance system charges. The system energy was minimized for 500 steps using steepest descending method. The initial velocities were assigned to produce a Maxwell distribution at 298 K. After a 0.2 ns equilibrium, a 1 ns simulation was carried out with the NO3- ion frozen so that the MHC carbon chains would reach a balance. Finally, this system perform a 3 ns MD simulation. More details are in the SI.