Distance calculations suggest a salt bridge could form in mtre1, msctt and gtre1 model systems. Distances between oxygen atoms of the carboxylate group of aspartic acid (D) (OD1, OD2) and nitrogen atoms of the guanidium group of arginine (R) (NH1, NH2) are plotted. N-O distances of 3.2 Å (dotted lines) or less are capable of forming a salt bridge. The N-O distances are close enough to form a salt bridge transiently in mtre1 and msctt. There is potential for a salt bridge to be consistently present in the gtre1 model system. The N-O distances are too great in gsctt to form a salt bridge at any point during the simulation.