Different cations exhibit distinct numbers of coordination oxygens from amino acid carbonyls and water. (A) Time-averaged number of coordinating oxygen atoms for sampled cation species in different SF binding sites drawn from MD simulations conducted at voltages between −700 and −250 mV in pure-cationic solution (900 mM): −700 mV (n = 4) for Li⁺, −500 mV (n = 2) for Na⁺, −250 mV (n = 3), and −500 mV (n = 4) for K⁺, −250 mV (n = 3), −500 mV (n = 4), and −700 mV (n = 1) for Rb⁺, −150 mV (n = 2), −250 mV (n = 2), −500 mV (n = 3), and −700 mV (n = 2) for Cs⁺. For further sampling with Li⁺ and Na⁺, a subset of mixed-cationic simulations was also included: −700 mV (n = 1) for Li⁺/Na⁺, −700 mV (n = 3) for Li⁺/Na⁺/K⁺, and −500 as well as −700 mV (n = 1 each) with Na⁺/K⁺. Error bars indicate the standard deviation. (B–E) Simultaneous binding by carbonyl oxygens and H₂O oxygens in Sc is depicted by representative trajectory frames for Cs⁺ in Sa (B), Li⁺ (C), and Rb⁺ I (D), as well as Cs⁺ in Sb (E). Two opposite monomers are shown in the tube representation, carbonyl oxygens involved in cation binding are depicted as red spheres. Dashed gray lines indicate the oxygen plane.