Distributions of polyamine nitrogen atoms and glutamate side chain oxygen atoms at the cytosolic end of the MthK pore, when N_L is interacting with T59-OH. Atom occupancy histograms corresponding to the trajectories in Fig. 8, with polyamine and voltage as indicated. These illustrate that SPD³⁺ is typically within hydrogen bonding distance of only E92 at either 0 mV (top left) or 500 mV (top right), based on differences in the peak positions (<3 Å). In contrast, SPM⁴⁺ is typically within hydrogen bonding distance of both E92 and E96 at either 0 mV (bottom left) or 500 mV (bottom right). These results are consistent with electrostatic bonds between MthK pore-lining side chains and SPM⁴⁺ that are greater than those with SPD³⁺. Dashed lines indicate error estimates.