State-dependent salt-bridge network analysis for R-to-Q mutants in VSD _I and VSD _II . (A and B) Comprehensive analysis for WT and mutants (R1, R2, and R3) in VSD_I (A) and VSD_II (B). Each panel depicts the map of electrostatic interactions at various states (up, intermediate, and down) within each VSD. Nodes represent residues of gating charges (blue), mutated residues (orange), countercharges (red), HCS (green), and lipids (black). Edges represent the salt bridges, with numbers indicating the occupancy percentage of the interaction during each state. Countercharges are labeled with residue IDs from Na_v1.5.