Conformational differences in the selectivity filter region of Na_VSp1 and Na_VSp1-P. Spin-normalized CW-EPR spectra for positions along the turret loop, P1 helix, and P2 helix in Na_VSp1 (red) and Na_VSp1-P (black). These regions are color-coded on the Na_VAb structure (PDB ID 3RVY). Dotted lines marked as “i” and “m” represent the immobile and mobile components of the spectra, respectively. The two components may arise from two different rotameric orientations of the spin labels and/or from two conformational states of the protein in equilibrium. Arrows highlight positions with the most prominent differences in line shapes in the two channels.