Fig. 2.

Environment of the S4 segment (A) Profile of changes in the ΠO₂ parameters reflects a break in periodicity. Fourier transform power spectra of ΠO₂ shows that the peak angular frequency occurs at ∼99.8° for the upper-half of S4 corresponding to an α-helix, while it is at ∼118.1° for the lower-half suggesting a 3₁₀ helical structure. (B) An overlap of representative X-band CW-EPR spectra of spin-labeled mutants at corresponding arginines in S4 for the VSD from NaChBac (red) and KvAP (black). Blue line shows the location of immobile component of the spectra (Left). Position of arginine side-chains as seen in the crystal structure of the isolated-VSD of KvAP (center). An overlap of EPR spectra of the corresponding positions based on the alignment of NaChBac and KvAP VSDs (Right). The shaded boxes highlight positions that show maximal change in line shape.