Figure 2.

The variation in V_rev (A), I_6.2/I_7.5 (B), Inh (C), and PI(n) (D) as a function of position in the TM helix sequence. The smooth curve through the PI(n) data (D) represents the best fit to a cosine function (Eq. 2) generated using kaleidagraph with initial parameters A = 0.5, B = 1.0, C = 0, and P = 3.6. The dashed lines in A–C show peak positions expected for a 3.5-residue periodicity. Labels a and d in D represent heptad positions in the sequence. The perturbational index [PI(n)] is calculated from the three individual parameters. To combine these three parameters into PI(n), the individual values were normalized so their average values would be zero, then weighted according to their standard deviations. The values of the three parameters for each position were then averaged, and the combined PI(n) scale was normalized to range from 0 to 1. Because the periodic variation of Inh was limited to approximately the N-terminal half of the TM region, only residues 26–38 were included in calculating PI(n).