Figure 4.

Electrondensity mapsandB-factor plotsof helix X in theR∗state. A, the F_O–F_C map obtained by modeling the R∗ structure with the closed conformer (green sticks) shown in (A) indicates that the electron density associated with helix X cannot be fully accounted by the pure closed conformer. The map is countered at 3.0 σ; the positive and negative features are shown in green and red, respectively. The residual positive features (green) were reproduced in (B) to demonstrate that they coincide well with the open conformer (blue sticks), indicating the coexist of the two conformers. The 2F_O–F_C map (countered at 1.25 σ) shown in (C) was obtained with the R∗ structure modeled with a mixed open conformer (55%) and closed conformer (45%); the absence of significant features in the F_O–F_C map (contoured at 3.0 σ, not shown) supports the reliability of the modeling. B, the average backbone B-factors (in Ų) of the R∗ structure obtained by modeling with the closed conformer (B) or with a mixed open (55%) and closed (45%) conformer (C). The comparable B-factor plot of the O∗ structure modeled with the open conformer is shown in (A) as a reference. The right inset shows a plot of ΔB as a function of the percent contribution of the closed conformer derived from the B-factor analysis described in the main text. The crossed dashed lines indicate that a mixed 55% open conformer and 45% closed conformer leads to ΔB = 0, where the averaged B-factors of the two conformers are comparable.