Figure 3.

The precession entropy gain associated with TM helix tilting in the membrane. (A) Schematic illustration of the spherical surface area corresponding to the precession entropy of a hypothetical helix that spans the membrane vertically. Helix librations around the membrane normal, with a maximum amplitude of β, generate rotational entropy that is proportional to the dark cap-like surface area. (B) The precession entropy of a helix, which is tilted at an angle α from the normal, is larger (larger area). Assuming that β is independent of α, the rotational entropy of the tilted helix corresponds to the dark belt-like area. The helix is represented as a cylinder with the hydrophobic core in purple and hydrophilic termini in white. The helix’s principal axis is marked by the solid line. L is the native (peptide-free) width of the hydrocarbon region of the membrane. P_eff is the length of the portion of the helix’s hydrophobic core that spans the hydrocarbon region of the membrane. R is the radius of the helix rotational sphere. H is the height of the dark sector of the sphere.