FIGURE 9.

GWALP23-H⁺ 12 exits the bilayer. A, model to illustrate the transition of GWALP23-H⁺12 from its tilted transmembrane orientation (above pH 3) to a primary orientation at the surface of DOPC bilayer membranes (as the pH is lowered below 3). Carbons are green and nitrogens are blue. The midpoint for this transition is between pH 2.3 and 2.6 (see “Results”). The observed azimuthal rotation from ²H NMR experiments is illustrated for each of the helix orientations. B, dependence of RMSD of the fit to the ²H NMR data upon azimuthal rotation of the GWALP23-H⁺12 helix on the surface of a DOPC bilayer at low pH. The best fit occurs when the azimuthal rotation ρ is 296°, and the corresponding surface-oriented helix in A has a tilt τ of 81°.