Charge-induced tilt in ordered-phase phosphatidylglycerol bilayers evidence from X-ray diffraction.

X-ray diffraction studies have been performed, as a function of water content, on dipalmitoyl phosphatidylglycerol bilayers, both in the charged state at pH 8.0 and in the protonated state at pH 1.5, using buffers of 1.5 M salt concentration. Measurements were made at 20 degrees C, and the high-angle reflections indicated that the bilayers were in the ordered phase at both pH values. Lamellar diffractions were observed under all conditions studied. THe lamellar repeat reached a limiting value of 62.4 A (6.24 nm) at a water/lipid ratio of 0.24 at pH 8.0, and a limiting value of 67.3 A (6.73 nm) at a water/lipid ratio of 0.22 at pH 1.5. The area per lipid molecule in the plane of the bilayer, deduced from the bilayer thickness and the lipid partial specific volume, is 48 A2 (0.48 nm2) at pH 8.0 and 37 A2 (0.37 nm2) at pH 1.5. The area per molecule in the plane perpendicular to the chain axes, deduced from the X-ray short spacings, is 40.5 A2 (0.405 nm2) at pH 8.0 and 39.2 A2 (0.392 nm2) at pH 1.5. Thus the lipid molecules are tilted by approx. 30 degrees relative to the bilayer normal at pH 8.0, but are essentially untilted at pH 1.5.