Acidic pH Modulates Headgroup Orientation and Packing in Bis(monoacylglycero)phosphate Bilayers

Bis-(monoacylglycero)-phosphate (BMP) possesses an atypical headgroup structure, and its naturally occurring 2,2'-isomer rapidly rearranges to the more stable 3,3'-form in aqueous buffers. Because the BMP is a major component of endosomal and lysosomal membranes, understanding its intrinsic physicochemical behavior is broadly relevant to the organization of acidic intracellular interfaces. To quantify how acidity modulates the physicochemical organization of 3,3'-BMP membranes, we characterized model bilayers across a controlled pH range by using electrophoretic mobility, fluorescence spectroscopy, optical microscopy, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and atomistic molecular dynamics (MD) simulations. Below pH 5, BMP vesicles show a marked decrease in surface charge, a reduction in area per lipid, and diminished hydration at the polar-apolar interface. Simulations reveal that protonation enables transient flipping of the phosphate group toward the hydrophobic core, generating a pH-dependent coexistence of headgroup orientations absent in the deprotonated state. Experimentally, these changes manifest as reduced vesicle sizes and the emergence of multilamellar protrusions and locally folded membrane regions. The combined results demonstrate how small variations in the BMP charge state generate amplified structural responses at the bilayer level, establishing a molecular mechanism by which protonation governs packing, hydration, and curvature in BMP-rich membranes.