Topological Changes in Lipid Membrane Surfaces Induced by Lowering of pH in Isotonic Conditions

Through the use of fluorescence probes, FTIR spectroscopy, zeta potential measurements, and surface pressure–area isotherms, it is demonstrated that lowering the pH increases the hydrophobicity of DPPC lipid vesicles in the gel state. This effect is noticeable at pH 2. The increase in hydrophobicity is comparable to that produced by hypertonic stress, in which vesicles shrink due to a decrease in volume and an increase in curvature. However, in the case of pH, no effect on vesicle size, area per lipid, and hydrocarbon chain packing is observed. It is concluded that the protonation of phosphate groups alters the electrical repulsion affecting the dipole potential due to the alignment of CO and water dipoles without deforming the bilayer, which results in the exposure of membrane hydrophobic regions, as detected by fluorescent probes. These findings highlight the role of proton-lipid interactions in membrane function under extreme conditions and have potential applications in biotechnology and medicine.