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Solution pH Alters Mechanical and Electrical Properties of Phosphatidylcholine Membranes: Relation between Interfacial Electrostatics, Intramembrane Potential, and Bending Elasticity

Yong Zhou ^* and Robert M. Raphael 

^* Department of Biochemistry and Cell Biology, Department of Bioengineering, Rice University, Houston, Texas

Correspondence: Address reprint requests to R. M. Raphael, Tel.: 713-348-3494; E-mail: rraphael{at}rice.edu.

Solution pH affects numerous biological processes and some biological membranes are exposed to extreme pH environments. We utilized micropipette aspiration of giant unilamellar vesicles composed of 1-stearoyl-2-oleoyl-phosphatidylcholine to characterize the effect of solution pH (2–9) on membrane mechanical properties. The elastic area compressibility modulus was unaffected between pH 3 and 9 but was reduced by 30% at pH 2. Fluorescence experiments utilizing the phase-sensitive probe Laurdan confirmed gel-phase characteristics at pH 2, explaining the reduction of membrane elasticity. The membrane bending stiffness, k_c, increased by 40% at pH 4 and pH 9 over the control value at pH 6.5. Electrophoretic mobility measurements indicate that these changes are qualitatively consistent with theoretical models that predict the effect of membrane surface charge density and Debye length on k_c, substantiating a coupling between the mechanical and interfacial electrical properties of the membrane. The effect of pH on intramembrane electrical properties was examined by studying the spectral shifts of the potentiometric probe di-8 ANEPPS. The intramembrane (dipole) potential (_d) increased linearly as the solution pH decreased in a manner consistent with the partitioning of hydroxide ions into the membrane. However, changes in _d did not correlate with changes in k_c. These mechanical and electrical studies lead to the conclusion that the effect of pH on membrane bending stiffness results from alterations in interfacial, as opposed to intramembrane, electrostatics.