Evidence for the Key Role of H3O(+) in Phospholipid Membrane Morphology.

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Journal Article
Langmuir : the ACS journal of surfaces and colloids, 2016
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This study identifies the importance of the phosphate moiety and H3O(+) in controlling the ionic flux through phospholipid membranes. We show that despite increasing the H3O(+) concentration when lowering the pH, the ionic conduction through phospholipid bilayers is reduced. Through modifying the lipid structure we show the dominant determinant of membrane conduction is hydrogen bonds between the phosphate oxygens on adjacent phospholipids. The modulation of conduction with pH is proposed to arise from the varying H3O(+) concentrations altering the molecular area per lipid and modifying the geometry of conductive defects already present in the membrane. Given the geometrical constraints that control the lipid phase structure of membranes, these area changes predict that organisms evolving in environments of different pH will select for different phospholipid chain lengths, such as is found for organisms near highly acidic volcanic vents (short chains) or in highly alkaline salt lakes (long chains). The stabilizing effect of the hydration shells around phosphate groups also accounts for the prevalence of phospholipids across biology. Measurement of ion permeation through lipid bilayers was made tractable using sparsely tethered bilayer lipid membranes (tBLMs) with swept frequency electrical impedance spectroscopy (EIS) and ramped DC amperometry. Additional evidence for the effect of pH change on lipid packing density is obtained from neutron reflectometry data of tethered membranes containing perdeuterated lipids.
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