A topic emerging roughly 30 years ago and engendering an incompletely resolved controversy is reviewed in this article: the relatively high permeability and pH independence associated with H+/OH− passive movements across lipid membranes. We summarize the expected characteristics of simple H+/OH− diffusion and those of a reaction between H+ and OH− being attracted from opposite surfaces and condensing in an interfacial zone of the membrane. An interfacial H+/OH− reaction mechanism gives the experimentally observed behavior of an H+/OH− flux that is independent of the pH measurement range. This mechanism assumes that H+ and OH− within the interfacial zone become electrostatically aligned on opposite sides of the hydrophobic membrane core. Electrostatic attraction and charge delocalization among a small cluster of water molecules surrounding the ions reduce the Born energy for H+/OH− insertion into lipid. This transmembrane condensation model predicts the magnitude of the experimentally determined H+/OH− flux, which is significantly greater than that of other monovalent ions. The consequences of an elevated H+/OH− permeability compared to other ions and the relative pH independence of this flux have consequences for understanding the chemical evolution of life.
The Journal of Membrane Biology – Springer Journals
Published: Sep 25, 2010
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