Abstract
A topic emerging roughly thirty years ago and engendering an incompletely resolved controversy is addressed: the relatively high permeability and pH independence associated with H+/OH- passive movements across lipid membranes. 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 region of the membrane are considered. An interfacial H+/OH- reaction mechanism predicts the experimentally observed behavior of a H+/OH- flux that is independent of the pH measurement range. In order to obtain the correct magnitude of flux, it is assumed that H+ and OH- within the interfacial zone become electrostatically aligned on opposite sides of the hydrophobic membrane core. Electrostatic attraction combined with charge delocalization among a small cluster of water molecules surrounding the ions sufficiently reduce the Born energy for insertion into lipid, accounting for the experimentally determined magnitude of this flux. The pH independence associated with H+/OH- passive movements across membranes could have satisfied a requirement for pH homeostasis in emerging life forms and provided stability for natural selection.
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