title: Controlled ionic condensation at the surface of a native extremophile membrane.
abstract: At the nanoscale level biological membranes present a complex interface with the solvent. The functional dynamics and relative flexibility of membrane components together with the presence of specific ionic effects can combine to create exciting new phenomena that challenge traditional theories such as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory or models interpreting the role of ions in terms of their ability to structure water (structure making/breaking). Here we investigate ionic effects at the surface of a highly charged extremophile membrane composed of a proton pump (bacteriorhodopsin) and archaeal lipids naturally assembled into a 2D crystal. Using amplitude-modulation atomic force microscopy (AM-AFM) in solution, we obtained sub-molecular resolution images of ion-induced surface restructuring of the membrane. We demonstrate the presence of a stiff cationic layer condensed at its extracellular surface. This layer cannot be explained by traditional continuum theories. Dynamic force spectroscopy experiments suggest that it is produced by electrostatic correlation mediated by a Manning-type condensation of ions. In contrast, the cytoplasmic surface is dominated by short-range repulsive hydration forces. These findings are relevant to archaeal bioenergetics and halophilic adaptation. Importantly, they present experimental evidence of a natural system that locally controls its interactions with the surrounding medium and challenges our current understanding of biological interfaces.
authors.affiliation: University of Oxford, Bionanotechnology IRC, Clarendon Laboratory, Physics Department, Parks Road, OX1 3PU, Oxford, UK. s.antoranzcontera1@physics.ox.ac.uk
authors.forename: Sonia
authors.initials: S
authors.lastname: Antoranz Contera
authors.forename: Kislon
authors.initials: K
authors.lastname: Voïtchovsky
authors.forename: John F
authors.initials: JF
authors.lastname: Ryan
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry; D007477:Ions / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D018717:Purple Membrane / Q000737:chemistry; D055672:Static Electricity; D013499:Surface Properties
chemical_list: D007477:Ions; D001436:Bacteriorhodopsins