Salt screening and specific ion adsorption determine neutral-lipid membrane interactions

Abstract

The simplest, single-component biological membrane challenges accepted models of macromolecular interactions: lipid lamellar phases swell when immersed in monovalent salt solutions. Moreover, typical of a Hofmeister series, Br salts swell multilayers more than Cl salts, offering an excellent opportunity to investigate long-standing questions of ionic specificity. In accord with earlier measurements of liposome mobilities in electric fields, we find an added electrostatic repulsion of membranes due to anion binding, with a much stronger Br binding compared with Cl. However, contrary to the expectation that electrostatic repulsion should vanish in high salinity, swelling of lipid multilayers is monotonic with increasing salt concentration for both Br and Cl salts. The apparent contradiction is resolved by recognizing that although the electrostatic repulsion is progressively screened by increasing salt concentration, so is the van der Waals (vdW) attraction. Negligible in low salt, weakening of vdW forces becomes significant by the time electrostatic forces vanish. The result is a smooth monotonic swelling curve with no apparent distinction between low and high salt concentration regimes. Furthermore, when compared with theoretical predictions, measured vdW forces decay much too slowly with added salt. However, by accounting for the recently measured salt deficit near lipid bilayers, the expected scaling with Debye screening length is recovered. The combination of ion-specific binding and nonspecific ionic screening of lowfrequency fluctuations explains salt effects on lipid membrane interactions and, by extension, explains specific (Hofmeister) effects at macromolecular interfaces between low and high dielectric.electrostatics ͉ halides ͉ ion binding ͉ van der Waals ͉ hydration

Controlled ionic condensation at the surface of a native extremophilemembrane

“…At 30 mM KCl, k ts reflects the asymmetry of the membrane: the extracellular leaflet is less charged than the cytoplasmic leaflet and exhibits longer range interactions. Both leaflets show a strictly monotonic repulsive interaction, which 15 validate the assumption of a locally parabolic potential. The bare mica surface, on the other hand, shows an additional attractive region.…”

“…In this way we could calculate the inverse optical lever sensitivity (nm/V) and ensure stability of the system throughout our measurements. 15 To avoid systematic errors, each set of measurements carried out in a specific buffer condition was made in a random order.…”

“…potassium over sodium. 12,15 range hydration forces and the discrete effects of charges, ions and water molecules are not taken into account in DLVO either. Finally, we note that electrostatic interactions of highly…”

“…18 It is composed of seven trans-membrane α-helices which enclose a retinal chromophore. An enormous 15 amount of biochemical, genetic and spectroscopic investigations have been dedicated to understand the vectorial proton pumping mechanism of bR, [19][20][21][22][23][24][25] and although there are different conflicting data sets and interpretations [26][27][28] a coherent picture of a ~10 ms photocycle process has emerged: 20 Photo-isomerization of the retinal produces a steric and electrostatic mismatch that is relaxed through local conformational changes within bR. These changes become larger throughout the photocycle and spread gradually to the membrane surface.…”

Ion Dynamics in Cationic Lipid Bilayer Systems in Saline Solutions

“…Experiments have shown that anionic 4, [22][23][24][25] and cationic 26,27 membranes interact readily with their counterions (especially divalent ones), whereas the interactions of zwitterionic lipid bilayers with salt ions appear rather sensitive to the size and valency of ions. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] As for molecular-level computational studies, the increase in computing power in the past few years has made it possible to extend computer simulations beyond the relatively long relaxation times of tens to hundreds of nanoseconds required for equilibration of ions in lipid/water systems. Although most computational studies by far have focused on the effects of salt ions on zwitterionic (neutral) lipid bilayers, 33,[42][43][44][45][46][47][48][49][50][51][52][53] there is also an increasing number of studies on anionic [54][55][56][57][58][59] and cationic 60,61 lipid bilayers.…”