A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains

Abstract

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.

Interfacial Positioning and Stability of Transmembrane Peptides in Lipid Bilayers Studied by Combining Hydrogen/Deuterium Exchange and Mass Spectrometry

“…The remaining hydrogens, including the ones in the hydrophobic core have rate constants of (much) smaller than 0.10 min Ϫ1 . These slow exchange rates are a result of both the extremely low partition coefficient of D 2 O in a lipid bilayer (19) and the involvement of the amide protons in hydrogen bonding because of ␣-helix formation. The amount of fast exchanged hydrogens increases with the length of the peptides but does not increase quantitatively with the numbers of exchangeable hydrogens.…”

Fast solute release from photosensitive liposomes: an alternative to ‘caged’ reagents for use in biological systems

“…The behaviour in a lipid bilayer host of azobenzene-containing lipids similar to those used in this study has been investigated by Song et al [12] who find spectroscopic evidence of aggregate formation in some circumstances. It is well known that solute release from liposomes of pure gel-phase lipids is markedly increased at the midpoint of a phase transition [13] where regions of ordered and disordered lipid coexist, and indeed this has been proposed as a method to control localised delivery of soluble drugs trapped within liposomes [14]. Millisecond release of liposome contents is unlikely to be caused by an overall transition in the membrane to the fluid, liquid crystalline state, since these transitions in pure DPPC bilayers occur on a time-scale of tens of milliseconds [15].…”

Cell membrane electroporation- Part 1: The phenomenon

“…Because of its nonpolar interior, it is also an almost impenetrable barrier for polar molecules dissolved in the aqueous electrolyte on both its sides. Nevertheless, water and some monoatomic ions permeate through it at rates too high to be explainable by diffusion through an entirely intact bilayer [9], [10]. Under certain conditions, e.g., sufficiently high temperature, surface tension, or both, this permeation can be attributed to the formation and rapid resealing of very small aqueous pores in the lipid bilayer, with radii below a nanometer and lifetimes below a nanosecond; they form and reseal because of thermal and mechanical fluctuations.…”