The effect of gramicidin A on the temperature dependence of water permeation through liposomal membranes prepared from phosphatidylcholines with different chains lengths

The energetic barrier to single-file water flow through narrow channels

Various nanoscopic channels of roughly equal diameter and length facilitate single-file diffusion at vastly different rates. The underlying variance of the energetic barriers to transport is poorly understood. First, water partitioning into channels so narrow that individual molecules cannot overtake each other incurs an energetic penalty. Corresponding estimates vary widely depending on how the sacrifice of two out of four hydrogen bonds is accounted for. Second, entropy differences between luminal and bulk water may arise: additional degrees of freedom caused by dangling OH-bonds increase entropy. At the same time, long-range dipolar water interactions decrease entropy. Here, we dissect different contributions to Gibbs free energy of activation, ΔG ^‡, for single-file water transport through narrow channels by analyzing experimental results from water permeability measurements on both bare lipid bilayers and biological water channels that (i) consider unstirred layer effects and (ii) adequately count the channels in reconstitution experiments. First, the functional relationship between water permeabilities and Arrhenius activation energies indicates negligible differences between the entropies of intraluminal water and bulk water. Second, we calculate ΔG ^‡ from unitary water channel permeabilities using transition state theory. Plotting ΔG ^‡ as a function of the number of H-bond donating or accepting pore-lining residues results in a 0.1 kcal/mol contribution per residue. The resulting upper limit for partial water dehydration amounts to 2 kcal/mol. In the framework of biomimicry, our analysis provides valuable insights for the design of synthetic water channels. It thus may aid in the urgent endeavor towards combating global water scarcity.

Single-file transport of water through membrane channels

Water at interfaces governs many processes on the molecular scale from electrochemical and enzymatic reactions to protein folding. Here we focus on water transport through proteinaceous pores that are so narrow that the water molecules cannot overtake each other in the pore. After a short introduction into the single-file transport theory, we analyze experiments in which the unitary water permeability, pf, of water channel proteins (aquaporins, AQPs), potassium channels (KcsA), and antibiotics (gramicidin-A derivatives) has been obtained. A short outline of the underlying methods (scanning electrochemical microscopy, fluorescence correlation spectroscopy, measurements of vesicle light scattering) is also provided. We conclude that pf increases exponentially with a decreasing number NH of hydrogen bond donating or accepting residues in the channel wall. The variance in NH is responsible for a more than hundredfold change in pf. The dehydration penalty at the channel mouth has a smaller effect on pf. The intricate link between pf and the Gibbs activation energy barrier, ΔG‡t, for water flow suggests that conformational transitions of water channels act as a third determinant of pf.

Osmotic behaviour and permeability properties of liposomes

In this overview liposomes are described as bilayer-bounded vesicles which under defined conditions act as ideal osmometers according to the Boyle van't Hoff law. Investigations on osmotic volume changes, directly or indirectly by taking advantage of changes in light scattering, are considered and applications in permeability measurements are discussed. Solute-solvent interactions occurring in isotonic swelling experiments are analysed in view of an irreversible thermodynamic description. In a second part liquid crystalline lipid bilayers are characterized as highly selective permeability bilayers and the physical principles underlying this selectivity are considered. Attention is given to special physical and chemical conditions that may cause structural defects in the bilayer organization and can affect the selective permeability properties of the bilayer or completely deteriorate its barrier function. Finally an evaluation is given of intrinsic ionophoretic activity in lipid bilayers containing negatively charged lipids.

Relationship between lipid fluidity and water permeability of bovine tracheal epithelial cell apical membranes

Apical membrane vesicles were prepared from bovine tracheal epithelial cells. These membranes were enriched in alkaline phosphatase specific activity 35-fold compared to cellular homogenates. Steady-state fluorescence polarization studies of these membranes, using three fluorophores, demonstrated that they possessed a relatively low fluidity. Studies using the probe 1,6-diphenyl-1,3,5-hexatriene detected thermotropic transitions at 25.7 +/- 0.4 and 26.8 +/- 0.6 degrees C in these membranes and their liposomes, respectively. Analysis of the composition of these membranes revealed a fatty acyl saturation index of 0.59 +/- 0.02, a protein/lipid ratio (w/w) of 0.60 +/- 0.06, a cholesterol/phospholipid ratio (mol/mol) of 0.83 +/- 0.11, and a sphingomyelin/lecithin ratio (mol/mol) of 0.64 +/- 0.10. Membrane vesicles were osmotically active when studied by a stopped-flow nephelometric technique. Arrhenius plots of rates of osmotic water efflux demonstrated break points at approximately 28 and 18 degrees C, with activation energies of 16.7 +/- 0.2 kcal mol-1 from 35 to 28 degrees C, 8.3 +/- 0.5 kcal mol-1 from 28 to 18 degrees C, and approximately 3.0 kcal mol-1 below 18 degrees C. Treatment of membrane vesicles with benzyl alcohol, a known fluidizer, decreased lipid order (increased fluidity) and increased the rate of osmotic water efflux. The present results suggest that water crosses tracheal epithelial cell apical membranes by solubility-diffusion across the lipid domain and that increases in fluidity correlate with increases in the water permeability of these membranes.

Osmotic water permeability of small intestinal brush-border membranes

A stopped-flow nephelometric technique was used to examine osmotic water flow across small intestinal brush-border membranes. Brush-border membrane vesicles (BBMV) were prepared from rat small intestine by calcium precipitation. Scattered 500 nm light intensity at 90 degrees to incident was a linear function of the number of vesicles in suspension, and of the reciprocal of the suspending medium osmolality. When BBMV were mixed with hyperosmotic mannitol solutions there was a rapid increase in the intensity of scattered light that could be fit to a single exponential function. The rate constant for vesicle shrinking varied with temperature and the size of the imposed osmotic gradient. At 25 degrees C and an initial osmotic gradient of 50 mOsm, the rate constant was 1.43 +/- 0.044 sec-1. An Arrhenius plot of the temperature dependence of vesicle shrinking showed a break at about 25 degrees C with an activation energy of 9.75 +/- 1.04 kcal/mole from 11 to 25 degrees C and 17.2 +/- 0.55 kcal/mole from 25 to 37 degrees C. The pore-forming antibiotic gramicidin increased the rate of osmotically driven water efflux and decreased the activation energy of the process to 4.51 +/- 0.25 kcal/mole. Gramicidin also increased the sodium permeability of these membranes as measured by the rate of vesicle reswelling in hyperosmotic NaSCN medium. Gramicidin had no effect on mannitol permeability. Assuming spherical vesicles of 0.1 micron radius, an osmotic permeability coefficient of 1.2 X 10(-3) cm/sec can be estimated for the native brush-border membranes at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Stopped-flow study of anesthetic effect on water-transport kinetics through phospholipid membranes. Interfacial versus lipid core ligands

We have compared ligand effects between polar and apolar anesthetic molecules upon water transport across phospholipid membranes by kinetic analysis of the osmotic swelling rate, using a stopped-flow technique. Chloroform and 1-hexanol were used as interfacial ligands, and carbon tetrachloride and n-hexane were used as their counterparts, representing lipid core action. Because anesthetics transform the solid-gel membrane into a liquid-crystalline state, and because phospholipid membranes display an anomaly in permeability at the phase transition, dimyristoylphosphatidylcholine vesicles were studied at temperatures above the main phase transition to avoid this anomaly. All these molecules increased the osmotic swelling rate. However, a significant difference was observed in the activation energy, delta Ep, between polar and apolar molecules; delta Ep was almost unaltered by the addition of polar molecules (chloroform and 1-hexanol), whereas it was decreased by apolar molecules (carbon tetrachloride and n-hexane). The obtained results were analyzed in terms of the dissolution-diffusion mechanism for water permeation across the lipid membrane. It is suggested that polar molecules affect water permeability by altering the partition of water between the membrane interior and water phase, and apolar molecules affect it by altering both the partition and the diffusion of water within the membrane interior.

Effect of anions on the cation selectivity of gramicidin-containing liposomes

An osmotic method was used to study the salt permeability induced by gramicidin A in liposomes. Sequences of cation permeation were obtained for iodide, salycilate, acetate and formate salts in liposomes below and above their transition temperature. Salycilate and formate salts, unlike acetate and iodide salts, exhibit the same sequences for cation selectivity in liposomes below and above their transition temperature. These results can be explained by assuming three mechanisms for salt permeation across gramicidin-containing liposomes: (i) the anion moves by the lipid part of the membrane whereas the cation moves by the gramicidin channel, (ii) movement of the undissociated acid species occurs through the lipid part of the membrane followed by cation-proton exchange via the gramicidin channel and (iii) the cation and anion may move simultaneously via the gramicidin channel. When the movement of the anion or undissociated acid across the lipid part of the membrane is not rate limiting the permeation process, the cation selectivity obtained agrees with the cation selectivity of the gramicidin A channel, as determined by others using independent measurements.

Phospholipid-dependent assembly of mitochondrial ATPase complex

1. Phosphatidylcholines of different acyl-chain composition and a preparation of ATPase complex depleted of phospholipids have been employed in order to evaluate the contribution of lipid bilayer to the assembly of this multi-subunit component of mitochondrial membrane. 2. At the minimal requirement for bilayer assembly (dinonanoylphosphatidylcholine, mixtures of lysophosphatidylcholine and phosphatidylcholine), fragments with oligomycin-insensitive ATPase activity are reconstituted. Conformational changes with dislocation of ATPase complex subunits may explain these results. 3. At increased strength of acyl-chain interaction (dilauroylphosphatidylcholine and higher homologues), the damage to the ATPase complex is prevented but this is not sufficient to achieve functional restoration. Bilayers with a tendency to coalesce and fuse aggregate in large amounts with the complex and yield low ATPase reactivation. Bilayers of high stability yield complexes with physiological content of phospholipids and efficient ATPase activity. Transition between these two possibilities is found at sixteen carbon acyl-chains. Only at this chain length does the cholate dialysis procedure of reconstitution become feasible. 4. It is concluded that a minimum of 16 carbon atoms in each chain are required to organize a bilayer structurable to maintain the ATPase complex conformation and to sustain the transmembrane position of the whole assembly.