The water permeability of lipid bilayer membranes

Interfacial Chemistry of Conical Fullerene Amphiphiles in Water

Amphiphiles are used for a variety of applications in our daily life and in industrial processes. They typically possess hydrophobic and hydrophilic moieties within the molecule, thereby performing a myriad of functions through the formation of two- and three-dimensional assemblies in water, such as Gibbs monolayers and micelles. However, these functions are often inseparable because they emerge from the same structural feature of the molecule, and are difficult to control because the structural diversity is limited to either long-chain hydrocarbons bearing a polar end group(s) or polymers bearing polar groups exposed to the exterior surface. In this Account, we describe the chemistry of a new class of amphiphiles, conical fullerene amphiphiles (CFAs), utilizing a superhydrophobic [60]fullerene group as a nonpolar apex with added structural features to make it soluble in water. By selective functionalization of only one side of the fullerene molecule, the CFA molecules spontaneously assemble in water through strong hydrophobic interactions among the fullerene apexes and exhibit unusual supramolecular and interfacial behavior. They form unilamellar micelles and vesicles at a critical aggregation concentration as low as micromolar, not showing any air-water and oil-water interfacial activity. The strong preference for self-assembly in water over monolayer formation at an air-water interface makes CFAs unique among conventional nonpolymeric surfactants. The CFA assemblies are often so mechanically robust that they can be transferred to the surface of a solid substrate and analyzed by high-resolution microscopy. Because of this rigid conical structure of a few nanometers in size, CFA molecules aggregate readily in water to form a hierarchical assembly with biomolecules and nanomaterials while maintaining the structural integrity of the CFA aggregate to form multicomponent agglomerates of controllable structural features. For instance, tissue-selective in vivo transport of DNA and siRNA has been achieved. Hybridization of a CFA vesicle with a transition metal catalyst enables the construction of a structurally defined nanospace and an interface for precise control of the nanoscale morphology of polymers. Solubilization of hydrophobic nanocarbons and nanoparticles is also achieved through hemimicelle formation on solid surfaces. The examples reported here illustrate the potential of the conical fullerene motif for the design of amphiphiles as well as supramolecular structures at molecular and tens of nanometers scale.

Water transport and homeostasis as a major function of erythrocytes

Erythrocytes have long been known to change volumes and shapes in response to different salt concentrations. Aquaporin-1 (AQP1) was discovered in their membranes more than 20 yr ago. The physiological roles of volume changes and AQP1 expression, however, have remained unclear. We propose that rapid water exchange through AQP1 coupled with large capacity for volume change may allow erythrocytes to play an important role in water regulation. In this study, we showed that erythrocytes in situ gradually reduced their volumes by 39% in response to the hyperosmotic corticomedullary gradient within mouse kidneys. AQP1 knockout (KO) erythrocytes, however, displayed only minimal reduction. Constructing a microfluidic device resembling capillary flow with an extracellular fluorescent reporter demonstrated that water exchanges between erythrocytes and their hypotonic or hypertonic surroundings in vitro reached steady state in ~60 ms. AQP1 KO erythrocytes, however, did not show significant change. To simulate the water transport in circulation, we built basic units consisting of three compartments (i.e., erythrocyte, plasma, and interstitial fluid) using Kedem-Katchalsky equations for membrane transport, and connected multiple units to account for the blood flow. These simulations agreed with experimental results. Importantly, volume-changing erythrocytes in capillaries always "increase" the osmotic gradient between plasma and interstitial fluid, making them function as "micropumps" to speed up the regulation of local osmolarity. Trillions of these micropumps, mobile throughout the body, may further contribute to water homeostasis. These insights suggest that the enhanced exchange of water, in addition to O₂ and CO₂, may well be the third major function of erythrocytes. NEW & NOTEWORTHY Physiological roles of erythrocyte volume change and aquaporin-1 were proposed and investigated here. We conclude that fast water transport by aquaporin-1 coupled with large volume-change capacity allows erythrocytes to enhance water exchange with local tissues. Furthermore, their huge number and mobility allow them to contribute to body water homeostasis.

Optical Trapping of Unilamellar Phospholipid Vesicles: Investigation of the Effect of Optical Forces on the Lipid Membrane Shape by Confocal-Raman Microscopy

Optical trapping of liposomes is a useful tool for manipulating these lipid vesicles for sampling, mechanical testing, spectroscopic observation, and chemical analysis. Through the use of confocal Raman microscopy, this study addresses the effects of optical forces on the structure of unilamellar, dipalmitoylphosphatidylcholine (DPPC) vesicles, both optically trapped in solution and adhered to a coverslip. The energy and forces involved in optical trapping of lipid vesicles were derived in terms of the dielectric contrast between the phospholipid membrane and the surrounding solution; reflection forces at the membrane/water interface were found to be negligible. At optical powers of 9 mW and greater, unilamellar liposomes trapped in bulk solution experience a gradient force sufficiently strong to bend the vesicle membrane, so that a second bilayer from the same vesicle is drawn into the optical trap, with an energy of approximately 6 x 10(-13) erg. For vesicles adhered to a coverslip, the confocal probe can be scanned through the attached vesicle. Optical forces are insufficient to detach the bilayer that is adhered to the glass; however, the upper DPPC bilayer can be manipulated by the optical trap and the shape of the vesicle distorted from a spherical geometry. The effect of calcium ion on the flexibility of membrane bilayers was also tested; with 5 mM calcium ion in solution, the lipid bilayer of a surface-attached liposome is sufficiently rigid so that it cannot be distorted at moderate laser powers.

Permeability of liposomal membranes to water: results from the magnetic field dependence of T1 of solvent protons in suspensions of vesicles with entrapped paramagnetic ions

The diffusive permeability to water molecules, Pd, of lipid vesicles with entrapped paramagnetic solute ions can be determined rapidly from analysis of the magnetic field dependence (nuclear magnetic relaxation dispersion, or NMRD profile) of T1 of exterior solvent water protons. Such data yield tau, the mean lifetime of solvent molecules inside the vesicles, from tau = (fT1Para) - T1Ves, where f is the volume fraction of entrapped water, T1Para is the observed T1 corrected for buffer background, and T1Ves is the relaxation time of water protons in the entrapped solution. For small spherical unilamellar vesicles of inner radius R, Pd = R/3 tau, f can be obtained accurately from knowledge of both the concentration of Gd(DTPA)2- in the solution in which the vesicles were formed and the average concentration of ions in the final sample. At low temperatures, in the limit of slow exchange, T1Para becomes independent of field and tau = fT1Para; the observation of a field-independent profile is a control that confirms that no paramagnetic material is external to the vesicles. We have measured T1Para, using a field-cycling relaxometer, for suspensions of POPC (1-palmitoyl-2-oleoyl-lecithin) vesicles with 100-500 mM entrapped Gd(DTPA)2- and membrane concentrations of cholesterol ranging from 0 to 40 mol %. These profiles, which span the field range 0.01-50 MHz proton Larmor frequency, were taken at 5, 15, 25, and 35 degrees C. Concentrations of Gd(DTPA)2- were determined independently by both ICP analyses and NMRD methods. Values for Pd for vesicles with 100 mM Gd(DTPA)2- and outer diameters 100 nm +/- 20%, as determined by quasielastic light scattering, are 63, 47, 24, 16, and 8.7 x 10(-4) cm s-1, at 25 degrees C, for cholesterol concentrations of 0, 10, 20, 30, and 40%, respectively. The corresponding activation enthalpies are 14, 14, 14, 17, and 17 kcal/M. Comparison with 2H NMR studies of deuterated POPC vesicles with no cholesterol at 20 degrees C, and with 10% at 40 degrees C, which yielded the same order parameter for the palmitoyl acyl chains, gives no indication of a correlation between order parameter and permeability.

Plasma membrane alterations as a result of heat activation in Dictyostelium spores

At the end of heat activation the distribution of spore plasma membrane particles between the two fracture faces (PF and EF) is drastically changed. While in dormant spores the particle number ratio of PF/EF was about 1;1, it increased up to 9:1 in heat activated sproes, indicating a subtle change in plasma membrane properties. The permeability of spores increased within 30 min following heat activation as determined by efflux measurements of radioactively labelled spores. At the onset of swelling this efflux was accelerated. During germination the osmotically active material within the spores increased, part of which could be recovered from the supernatant. The combined experiments point to the plasma membrane as possible target site of heat activation in this system.

Membranes of Valonia ventricosa: apparent absence of water-filled pores

Osmotic and diffusional permeabilities to water have been measured in internally perfused cells of Valonia ventricosa. The osmotic and diffusional permeability coefficients for the protoplast are identical, 2.4 x 10(-4) centimeter per second. Thus, both osmotic and diffusional flows can occur by the same mechanism, that is, by diffusion; and there is no need to postulate the existence of water-filled pores in the membranes of this cell. Supporting evidence for this conclusion is the absence of solvent-solute interactions, that is, "solvent drag," for water, urea, and methanol crossing the protoplast of Valonia.

Permeation of water through cation exchange membranes

Water permeabilities as well as other membrane parameters, such as exchange capacity, water content, and specific conductance, have been measured for two cation exchange membranes in the H form. The conductance of membrane with low water content was less than that of the membrane with high water content. These data have been discussed in the light of an existing theory and found inadequate to explain the results in a quantitative way. Water permeability of the membranes subject to mechanical pressure was found to be higher than their isotopic water permeability, according to expectation. These data have been examined from the standpoint of thermodynamic and kinetic theories of water flow in membranes and used to estimate the average size of membrane pores.