Unilamellar Vesicles Of Egg Phosphatidylcholine Research Articles

Surfactants, Salt, and pH Alter Nanoparticle-Model Cell Membrane Interactions

Due to their small size, nanoparticles have the ability to penetrate pulmonary and vascular tissue, and as a result, are classified as potential human carcinogens. On the other hand, nanoparticle insertion into targeted cells can play a key role in drug delivery and gene therapy applications, prompting a need to more thoroughly characterize nanoparticle/membrane interactions. Polystyrene nanoparticles with modifications in surface functionalization and detergent conditions remain monodisperse in a variety of aqueous solutions as measured by dynamic light scattering, but tend to aggregate in phosphate buffered saline at neutral pH potentially due to electrostatic screening effects. Calcein leakage assays with small unilamellar egg phosphatidylcholine vesicles were run to measure their interactions with nanoparticles and determine applicability of previous experiments performed with lipid monolayers at the air-water interface that modeled the outer leaflet of a cell membrane. In pure water, adding surfactant to detergent-free nanoparticle solutions increased the magnitude of membrane permeabilization compared to nanoparticles alone. In a buffered solution, the opposite was true; addition of surfactant decreased the nanoparticle induced membrane permeabilization. To better understand nanoparticle-detergent-membrane interactions, either nanoparticles or detergents were individually introduced to the vesicles, and following a time lapse, the other component was introduced. A model of detergent sequestration by the polystyrene nanoparticles explains these results and provides insight to the mechanism of vesicle leakage caused by surfactant-nanoparticle mixtures.

Binding of Hydrophobic Fluorinated Bisphenol A to Large Unilamellar Vesicles of Egg Phosphatidylcholine

The kinetics of membrane binding and dissociation of fluorinated bisphenol A (FBPA, (CF(3))(2)C(C(6)H(4)OH)(2)) is quantified by 1D (19)F NMR spectra in situ. Although the bound and free components are in fast exchange, the rate constants and bound fraction is nonetheless determined from an analysis of the spectra. The analysis relies on the expression of 1D NMR signal intensity by a set of Bloch equations with exchange terms. The time span of the binding and dissociation of hydrophobic FBPA to large unilamellar vesicles of egg phosphatidylcholine (EPC) is 10(-3) to 10(-2) s. The rates of FBPA binding and dissociation are kept constant per EPC molecule even at different concentrations of the vesicle. The free energy of FBPA transfer is -20 ± 2 kJ/mol at 303 K. The process is entropy-driven. The efficiency of FBPA transfer is enhanced by a factor of 7 × 10(4), as compared with the hydrophilic 5-fluorouracil.

Novel Method for Obtaining Homogeneous Giant Vesicles from a Monodisperse Water-in-Oil Emulsion Prepared with a Microfluidic Device

A novel technique called the "lipid-coated ice droplet hydration method" is presented for the preparation of giant vesicles with a controlled size between 4 and 20 microm and entrapment yields for water-soluble molecules of up to about 30%. The method consists of three main steps. In the first step, a monodisperse water-in-oil emulsion with a predetermined average droplet diameter between 4 and 20 microm is prepared by microchannel emulsification, using sorbitan monooleate (Span 80) and stearylamine as emulsifiers and hexane as oil. In the second step, the water droplets of the emulsion are frozen and separated from the supernatant hexane solution by precipitation, followed by a removal of the supernatant and followed by the replacement of Span 80 by using a hexane solution containing egg yolk phosphatidylcholine, cholesterol, and stearylamine (5:5:1, molar ratio). This procedure is performed at -10 degrees C to keep the water droplets of the emulsion in a frozen state and thereby to avoid extensive water droplet coalescence. In the third step, hexane is evaporated at -4 to -7 degrees C and an external water phase is added to the remaining mixture of lipids and water droplets to form giant vesicles that have an average size in the range of that of the initial emulsion droplets (4-20 microm). The entrapment yield and the lamellarity of the vesicles obtained depend on the lipid/water droplet ratio and on the composition of the external water phase. At high lipid/water droplet ratio, the giant vesicles have a thicker membrane (indicating multilamellarity) and a higher entrapment yield than in the case of a low lipid/water droplet ratio. The highest entrapment yield ( approximately 35%) is obtained if the added external water phase contains preformed unilamellar egg phosphatidylcholine vesicles with an average diameter of 50 nm. The addition of these small vesicles minimizes the water droplet coalescence during the third step of the vesicle preparation, thereby decreasing the extent of release of water-soluble molecules originally present in the water droplets. The GVs prepared can be extruded through polycarbonate membranes to yield large unilamellar vesicles with about 100 nm diameter. This size reduction, however, leads to a decrease in the entrapment yield to about 12% due to solute leakage from the vesicles during the extrusion process.

Supported Bilayer Lipid Membrane Arrays on Photopatterned Self‐Assembled Monolayers

This work demonstrates the use of photocleavable cholesterol derivatives to create supported bilayer lipid membrane arrays on silica. The photocleavable cholesteryl tether is attached to the surface by using the reaction of an amine-functionalized self-assembled monolayer (SAM) and the N-hydroxysuccinimide-based reagent 9. The resultant SAM contains an ortho-nitrobenzyl residue that can be cleaved by photolysis by using soft (365 nm) UV light regenerating the original amine surface, and which can be patterned using a mask. The photoreaction yield was approximately 75 % which was significantly higher than previously found for related ortho-nitrobenzyl photochemistry on gold substrates. The SAMs were characterized by means of contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. Patterned surfaces were characterized with SEM and AFM. After immersing the patterned surface into a solution containing small unilamellar vesicles of egg phosphatidylcholine (PC), supported lipid membranes were formed comprised of lipid bilayer over the amine functionalized "hydrophilic" regions and lipid monolayer over the cholesteryl "hydrophobic" regions. This was confirmed by fluorescence microscopy and AFM. FRAP studies yielded a lateral diffusion coefficient for the probe molecule of 0.14+/-0.05 microm(2) s(-1) in the bilayer regions and approximately 0.01 microm(2) s(-1) in the monolayer regions. This order of magnitude difference in diffusion coefficients effectively serves to isolate the bilayer regions from one another, thus creating a bilayer array.

Generation of Superparamagnetic Liposomes Revealed as Highly Efficient MRI Contrast Agents for in Vivo Imaging

Maghemite (gamma-Fe2O3) nanocrystals stable at neutral pH and in isotonic aqueous media were synthesized and encapsulated within large unilamellar vesicles of egg phosphatidylcholine (EPC) and distearoyl-SN-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG(2000), 5 mol %), formed by film hydration coupled with sequential extrusion. The nonentrapped particles were removed by flash gel exclusion chromatography. The magnetic-fluid-loaded liposomes (MFLs) were homogeneous in size (195 +/- 33 hydrodynamic diameters from quasi-elastic light scattering). Iron loading was varied from 35 up to 167 Fe(III)/lipid mol %. Physical and superparamagnetic characteristics of the iron oxide particles were preserved after liposome encapsulation as shown by cryogenic transmission electron microscopy and magnetization curve recording. In biological media, MFLs were highly stable and avoided ferrofluid flocculation while being nontoxic toward the J774 macrophage cell line. Moreover, steric stabilization ensured by PEG-surface-grafting significantly reduced liposome association with the macrophages. The ratios of the transversal (r2) and longitudinal (r1) magnetic resonance (MR) relaxivities of water protons in MFL dispersions (6 < r2/r1 < 18) ranked them among the best T2 contrast agents, the higher iron loading the better the T2 contrast enhancement. Magnetophoresis demonstrated the possible guidance of MFLs by applying a magnetic field gradient. Mouse MR imaging assessed MFLs efficiency as contrast agents in vivo: MR angiography performed 24 h after intravenous injection of the contrast agent provided the first direct evidence of the stealthiness of PEG-ylated magnetic-fluid-loaded liposomes.

Influence of cetyltrimethylammonium bromide on phosphatidylcholine-coated capillaries.

Large unilamellar vesicles of egg-phosphatidylcholine (eggPC), a naturally occurring phospholipid, were used in capillary electrophoresis (CE) for semi-permanent coating of fused silica capillaries. The stability of the phospholipid coating was tested at different cetyltrimethylammonium bromide (CTAB) concentrations with and without CaCl(2) present in the coating solution. The effect of physical factors influencing the coating stability (e.g. duration of the coating time, storage temperature of the coating solution) were also studied. Standing overnight in background electrolyte (BGE) solution did not alter the eggPC phospholipid coating noticeably. The performance of the coating was tested with a mixture of basic proteins (lysozyme, ribonuclease A and alpha-chymotrypsinogen A). Highest efficiencies (over 200,000 plates m(-1)) were achieved when the capillary was filled for 15 h with a liposome solution containing both CTAB and CaCl(2).

A comparative study of sugar-based surfactants for the solubilization of phosphatidylcholine vesicles

The solubilization of small unilamellar vesicles of egg phosphatidylcholine (PC) by a series of sugar-head surfactants was investigated. The experiments consisted of recording turbidity at 25°C, during continuous addition of surfactant solutions to PC vesicles of different initial concentrations. Surfactant-to-phospholipid molar ratios in the molecular aggregates [Surf/PC] ag as well as the surfactant concentrations in the aqueous phase [Surf] bulk were determined at the total micellization stage. The values of [Surf/PC] ag varied over a factor often, from 0.44 to 5.20, pointing out a significant variability of the solubilizing properties of the sugar derivatives. This was more likely due to the headgroup conformation of the surfactants rather than to their water-solubility.

Effect of monogalactosyldiacylglycerol and other thylakoid lipids on violaxanthin de-epoxidation in liposomes

In this study we present evidence that one of two reactions of the xanthophyll cycle, violaxanthin de-epoxidation, may occur in unilamellar egg phosphatidylcholine vesicles supplemented with monogalactosyldiacylglycerol (MGDG). Activity of violaxanthin de-epoxidase (VDE) in this system was found to be strongly dependent on the content of MGDG in the membrane; however, only to a level of 30 mol%. Above this concentration the rate of violaxanthin de-epoxidation decreased. The effect of individual thylakoid lipids on VDE-independent violaxanthin transformation was also investigated and unspecific effects of phosphatidylglycerol and sulphoquinovosyldiacyglycerol, probably related to the acidic character of these lipids, were found. The presented results suggest that violaxanthin de-epoxidation most probably takes place inside MGDG-rich domains of the thylakoid membrane. The described activity of the violaxanthin de-epoxidation reaction in liposomes opens new possibilities in the investigation of the xanthophyll cycle and may contribute to a better understanding of this process.

Effect of monogalactosyldiacylglycerol and other thylakoid lipids on violaxanthin de-epoxidation in liposomes

In this study we present evidence that one of two reactions of the xanthophyll cycle, violaxanthin de-epoxidation, may occur in unilamellar egg phosphatidylcholine vesicles supplemented with monogalactosyldiacylglycerol (MGDG). Activity of violaxanthin de-epoxidase (VDE) in this system was found to be strongly dependent on the content of MGDG in the membrane; however, only to a level of 30 mol%. Above this concentration the rate of violaxanthin de-epoxidation decreased. The effect of individual thylakoid lipids on VDE-independent violaxanthin transformation was also investigated and unspecific effects of phosphatidylglycerol and sulphoquinovosyldiacyglycerol, probably related to the acidic character of these lipids, were found. The presented results suggest that violaxanthin de-epoxidation most probably takes place inside MGDG-rich domains of the thylakoid membrane. The described activity of the violaxanthin de-epoxidation reaction in liposomes opens new possibilities in the investigation of the xanthophyll cycle and may contribute to a better understanding of this process.

Lipids favoring inverted phase enhance the ability of aerolysin to permeabilize liposome bilayers.

Channel formation by the bacterial toxin aerolysin follows oligomerization of the protein to produce heptamers that are capable of inserting into lipid bilayers. How insertion occurs is not understood, not only for aerolysin but also for other proteins that can penetrate membranes. We have studied aerolysin channel formation by measuring dye leakage from large unilamellar egg phosphatidylcholine vesicles containing varying amounts of other lipids. The rate of leakage was enhanced in a dose-dependent manner by the presence of phosphatidylethanolamine, diacylglycerol, cholesterol, or hexadecane, all of which are known to favor a lamellar-to-inverted hexagonal (L-H) phase transition. Phosphatidylethanolamine molecular species with low L-H transition temperatures had the largest effects on aerolysin activity. In contrast, the presence in the egg phosphatidylcholine liposomes of lipids that are known to stabilize the lamellar phase, such as sphingomyelin and saturated phosphatidylcholines, reduced the rate of channel formation, as did the presence of lysophosphatidylcholine, which favors positive membrane curvature. When two different lipids that favor hexagonal phase were present with egg PC in the liposomes, their stimulatory effects were additive. Phosphatidylethanolamine and lysophosphatidylcholine canceled each other's effect on channel formation.

Effect of N-acyl-phosphatidylethanolamine on the membrane fusion between Sendai virus and liposome.

We have compared the properties of two N-acyl derivatives of dilauryl phosphatidylethanolamine on lipid polymorphism, vesicle leakage and Sendai virus fusion. The derivatives contained either an N-lauroyl group (NLPE) or an N-acetyl group (NAcPE). Only the NAcPE markedly affected the bilayer to hexagonal transition temperature of dielaidoyl phosphatidylethanolamine, shifting it to higher values. In contrast the NLPE slightly lowered this phase transition temperature. The two lipids also have opposite effects on leakage from small unilamellar vesicles of egg phosphatidylcholine. The NLPE inhibits leakage, while the NAcPE promotes it. This vesicle stabilizing effect of NLPE against leakage is not manifested in alterations of rates or extents of Sendai virus fusion to liposomes of egg phosphatidylethanolamine plus 2% ganglioside GD1a. The NLPE has no effect, while the NAcPE reduces the observed fusion, at least in part as a consequence of a reduction in the final extent of fusion. These results demonstrate that the bilayer stabilizing effects of NLPE do not result in a lower rate of viral fusion. Furthermore, these bilayer stabilizing effects against leakage are not solely a function of the lipid headgroup but also require a structure with three long acyl chains. The reduced leakage is not related to a loss in monolayer curvature strain.

Lysis of Large Unilamellar Vesicles Induced by Analogs of the Fusion Peptide of Influenza Virus Hemagglutinin

We have developed a micropipette aspiration assay to observe the lysis of large (20–30 μm diameter) vesicles aspirated using micropipettes. Single membrane lysis events can be seen with the light microscope and are followed using fluorescence assays and video microscopy. In this study, we have examined the ability of two analogs of the fusion peptide from influenza virus hemagglutinin to induce the lysis of large unilamellar egg phosphatidylcholine vesicles, as a function of peptide concentration and pH. X31 is a wild-type peptide from one strain of Influenza A, and E5 is an analogue which has several residues replaced by glutamate residues. Both peptides were found to induce lysis of large vesicles in a pH-dependent manner. Both peptides exhibited maximal activity at pH 5, measured in terms of both rate and extent of lysis. E5 was active at much lower concentrations than X31. Our results with both peptides are compared to results published from other laboratories.

Timemresolved fluorescence of S-100a protein: effect of Ca +, Mg 2+ and unilamellar vesicles of egg phosphatidylcholine

Phase-modulation fluorescence lifetime measurements were used to study the single Trp residue of the Ca 2+-binding protein S-100a both in the absence and in the presence of Ca 2+ and/or Mg 2+. Trp fluorescence decay for the protein was satisfactorily described by Lorentzian lifetime distributions centered around two components (approximately 4 ns and 0.5 ns). Lifetime values were unchanged by 2 mM Ca 2+, but the fractional intensity associated with longer lifetime increased up to 75%. In the presence of Mg 2+, the Ca 2+ induced increase of the fractional intensity associated with longer lifetime was only 57%. For the protein in buffer, about the 85% of the recovered anisotropy was associated to a rotational correlation time of 6.7 ns. After the addition of Ca 2+, this value was increased to 16.08 ns. In the presence of Mg 2+, Ca +2 increased the rotational correlation time to 33.75 ns. Similar studies were performed with S-100a interacting with egg phosphatidylcholine vesicles (SUV). Our data suggest that the conformation of the protein may be influenced by structural features of the lipidic membrane. Moreover, data obtained in the presence of Mg 2+ indicate some interaction between lipids and S-100, likely mediated by this ion.

Specific Targeting of Phototoxic Haptenated Liposomes to a Hapten-Specific B Cell Lymphoma

A method is reported to eliminate B lymphocytes specific for a haptenated lipid by using the lipid hapten to target a photosensitive drug to them. The photosensitizer eosin was coupled to a phospholipid and incorporated into trinitrophenol (TNP)-bearing small unilamellar vesicles of egg phosphatidylcholine (PC) and cholesterol in order to target the photosensitizer to B lymphoma cells (A20-HL) with TNP-specific membrane IgM receptorsin vitro.Exposure of the treated cells to visible light led to an antigen-specific toxic effect indicated by inhibition of cell proliferation. A significantly higher concentration of liposomal eosin was required to inhibit control B cells. These were genetically identical B lymphoma cells (A20-2J) which lack only the DNA for the surface antigen receptor. Furthermore, pretreatment with TNP-conjugated keyhole limpet hemocyanin or anti-IgM antibody abolished the antigen-specific toxic effect, confirming that the TNP-targeted liposomal eosin mediates its effect by binding to the Ig antigen receptors on TNP-specific B cells. Incubation of cells with the TNP-bearing phototoxic liposomes at 4°C instead of 37°C did not alter the antigen-specific targeting effect, suggesting that uptake of the liposomal drug into the cells is not necessary for its toxic effect. Replacement of the liposomal phospholipid (egg PC) with saturated species of PC having higher phase transition temperatures or with sphingomyelin caused a decrease of the antigen-specific effect. These results demonstrate the potential use of antigen-bearing liposomal phototoxic drugs for the purpose of targeting and eliminating B cells with antigen-specific surface Ig receptors.

Origin of the lag period in the phospholipase C cleavage of phospholipids in membranes. Concomitant vesicle aggregation and enzyme activation.

When phospholipase C is added to a suspension of large unilamellar vesicles of egg phosphatidylcholine, maximal rates of hydrolysis occur only after a latency period. No lag period is seen when the substrate is in the form of small (sonicated) vesicles, or of short-chain phosphatidylcholine monomers. For a given vesicle concentration, the lag time may vary as a function of Ca2+, enzyme concentration, or temperature, but activation occurs at a fixed molar fraction of diacylglycerol produced. Lag times decrease gradually with vesicle size, and also with the amount of diacylglycerol present in the bilayers when it is mixed with phospholipid prior to enzyme addition. Parallel recordings of enzyme activity and suspension turbidity reveal that in all cases the latency period ends concomitantly with the start of a process of vesicle aggregation. Both the lag time and the amount of diacylglycerol formed before activation decrease with vesicle concentration, suggesting that enzyme activation is somehow related to vesicle aggregation. The latency period of phospholipase C may be explained in terms of a hypothesis according to which (a) full enzyme activity requires the presence of membrane surface irregularities or defects, (b) the diacylglycerol generated in the lag phase produces some kind of phase separation, with the formation of diacylglycerol-rich "patches" or domains, (c) vesicles aggregate through contacts between those patches, and (d) aggregation causes (and/or increases, and/or stabilizes) the surface inhomogeneities that allow fast enzyme activity. These data and suggestions may be relevant to the process of model membrane fusion promoted by phospholipase C.

Effect of I.V. Injection of Small Unilamellar Liposomes of Egg Phosphatidylcholine on Cholesterol in Plasma and Erythrocytes, Serum Enzymes and Liver Function in Dogs

AbstractSmall unilamellar vesicles of egg phosphatidylcholine were prepared and injected intravenously at dose levels of 250 or 625 mg/kg into 4 adult Beagle dogs 3 times for each dose, once every other day. Plasma unesterified cholesterol increased and RBC unesterified cholesterol decreased in a dose-related manner. At the larger, but not the smaller dose, there was a reversible rise in some liver enzymes in the serum: alanine aminotransferase > > aspartate aminotransferase = sorbitol dehydrogenase > alkaline phosphatase at their peak levels. Normal bromosulfophthalein clearance, metabolite levels, and plasma protein profiles were observed following both doses of liposomes, indicating normal liver function. The rise in liver enzymes in serum may be due to changes in hepatocyte membrane permeability caused by a loss of cholesterol, resulting in enzyme leakage from the cells.

Cholesterol distribution in rat heart myocytes

Cholesterol oxidase was used to investigate the distribution of free cholesterol between plasma membrane and intracellular pools in cultured neonatal rat heart myocytes. Only 20% of the total unesterified cholesterol was converted to delta 4-cholestenone by cholesterol oxidase in intact cells. With increasing age in culture and concurrent hypertrophy, there was an increase in unesterified cellular cholesterol and plasma membrane cholesterol; their relative distribution remained unchanged. Electron micrographs of negatively stained samples of day 4 cytosol revealed the presence of vesicles 50-200 nm in diameter. Cholesterol monohydrate crystals were found in the cytosol of hypertrophic day 14 cells. Treatment of day 14 cells with small unilamellar vesicles of egg phosphatidylcholine reduced plasma membrane and intracellular cholesterol levels, resulting in the disappearance of the cholesterol monohydrate crystals and the formation of vesicles smaller than those observed in day 4 cultures.

Absorption of Monoacylglycerols by Small Intestinal Brush Border Membrane

The absorption of monoacylglycerol by small intestinal brush border membrane is a passive process, i.e., the movement of monoacylglycerol from small unilamellar phospholipid vesicles as donor particles through the aqueous medium and the incorporation into the outer monolayer of the lipid bilayer of the brush border membrane are passive processes involving diffusion of the lipid along a concentration gradient. Small unilamellar vesicles of egg phosphatidylcholine containing 1 mol% of radiolabeled hexadecylglycerol were used as donor, and rabbit small intestinal brush border membrane vesicles or intact enterocytes isolated from pig jejunum, as acceptor. Hexadecylglycerol was employed as a lipase-resistant model compound for monoacylglycerols. Both acceptor membranes behave similarly in terms of hexadecylglycerol absorption: the kinetics of hexadecylglycerol absorption are biphasic. The initial fast phase is due to the movement of hexadecylglycerol from the donor particle through the aqueous medium to the outer lipid monolayer of the acceptor membrane, and the second slow phase probably involves the flip-flop motion of hexadecylglycerol from the outer to the inner monolayer of the acceptor membrane. The values for the pseudo-first-order rate constants of the initial fast phase for hexadecylglycerol absorption are relatively large and primarily determined by the high solubility (cmc) of hexadecylglycerol in aqueous media. The pseudo-first-order rate constants depend linearly on the protein (lipid) concentration of the acceptor membrane, indicating that the on rate of the hexadecylglycerol into the brush border membrane is rate limiting. The mechanism of the hexadecylglycerol absorption involves mainly monomer diffusion and probably collision-induced transfer.

Thermodynamic stability and osmotic sensitivity of small unilamellar phosphatidylcholine vesicles

Evidence is presented to show that small unilamellar phosphatidylcholine vesicles with a diameter of approx. 20 nm are osmotically sensitive. Such vesicles respond to osmotic pressure by swelling or shrinking depending on the direction of the applied salt gradient. This is true for small unilamellar vesicles of egg phosphatidylcholine and dimyristoylphosphatidylcholine below and above their crystal-to-liquid crystal transition temperature. At the transition temperature the vesicles are osmotically insensitive due to the increased bilayer permeability resulting in rapid dissipation of salt gradients. Positive salt gradients produce shrinking and collapse of spherical phospholipid vesicles to disks. Shrinking of vesicles is associated with H 2O and solute efflux, but only limited solute influx. Clustering of lipid molecules in the bilayers of the resulting disks can be detected by EPR spin labeling. Negative salt gradients produce swelling of vesicles which is associated with H 2O and solute influx. Our experiments are consistent with an osmotically perturbed bilayer. In the presence of osmotic gradients the influx and efflux of H 2O is coupled with the movement of ions and small molecules which in the absence of salt gradients or osmotic stress cannot pass the phospholipid bilayer. However, during osmotically induced shrinking and swelling of SUV the integrity of the phospholipid bilayer is maintained to the extent that vesicles do not break, and therefore equilibration between external medium and vesicle cavity does not take place.

Interbilayer transfer of phospholipid-anchored macromolecules via monomer diffusion.

A series of conjugates has been prepared by linking various hydrophilic macromolecules [poly-(ethylene glycols), polylysine, aminodextrans, or apotransferrin] to synthetic phosphatidylethanolamines via linker moieties incorporating a fluorescent bimane group. Using a fluorescence energy transfer-based assay, the rate of transfer of these species between phospholipid vesicles has been monitored as a function of the nature and size of the coupled macromolecule and of the acyl chain composition of the lipid anchor. Conjugates in which the phospholipid anchor is linked to a small hydrophilic terminal residue (e.g., ethanolamine or ethylenediamine) transfer between large unilamellar vesicles of egg phosphatidylcholine with half-times ranging from tens of minutes (for dimyristoyl lipid conjugates) to a few tens of hours (for dipalmitoyl and 1-palmitoyl-2-oleoyl lipid conjugates), in agreement with previous results for unlabeled phospholipids. Conjugation of these same lipid anchors to larger hydrophilic molecules markedly accelerates their rates of intermembrane transfer, by factors ranging from 5-7-fold (for conjugates with apotransferrin and aminodextrans of molecular weight 10,000-70,000) to over 25-fold [for conjugates with poly(ethylene glycol)-5000]. In all cases the observed transfer appears to reflect the diffusion of lipid monomers through the aqueous phase. Our results suggest that substantial intermembrane transfer can occur, on a time scale of several hours or less, for hydrophilic macromolecules conjugated to diacyl(/alkyl) lipids with 14- to 18-carbon chains unless portions of the conjugate other than the lipid anchor also interact strongly with the membrane.