Phospholipid Hydration Research Articles

An Oxygen-Sufficient Nanoplatform for Enhanced Imaging-Guided Microwave Dynamic Therapy Against Hypoxic Tumors.

Microwave dynamic therapy (MDT) as a novel reactive oxygen species (ROS)-based therapeutic modality has been explored as a promising modality for cancer treatment. However, the intrinsic hypoxic tumor microenvironment (TME) restricted the effectiveness of the MDT. The aim of this study is to develop an oxygen-sufficient nanoplatform with multi-modal imaging capability for enhanced MDT against hypoxic tumors. The liquid perfluorocarbon-based nanoplatform PFP@IR780@O2 was constructed by the phospholipid hydration and sonication method. Then, the characteristics, intracellular uptake process, and subcellular localization of PFP@IR780@O2 were verified. Additionally, the abilities of ROS generation, the anti-hypoxia capability, multi-mode imaging capabilities, and MDT efficacy of the nanoplatform were evaluated via in vitro and in vivo experiments. Finally, the in vivo biocompatibility and toxicity were also evaluated. The prepared nanoparticles PFP@IR780@O2 exhibited suitable size, improved stability, elevated dissolved oxygen level, enhanced cellular uptake, and mitochondria targeting capacity. Additionally, PFP@IR780@O2 demonstrated in vitro and in vivo multimodal imaging capabilities involving ultrasound, fluorescence, and photoacoustic imaging. In vivo studies also indicated that nanoparticles were safe and capable of accumulating in the tumor site after intravenous injection. Furthermore, the PFP@IR780@O2 nanoplatform mediated MDT could effectively alleviate the hypoxic TME, and elevate ROS concentration, thereby resulting in significant tumor growth inhibition. Overall, the oxygen-sufficient nanoplatform with multi-bimodal imaging capability demonstrated improved MDT efficiency, indicating a promising strategy for treating hypoxic tumors.

Electrospun Amphiphilic Nanofibers as Templates for In Situ Preparation of Chloramphenicol-Loaded Liposomes.

The hydration of phospholipids, electrospun into polymeric nanofibers and used as templates for liposome formation, offers pharmaceutical advantages as it avoids the storage of liposomes as aqueous dispersions. The objective of the present study was to electrospin and characterize amphiphilic nanofibers as templates for the preparation of antibiotic-loaded liposomes and compare this method with the conventional film-hydration method followed by extrusion. The comparison was based on particle size, encapsulation efficiency and drug-release behavior. Chloramphenicol (CAM) was used at different concentrations as a model antibacterial drug. Phosphatidylcoline (PC) with polyvinylpyrrolidone (PVP), using ethanol as a solvent, was found to be successful in fabricating the amphiphilic composite drug-loaded nanofibers as well as liposomes with both methods. The characterization of the nanofiber templates revealed that fiber diameter did not affect the liposome size. According to the optical microscopy results, the immediate hydration of phospholipids deposited on the amphiphilic nanofibers occurred within a few seconds, resulting in the formation of liposomes in water dispersions. The liposomes appeared to aggregate more readily in the concentrated than in the diluted solutions. The drug encapsulation efficiency for the fiber-hydrated liposomes varied between 14.9 and 28.1% and, for film-hydrated liposomes, between 22.0 and 77.1%, depending on the CAM concentrations and additional extrusion steps. The nanofiber hydration method was faster, as less steps were required for the in-situ liposome preparation than in the film-hydration method. The liposomes obtained using nanofiber hydration were smaller and more homogeneous than the conventional liposomes, but less drug was encapsulated.

Study of the interactions of gold nanoparticles functionalized with aminolevulinic acid in membrane models

Gold nanoparticles have been intensively studied in cancer therapy to improve drug release, increasing therapeutic action and reducing adverse effects. The interaction between gold nanoparticles and cell membranes can give information about the cell internalization. In this study, gold nanoparticles with aminolevulinic acid (5-ALA) were synthesized using the photoreduction method (5-ALA: AuNPs). The prodrug 5-ALA is responsible for protoporphyrin IX synthesis inside the cell and allows the use of therapies as photodynamic and sonodynamic therapies. The cytotoxicity test was performed on a breast cancer tumor line (MCF-7), and high Content Screening assay was applied to evaluate the entry of nanoparticles into cells. DPPS Langmuir monolayers were assembled at the air/water interface and employed as a simplified membrane model for half of a tumorigenic cell membrane. We assessed the molecular interactions between 5-ALA: AuNPs and phospholipids using tensiometry (π-A isotherms) and vibrational spectroscopy (PM-IRRAS) experiments. We found that the functionalized gold nanoparticles strongly interact with DPPS polar head groups (especially phosphate and carbonyl), changing the phospholipid hydration and leading to a general decrease in the monolayer conformational order. This work then probes that specific interaction between 5-ALA: AuNPs and the negatively charged phospholipid can be assessed using Langmuir monolayers as simplified biomembrane models.

Molecular interactions in reverse hexagonal mesophase in the presence of Cyclosporin A

The present work investigates the detailed molecular structure of the H II mesophase of GMO/tricaprylin/phosphatidylcholine/water system in the presence of hydrophobic model peptide Cyclosporin A (CSA) via ATR-FTIR analysis. The conformation of the peptide in the hexagonal mesophase, as well as its location and specific interactions with the components of the carrier, were studied. Incorporation of phosphatidylcholine to the ternary GMO/tricaprylin/water system caused competition for water binding between the hydroxyl groups of GMO and the phosphate groups of the phosphatidylcholine (PC) leading to dehydration of the GMO hydroxyls in favor of phospholipid hydration. Analysis of CSA solubilization effect on the H II mesophase revealed a significant increase in the strength of hydrogen bonding with surfactant hydrogen-bonded carbonyls, indicating interaction of the peptide with the C O groups of the surfactants. The peptide probably caused partial replacement of the intramolecular hydrogen bonds of the mesophase carbonyl groups with intermolecular hydrogen bonds of these carbonyl groups with the peptide. Furthermore, analysis of the Amide I’ peak in the FTIR spectra of the peptide demonstrated that two pairs of its internal hydrogen bonds are disrupted when it is incorporated. The partial disruption of the internal hydrogen bonds seems to cause an outward rotation of the peptide amide groups involved, resulting in more efficient intermolecular hydrogen-bonding ability. Apparently, this conformational change increased the hydrophilic properties of CSA, even making it susceptible to a weak interaction with the GMO hydroxyl groups in the interfacial region.

Properties of various phospholipid mixtures as emulsifiers or dispersing agents in nanoparticle drug carrier preparations

The characteristics of mixed phospholipids were examined when used as dispersing agents and emulsifiers. Synthesized phospholipids were mixed to investigate the potential effects of different hydrophilic or lipophilic groups on emulsification and dispersion. To examine the effects of the hydrophilic polar head group on the dispersing or emulsifying potency of phospholipids, l-α-phosphatidylcholine dimyristoyl (DMPC) and l-α-phosphatidylethanolamine dimyristoyl (DMPE) were mixed in various ratios. Moreover, all combinations of two kinds of phosphatidylcholines (PCs) out of l-α-phosphatidylcholine dilauroyl (DLPC), DMPC, l-α-phosphatidylcholine dipalmitoyl (DPPC) and l-α-phosphatidylcholine distearoyl (DSPC) were tested (50:50, w/w) to examine the effects of the hydrophobic carbon chains on the dispersing or emulsifying potency of phospholipids. Mean diameters of vesicles and O/W emulsions prepared by sonication were measured. Vesicles prepared with DMPC–DMPE mixtures gave larger particle sizes than those of DMPC alone. Particle sizes of vesicles prepared with a mixture of two kinds of PCs increased when adding a PC with a longer carbon chain, while particle sizes in a mixture with a PC having a shorter carbon chain was comparable to those in pure PC. In vesicles that were generated by hydration of phospholipids and had a bilayer form, the physical form of the phospholipids consisting of bilayers was thought to be an important factor influencing particle sizes. Among the emulsions, DMPC–DMPE mixtures gave a similar droplet size to DMPC alone. Droplet size in emulsions prepared with a mixture of two kinds of PCs had a strong positive correlation with the total number of carbons, which corresponds to hydrophilic–lipophilic balance (HLB). In O/W emulsions, in which phospholipids were absorbed at water–oil interfaces and which have a single layer form, HLB was thought to be a major factor in the determination of particle size; likewise with non-ionic emulsifiers.

The effect of the choline head group on phospholipid hydration

The hydration behavior of three naturally occurring sphingomyelins (SM) has been studied. Adsorption isotherms for these SM, singly and in combination with other lipids, were obtained and the isotherms analyzed by the application of BET theory. The results are compared with those found previously by us for the phosphatidylcholines (PC). We find that, depending on the SM studied, both ‘weak’ and ‘strong’ water adsorption can be observed and thus, the presence of choline in the phospholipid head group, does not guarantee ‘strong’ water adsorption. When the head group is removed from SM, the resulting ceramides are found to be very weak, water adsorbers. Cholesterol, when present in a mixture with SM, has a rather dramatic effect on the hydration of the SM. A mixed system of PC and SM exhibits water adsorption characteristics very similar to those exhibited by PC itself. We speculate that these hydration results may well play a role in the cell signaling action of SM and, moreover, may be closely associated with the hypothesized formation of lipid ‘rafts.’

A Calorimetric Study of Phospholipid Hydration. Simultaneous Monitoring of Enthalpy and Free Energy

We present a novel method for monitoring isothermal lipid hydration using a sorption microcalorimeter. A measuring cell of the double-twin calorimeter consists of two vessels connected by a stainless steel tube. The upper vessel contains pure water, and the bottom vessel is loaded with the lipid sample. This calorimeter allows for simultaneous measurement of the partial molar enthalpy and the chemical potential (or the partial molar free energy) of the water. The versatility of the method is demonstrated by studies of the hydration of the phospholipids dipalmitoyl phosphatyl choline (DPPC), dimyristoyl phosphatidyl choline (DMPC), and dilauroyl phosphatidyle choline (DLPC) at 25 and 27 °C. The measurements provide a relation between water content and water chemical potential, which, in these lamellar systems, is often recast as a force−distance relation and has been called the hydration force. Through the simultaneously monitored calorimetric values, the partial molar enthalpy of water is also obtained. The method consequently provides a rather unique combination of information on both partial molar enthalpy and partial molar free energy and thus also the partial molar entropy of the process. We find that the incorporation of the first three to four water molecules per lipid is exothermic. These water molecules presumably interact directly with oxygen atoms on the phosphate of the lipid headgroup. When the first waters have been added, the remaining ones are incorporated endothermically. This applies to the water molecules taken up both in the gel phase and in the liquid crystalline state. We also observe that the sorption process triggers a first-order phase change from a gel (Lβ‘) to a liquid crystalline (Lα) phase. For DLPC, this occurs at 25 °C at a relative humidity of 0.79 with an endothermic transition enthalpy of 42 ± 2 kJ/mol (DLPC) and, for DMPC, at 27 °C at 0.93 relative humidity with ΔH = 56 ± 5 kJ/mol (DMPC). We use a previously established model to quantitatively interpret these phase transitions. Furthermore, the observed endothermic nature of the sorption process above three to four waters per lipid is fully consistent with the suggestion that the negative free energy of the sorption (swelling) is due to increased thermal excitations and thus a positive entropy. It is more problematic to reconcile the data with models proposing structuring effects in the water as the main cause of the swelling.

Studies of Phospholipid Hydration by High-Resolution Magic-Angle Spinning Nuclear Magnetic Resonance

A sample preparation method using spherical glass ampoules has been used to achieve 1.5-Hz resolution in 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of aqueous multilamellar dispersions of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) and 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC), serving to differentiate between slowly exchanging interlamellar and bulk water and to reveal new molecular-level information about hydration phenomena in these model biological membranes. The average numbers of interlamellar water molecules in multilamellar vesicles (MLVs) of DOPC and POPC were found to be 37.5 ± 1 and 37.2 ± 1, respectively, at a spinning speed of 3 kHz. Even at speeds as high as 9 kHz, the number of interlamellar waters remained as high as 31, arguing against dehydration effects for DOPC and POPC. Both homonuclear and heteronuclear nuclear Overhauser enhancement spectroscopy (NOESY and HOESY) were used to establish the location of water near the headgroup of a PC bilayer. 1H NMR comparisons of DOPC with a lipid that can hydrogen bond (monomethyldioleoylphosphatidylethanolamine, MeDOPE) showed the following trends: 1) the interlamellar water resonance was shifted to lower frequency for DOPC but to higher frequency for MeDOPE, 2) the chemical shift variation with temperature for interlamellar water was less than that of bulk water for MeDOPE MLVs, 3) water exchange between the two lipids was rapid on the NMR time scale if they were mixed in the same bilayer, 4) water exchange was slow if they were present in separate MLVs, and 5) exchange between bulk and interlamellar water was found by two-dimensional exchange experiments to be slow, and the exchange rate should be less than 157 Hz. These results illustrate the utility of ultra-high-resolution 1H MAS NMR for determining the nature and extent of lipid hydration as well as the arrangement of nuclei at the membrane/water interface.

The hydration of phospholipids and its biological significance

The difference between the physical properties of water near the surface of certain materials and the same properties of solution water has been known for some time. The idea that this difference may also apply to the surfaces of biomolecules, cells, and tissues and, thus, may be involved in the structure/function relationships of these biological entities has only recently been appreciated. The purpose of this review is to discuss the nature of this water-surface interaction as it relates to biology and medicine, to illustrate some of the methods by which this water is studied and, perhaps most importantly, to inspire the reader to consider how this “bound water” concept might be applied to his or her research. Such research would include transport across membranes, lateral movement along the cell surface, and lipid-protein interactions, as well as many metabolic processes, both normal and pathological. Although the emphasis in this review is on the interaction of water with phospholipids, it is the author's experience that bound water is rapidly being accepted as a useful concept in the study of other macromolecular processes, such as radiation damage in DNA and bioenergetic mechanisms in proteins.

Direct in Situ Measurement of Phospholipid Hydration in an Aqueous Environment Using a Quartz Crystal Microbalance

Hydration of phospholipid head groups is essential for stabilizing bilayer structures in an aqueous environment. Here we describe a new, easy, and rapid method that can directly and quantitatively determine phospholipid hydration in both water vapor and liquid water ; i.e., a quartz crystal microbalance is employed which enables hydration to be determined in only 1 h using just 1 μg of phospholipids under various environmental conditions. Dipalmitoylphosphatidylethanolamine (DPPE) was examined with varying temperature, KCl concentration, and phase of water, where it was found that DPPE hydration in liquid water markedly increases upon raising the temperature, even when the phospholipid is in a gel phase, being maxima1 around its phase transition temperature (63.5 °C). Furthermore, KCl addition increased DPPE hydration in a temperature-dependent manner. The method presented is demonstrated to be capable of determining phospholipid hydration under physiological conditions ; hence, it is suitable for investigating hydration phenomena involved in biological processes.

Changes in the lipid dynamics of liposomal membranes induced by poly(ethylene glycol): free volume alterations revealed by inter- and intramolecular excimer-forming phospholipid analogs

Influence of osmotic shrinkage, swelling, and dehydration on large unilamellar liposomes (LUVs) of 1,2-dioleoylsn-glycero-3-phosphocholine (DOPC) was investigated using the fluorescent lipid probes 1-palmitoyl-2-[10-(pyren-1-yl)]-decanoyl-sn-glycero-3-phosphocholi ne (PPDPC) and 1,2-bis[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC). Increasing concentrations of poly(ethylene glycol) (PEG, average molecular weight of 6000) producing osmotic gradients delta omega up to 250 mOsm/kg were first added to the outside of LUV labeled with 0.1 mol% of either of the above fluorescent phospholipids. The resulting osmotic shrinkage was accompanied by a progressive reduction in the lateral diffusion of the membrane-incorporated PPDPC, evident as a decrease in the rate of its intermolecular excimer formation. In contrast, under the same conditions the rate of intramolecular excimer formation by bisPDPC increased. Notably, signals opposite to those described above were observed for both of the fluorescent probes upon osmotic swelling of DOPC liposomes with encapsulated PEG. The lateral diffusion of PPDPC became progressively reduced upon membrane dehydration due to increasing concentrations of symmetrically distributed PEG (with equal polymer concentrations inside and outside of the liposomes) when neither shrinkage nor swelling occurs while enhanced excimer formation by bisPDPC was evident. The later results were interpreted in terms of osmotically induced changes in the hydration of lipids. In brief, the removal of water from the phospholipid hydration shell diminishes the effective size of the polar headgroup, which subsequently allows for an enhanced lateral packing of the phospholipid acyl chains. Our findings are readily compatible with membrane free volume Vf changes due to osmotic forces under three different kinds of stress (shrinkage, swelling, and dehydration) applied on the lipid bilayers.

Infrared study of phospholipid hydration. New thermodynamic data about the main phase transition of saturated phosphatidylcholine water multidispersions

Infrared spectra measurements were performed with fully hydrated phospholipid multilamellar dispersions. The temperature profiles of the δ (OH) bending vibration at 1645 cm −1, correspondng to the water molecules, show three marked transitions in such temperature ranges that they can be respectively associated with the fusion of free water molecules, with pretransition phenomena and with the main phase transition. From absorbance variations and calorimetric data, main phase transition enthalpies and entropies were calculated for both the acyl chains and the polar head-water regions. Negative contributions of the polar head-water region are consistent with previous results, indicating a more ordered water layer in the fluid phase. On the other hand, a decrease of the amount of bound water molecules with increasing chain length is pointed out.

Infrared study of phospholipid hydration. Main phase transition of saturated phosphatidylcholine/water multilamellar samples

Infrared spectra of phosphatidylcholine (DMPC, DPPC, DSPC)/water multilamellar samples were recorded as a function of both water concentration and temperature. From wavenumber shifts, molecular interactions at the polar head/water interface are pointed out, particularly between the phosphate groups and the water molecules. Temperature profiles of the δ CH, (1465 cm −1) and δ OH (1645 cm −1) absorption bands provide the main transition temperatures of both the hydrocarbon chains of the phospholipid and the water layer. Experimental evidence of a re-ordering of the water layer before the well-known melting of the phospholipid chains are shown, whatever the water concentration and the chain length may be.

Purification of Band 3 from the Human Erythrocyte Membrane and its Incorporation into Liposome

Band 3, the predominent 95,000 dalton anion transport protein, is the major intrinsic glycoprotein of the human erythrocyte membrane. This anion carrier exists as a dimer and binds the cytoskeletons such as spectrin, ankyrin and actin. And the liposomes are vesicular structures which form spontaneouly upon hydration of phospholipids. These artificial lipid vesicles have been investigated as model of the biological membranes and as a mean of improving the delivery of nucleic acids, drugs, proteins and biological substances to specific target tissues and cells. In this study, we were purified Band 3 from the human erythrocyte membrane(ghost) was prepared by hemolysis of intact human erythrocyte with weak alkali-hypotonic solution. Band 6 was removed from ghost by extracting with solution of an ionic strength of 0.15. Band 3 and Band 4 were solubilized selectively by extracting Band 6-depleted ghosts with Triton X-100 under nondenaturing conditions. Band 3 was then purified from Triton X-100 extract treated with p-chloromercuribenzoate by sucrose density gradient ultracentrifugation. This purified Band 3 was incorporated into liposomes prepared by reverse-phase evaporation. Phosphatidyl L-serine and cholesterol(1 : 1 molar ratio) were dissolved in chloroform and then chloroform was removed by rotatory evaporation under reduced pressure. Band 3 solution without Triton X-100 was introduced into a mixture of lipids and diethylether. Diethylether was subsequently removed by evaporation. This purified Band 3 and its incorporation into liposomes were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Effect of poly(ethylene glycol) on phospholipid hydration and polarity of the external phase

The hydration properties of phosphatidylcholine (PC)/water dispersions on the addition of poly(ethylene glycol) were studied by means of 2H-NMR. The quadrupole splittings and their temperature dependences correspond to measurements of PC/water dispersions at low water content. It is concluded that the bound water is partly extracted by poly(ethylene glycol) but the binding properties of the water in the inner hydration shell of about five water molecules are not changed. The ability of some phospholipid/water dispersions to undergo phase transitions to nonlamellar structures upon dehydration is discussed. Dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine do not form nonlamellar structures on addition of purified poly(ethylene glycol), as was demonstrated by means of 31P-NMR. Poly(ethylene glycol) decreases the polarity of the aqueous phase and the partition of hydrophobic molecules between the membrane and the external phase is changed. This was demonstrated using the excimer fluorescence of pyrene in a ghost suspension. It is suggested that the changes in polarity and hydration on the addition of poly(ethylene glycol) can contribute to the alterations in the membrane surface observed under conditions of membrane contact and fusion.

Phospholipid‐ phospholipid interaction in soybean oil

AbstractPhospholipid‐phospholipid interaction in soybean oil is described. Phosphatidylcholine was effectively removed from soybean oil by degumming (water hydration), whereas phosphatidylethanolamine and phosphatidic acid were hardly hydratable. However, the degree of their hydration increased in the presence of phosphatidylcholine. The spectrophotometric assay based on charge transfer interaction between 7,7,8,8‐tetracyanoquinodimethane and phospholipids at 480 nm was used to determine the formation of phospholipid micelles in soybean oil. The critical micelle concentrations were 0.085, 0.84 and 2.6 mM for phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid, respectively. Phosphatidylcholine interacted with phosphatidylethanolamine or phosphatidic acid to form mixed micelles. The critical micelle concentrations of phosphatidylcholine‐phosphatidylethanolamine mixture and phosphatidylcholine‐phosphatidic acid mixture were 0.16 and 1.3 mM, respectively. The degree of hydration of phospholipids was related to their critical micelle concentrations. Interaction of phosphatidylcholine with phosphatidylethanolamine or phosphatidic acid was confirmed by determining the changes in the chemical shifts of 31PNMR spectra.

The hydration of phospholipids and phospholipid-cholesterol complexes

The hydration characteristics of phosphatidylcholines and the effect of cholesterol on these were studied with differential thermal analysis and water vapour adsorption experiments. Also the water adsorption of egg phosphatidylethanolamine and the effect of cholesterol on this was studied and compared with corresponding qualities of phosphatidylcholine. The differential thermal analysis study showed that the monohydrates of egg, dipalmitoyl, and dioleoyl phosphatidylcholine tightly bind ∼9 molecules of water per phosphatidylcholine molecule. Cholesterol is proved to somewhat increase the water binding of the phospholipids. Cholesterol is also shown to decrease the heat change of the chain melting transition of dioleoyl phosphatidylcholine, but not to abolish it completely. The water adsorption experiments indicate that the hydration of phosphatidylcholines takes place in two steps; a strong initial water binding and a second phase of weak binding. The adsorption isotherm of egg phosphatidylethanolamine is strikingly different from that of egg phosphatidylcholine. Cholesterol is shown, also by this method, to increase the hydration of phospholipids especially that of dipalmitoylphosphatidylcholine. The results in this study are in good agreement with those presented by many other authors. Starting with the accumulated information of the hydration characteristics of phosphatidylcholines the organization of the bound water around the polar group is discussed and the most probable model is evaluated.

Phospholipid hydration studied by deuteron magnetic resonance spectroscopy

Deuteron magnetic resonance spectra were obtained from 2H 2O in mixtures with egg lecithin, egg phosphatidylethanolamine, and ox brain sodium phosphatidylserine. The acid form of phosphatidylserine does not hydrate. Details of the different hydration “shells” were obtained by studying the spectral splittings as a function of 2H 2O concentration. Several different types of water are present, including bulk water (exchanging only slowly with water associated with the lipid), “trapped” water (not present with phosphatidylethanolamine), and up to three types of bound water. The spectral splittings characteristic of each water environment yielded information about the water binding energies and degrees of anisotropy of motion of the phospholipid polar groups; lecithin polar groups have least motional restriction and sodium phosphatidylserine most, while phosphatidylethanolamine binds water most tightly. Spectra of some lecithin and phosphatidylserine dispersions varied with time, due to a slow reorganization of randomly oriented multilamellar regions into longer, more ordered systems, with a length of about 1 μm. At −20°C the timescales of the change were of the order of a week and a month for lecithin and phosphatidylserine respectively. Complex changes in the spectra were observed as the temperature was raised; these are interpreted in terms of changes in the motions of the phospholipid molecules.

The hydration of phospholipids

The adsorption isotherms of egg phosphatidylcholine, egg phosphatidylethanolamine and bovine phosphatidylserine, in both their lyso and diacyl forms, have been determined in an examination of the hydration characteristics of films of these phospholipids. It appears that the diacyl phosphatidylcholine lipid has a higher affinity for water than diacyl phosphatidylethanolamine or diacyl phosphatidylserine. The lyso form of phosphatidylcholine and phosphatidylethanolamine display similar isotherms as compared to their respective diacyl counterparts. The lyso form of bovine phosphatidylserine, displays an isotherm which differs greatly from that of its diacyl form. Cholesterol is found to increase the hydration of phosphatidylcholine when complexdee with this phospholipid. Increasing the number of double bonds in the hydrocarbon chains of phosphatidylcholine is also found to increase the hydration of this phospholipid. The nature of the polar head-group and the hydrocarbon chain thus both have an effect on the hydration of phospholipids.