Search PubMed⌕ Search

Biomedical subjects

T J McIntosh

Publications and source records attributed to T J McIntosh.

At least 37 records · Page 2Linked to original sources
PubMed1997

Probing the ethanol-induced chain interdigitations in gel-state bilayers of mixed-chain phosphatidylcholines.

Using high-resolution differential scanning calorimetry (DSC), we have studied the effects of ethanol concentrations, [EtOH], on the main phase transition temperatures (T[m]) of the following mixed-chain phosphatidylcholines (PCs): C(15):C(17)PC, C(17):C(15)PC, and C(12):C(20)PC. These lipids have a common molecular weight; however, their apparent acyl chain-length differences between the sn-1 and sn-2 acyl chains, delta C, are distinctively different. The delta C values for these three mixed-chain PCs are, respectively, 0.5, 3.5, and 6.5 C-C bond lengths. DSC results show that the T(m) profiles for C(15):C(17)PC and C(17):C(15)PC bilayers in the plot of T(m) versus [EtOH] are V-shaped biphasic curves, with the minimum T(m) occurring at 50 and 73 mg/ml of ethanol, respectively. In contrast, the C(12):C(20)PC bilayer exhibits a nearly linear decrease in T(m) with increasing [EtOH]. In addition, x-ray diffraction experiments were also performed to assess the structural changes of these three mixed-chain PCs in the gel-state bilayers, at 20 degrees C, in response to high concentrations of ethanol. X-ray diffraction data indicate that, in the absence of ethanol, these three lamellar lipids are all packed in the normal (L beta') gel phase in aqueous media. In the presence of 120 mg/ml of ethanol, however, the C(15):C(17)PC and C(17):C(15)PC lamellae are packed in the fully interdigitated (L beta[I]) gel phase. The V-shaped T(m) curves detected calorimetrically for these two lipids in response to [EtOH] can thus be explained by the ethanol-induced L beta' --> L beta[I] isothermal phase transition. Interestingly, the results of x-ray diffraction study reveal, for the first time, that an ethanol-induced L beta' --> L(MI) (mixed interdigitated phase) isothermal phase transition occurs in the gel-state bilayer of highly asymmetrical C(12):C(20)PC. Therefore, the chain asymmetry is recognized to play an important role in the ethanol-induced chain interdigitation at T < T(m).

Biophysical Phenomena↗
PubMed1996

Hydration properties of lamellar and non-lamellar phases of phosphatidylcholine and phosphatidylethanolamine.

Two of the most common phospholipids in biological membranes are phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Over a wide range of temperatures the PCs found in biological membranes form lamellar (bilayer) phases when dispersed in excess water, whereas PEs form either lamellar or hexagonal phases depending on their hydrocarbon chain composition. This paper details the hydration properties of lamellar and hexagonal phases formed by PCs and PEs, focusing on the energetics of hydration of these phases. For the hexagonal phase, the energy of bending the lipid monolayer is a critical term, with other contributions arising from the energies of hydrating the lipid headgroups and filling voids in the interstices in the hydrocarbon region. For the lamellar phase of PC, the water content is determined by a balance between the attractive van der Waals pressure and repulsive hydration and entropic (steric) pressures. In the case of PE bilayers, recent experiments demonstrate the presence of an additional strong, short-range attractive interaction, possibly due to hydrogen-bonded water interactions between N+ H3 groups in one bilayer and the PO4- groups in the apposing bilayer. This additional attractive pressure causes apposing PE bilayers to adhere strongly and to imbibe considerably less water than PC bilayers.

Hydrogen Bonding↗
PubMed1996

X-ray diffraction analysis of isolated skin lipids: reconstitution of intercellular lipid domains.

Low- and wide-angle X-ray diffraction were used to determine the structural organization of lipids isolated from the stratum corneum extracellular matrix that forms the major water permeability barrier in mammalian epidermis. Hydrated pig skin ceramides gave a single low-angle reflection of about 62 angstroms and a wide-angle-reflection at 4.15 angstroms. The addition of either cholesterol or fatty acid, the other major lipid components of the skin stratum corneum extracellular matrix, modified this diffraction pattern, depending on the lipid mole ratios. In the absence of water, lipid mixtures exhibited lipid phase separation, as shown by low- and wide-angle reflections typical of a separate cholesterol phase. However, a hydrated 2:1:1 mole ratio of ceramide:cholesterol:palmitic acid (similar to that found in stratum corneum) produced a diffraction pattern with a single sharp wide-angle reflection at 4.10 angstroms and low-angle reflections which indexed as the first eight orders of a single repeat period of 130 angstroms. The repeat period and intensity distribution of the low-angle data were similar to those found in intact stratum corneum [White et al. (1988) Biochemistry 27, 3725-3732; Bouwstra et al. (1994) Biochim. Biophys. Acta 1212, 183-192]. Higher concentrations of cholesterol or palmitic acid resulted in lipid phase separations. The 130 angstrom repeat period decreased only about 3 angstroms as water was removed by incubation in low-relative humidity atmospheres. The 130 angstrom repeat period depended on the presence of a particular ceramide, N-(omega-acyloxy)-acylsphingosine, which is found only in the epidermis. In contrast, 2:1:1 mixtures of brain ceramide:cholesterol:palmitic acid gave reflections of 56 and 34 angstroms. These results indicate that a structure with dimensions similar to those of the lamellar repeating unit found in skin stratum corneum does not depend on the presence of protein but does depend on the presence of specific skin ceramides and appropriate concentrations of cholesterol and fatty acid.

Animals↗
PubMed1996

Structure and interactive properties of highly fluorinated phospholipid bilayers.

Because liposomes containing fluoroalkylated phospholipids are being developed for in vivo drug delivery, the structure and interactive properties of several fluoroalkylated glycerophosphocholines (PCs) were investigated by x-ray diffraction/osmotic stress, dipole potential, and hydrophobic ion binding measurements. The lipids included PCs with highly fluorinated tails on both alkyl chains and PCs with one hydrocarbon chain and one fluoroalkylated chain. Electron density profiles showed high electron density peaks in the center of the bilayer corresponding to the fluorine atoms. The height and width of these high density peaks varied systematically, depending on the number of fluorines and their position on the alkyl chains, and on whether the bilayer was in the gel or liquid crystalline phase. Wide-angle diffraction showed that in both gel and liquid crystalline bilayers the distance between adjacent alkyl chains was greater in fluoroalkylated PCs than in analogous hydrocarbon PCs. For interbilayer separations of less than about 8 A, pressure-distance relations for fluoroalkylated PCs were similar to those previously obtained from PC bilayers with hydrocarbon chains. However, for bilayer separations greater than 8A, the total repulsive pressure depended on whether the fluoroalkylated PC was in a gel or liquid-crystalline phase. We argue that these pressure-distance relations contain contributions from both hydration and entropic repulsive pressures. Dipole potentials ranged from -680 mV for PCs with both chains fluoroalkylated to -180 mV for PCs with one chain fluoroalkylated, compared to +415 mV for egg PC. The change in dipole potential as a function of subphase concentration of tetraphenyl-boron was much larger for egg PC than for fluorinated PC monolayers, indicating that the fluorine atoms modified the binding of this hydrophobic anion. Thus, compared to conventional liposomes, liposomes made from fluoroalkylated PCs have different binding properties, which may be relevant to their use as drug carriers.

Drug Carriers↗
PubMed1996

The interaction of polyphenols with bilayers: conditions for increasing bilayer adhesion.

Because proteins and other molecules with a high polyphenol content are commonly involved in adhesion processes, we are investigating the interactions between polyphenols and biological materials. A naturally occurring polyphenol that binds a variety of proteins and lipids is tannic acid (TA), which contains five digallic acid residues covalently linked to a central D-glucose. A previous study has shown that TA increases the adhesion between apposing phosphatidylcholine (PC) bilayers and over a very narrow concentration range collapses the interbilayer fluid space from about 15 A to 5 A. To determine the chemical requirements a polyphenolic molecule must possess to increase bilayer adhesion, we have synthesized several simpler TA analogs that vary in their size, shape, and number of gallic acid and hydroxyl groups. X-ray diffraction, absorbance, binding, and differential scanning calorimetry measurements were used to investigate the interaction of these polyphenolic molecules with egg PC (EPC) and dipalmitoyl PC (DPPC) bilayers. Of these synthetic polyphenols, only penta-O-galloyl-alpha-D-glucose (PGG) was able to completely mimic the effects of TA by collapsing the interbilayer fluid space from 15 A to 5 A, decreasing the dipole potential by about 300 mV, increasing the transition enthalpy of DPPC liposomes, and inducing an interdigitated phase in DPPC. Binding studies indicated that the fluid space was reduced to 5 A at an EPC:PGG mole ratio of 5:1. We conclude that these polyphenols collapse the fluid space of PC bilayers because they 1) are amphipathic and partition into the bilayers interfacial region, 2) are long enough to span the interbilayer space, 3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and 4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. Under these conditions we conclude that the polyphenols from interbilayer bridges. We argue that these bridges are stabilized by increased adhesion arising from an increased van der Waals interaction between apposing bilayers, electrostatic interactions between the pi electrons in the phenol ring and the -(N+CH3)3 groups on the PC headgroups, decreased hydration repulsion between bilayers, and hydrogen bonds between the H-bond-donating moieties on the polyphenols and H-bond-accepting groups in the bilayer.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1995

Experimental tests for protrusion and undulation pressures in phospholipid bilayers.

Theoretical treatments predict that strong entropic pressures between adjacent bilayer membranes can arise from out of plane motions caused by either thermally induced bending undulations of the entire bilayer [Harbich, W., & Helfrich, W. (1984) Chem. Phys. Lipids 36, 39-63; Evans, E. A., & Parsegian, V. A. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 7132-7136] or protrusions of individual lipid molecules from the bilayer surface [Israelachvili, J. N., & Wennerström, H. (1992) J. Phys. Chem. 96, 520-531]. To determine the relative contributions of these motions to the repulsive pressure between phospholipid bilayers, the osmotic stress/X-ray diffraction method was used to measure the range and magnitude of the total repulsive pressure, and micropipet methods were used to measure the bending moduli of phosphatidylcholine bilayers containing lysophosphatidylcholine and polyunsaturated diarachidonoylphosphatidylcholine (DAPC) bilayers. In the gel phase, incorporation of equimolar lysophosphatidylcholine into phosphatidylcholine bilayers caused the hydrocarbon chains from apposing monolayers to interdigitate, but did not appreciably change the equilibrium fluid spacing in excess buffer from its control value of 12 A. In contrast, the incorporation of equimolar lysophosphatidylcholine into liquid-crystalline phase phosphatidylcholine bilayers markedly increased the range of the repulsive pressure so that equilibrium fluid separation increased from 15 to 28 A, and also decreased the bilayer bending modulus from 5.1 x 10(-13) to 1.3 x 10(-13) erg. Liquid-crystalline DAPC bilayers had intermediate values of both equilibrium fluid separation (20 A) and bending modulus (2.8 x 10(-13) erg). Analysis of these data indicates that (1) the relative importance of entropic pressures compared to the hydration pressure depends strongly on the composition and structure of the bilayer, (2) the protrusion pressure may contribute to the total repulsive pressure at large pressures or small fluid spacings, and (3) the repulsive undulation pressure, together with the attractive van der Waals pressure, is a primary factor in determining the fluid spacing at low and/or zero applied pressures in liquid-crystalline bilayers.

Electrons↗
PubMed1995

Temperature dependence of the repulsive pressure between phosphatidylcholine bilayers.

Bilayer structure and interbilayer repulsive pressure were measured from 5 to 50 degrees C by the osmotic stress/x-ray diffraction method for both gel and liquid crystalline phase lipid bilayers. For gel phase dibehenoylphosphatidylcholine (DBPC) the bilayer thickness and pressure-distance relations were nearly temperature-independent, and at full hydration the equilibrium fluid spacing increased approximately 1 A, from 10 A at 5 degrees C to 11 A at 50 degrees C. In contrast, for liquid crystalline phase egg phosphatidylcholine (EPC), the bilayer thickness, equilibrium fluid spacing, and pressure-distance relation were all markedly temperature-dependent. As the temperature was increased from 5 to 50 degrees C the EPC bilayer thickness decreased approximately 4 A, and the equilibrium fluid spacing increased from 14 to 21 A. Over this temperature range there was little change in the pressure-distance relation for fluid spacings less than approximately 10 A, but a substantial increase in the total pressure for fluid spacings greater than 10 A. These data show that for both gel and liquid crystalline bilayers there is a short-range repulsive pressure that is nearly temperature-independent, whereas for liquid crystalline bilayers there is also a longer-range pressure that increases with temperature. From analysis of the energetics of dehydration we argue that the temperature-independent short-range pressure is consistent with a hydration pressure due to polarization or electrostriction of water molecules by the phosphorylcholine moiety. For the liquid crystalline phase, the 7 A increase in equilibrium fluid spacing with increasing temperature can be predicted by an increase in the undulation pressure as a consequence of a temperature-dependent decrease in bilayer bending modulus.

Gels↗
PubMed1995

Structure and phase behavior of lipid suspensions containing phospholipids with covalently attached poly(ethylene glycol).

Liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-lipids) are being developed for in vivo drug delivery. In this paper we determine the structure and phase behavior of fully hydrated distearoylphosphatidylcholine (DSPC) suspensions containing PEG-lipids composed of distearoylphosphatidylethanolamine with attached PEGs of molecular weights ranging from 350 to 5000. For DSPC:PEG-lipid suspensions containing 0-60 mol % PEG-lipid, differential scanning calorimetry shows main endothermic transitions ranging from 55 to 64 degrees C, depending on the size of the PEG and concentration of PEG-lipid. The enthalpy of this main transition remains constant for all PEG-350 concentrations but decreases with increasing amounts of PEG-750, PEG-2000, or PEG-5000, ultimately disappearing at PEG-lipid concentrations greater than about 60 mol %. Low-angle and wide-angle x-ray diffraction show that tilted gel (L beta') phase bilayers are formed for all PEG-lipid molecular weights at concentrations of about 10 mol % or less, with the distance between bilayers depending on PEG molecular weight and PEG-lipid concentration. At PEG-lipid concentrations greater than 10 mol %, the lipid structure depends on the size of the PEG moiety. X-ray diffraction analysis shows that untilted interdigitated (L beta I) gel phase bilayers form with the incorporation of 40-100 mol % PEG-350 or 20-70 mol % PEG-750, and untilted gel (L beta) phase bilayers are formed in the presence of about 20-60 mol % PEG-2000 and PEG-5000. Light microscopy, turbidity measurements, x-ray diffraction, and 1H-NMR indicate that a pure micellar phase forms in the presence of greater than about 60% PEG-750, PEG-2000, or PEG-5000.

Calorimetry, Differential Scanning↗
PubMed1995

Range and magnitude of the steric pressure between bilayers containing phospholipids with covalently attached poly(ethylene glycol).

The interactive properties of liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-lipids) are of interest because such liposomes are being developed as drug delivery vehicles and also are ideal model systems for measuring the properties of surface-grafted polymers. For bilayers containing PEG-lipids with PEG molecular weights of 350, 750, 2000, and 5000, pressure-distance relations have been measured by X-ray diffraction analysis of liposomes subjected to known applied osmotic pressures. The distance between apposing bilayers decreased monotonically with increasing applied pressure for each concentration of a given PEG-lipid. Although for bilayers containing PEG-350 and PEG-750 the contribution of electrostatic repulsion to interbilayer interactions was significant, for bilayers containing PEG-2000 and PEG-5000 the major repulsive pressure between bilayers was a steric pressure due to the attached PEG. The range and magnitude of this steric pressure increased both with increasing PEG-lipid concentration and PEG size, and the extension length of the PEG from the bilayer surface at maximum PEG-lipid concentration depended strongly on the size of the PEG, being less than 35 A for PEG-750, and about 65 A for PEG-2000 and 115 A for PEG-5000. The measured pressure-distance relations have been modeled in terms of current theories (deGennes, 1987; Milner et al., 1988b) for the steric pressure produced by surface-grafted polymers, as modified by us to take into account the effects of polymer polydispersity and the possibility that, at low grafting densities, polymers from apposing bilayers surfaces can interpenetrate or interdigitate. No one theoretical scheme is sufficient to account for all the experimental results. However, for a given pressure regime, PEG-lipid size, and PEG-lipid surface density, the appropriately modified theoretical treatment gives a reasonable fit to the pressure-distance data.

Kinetics↗
PubMed1994

Long- and short-range interactions between phospholipid/ganglioside GM1 bilayers.

The structure and interactive properties of liquid-crystalline egg phosphatidylcholine (EPC) bilayers containing the ganglioside GM1 and its uncharged analogue, asialoGM1 (AGM1), have been obtained by X-ray diffraction analysis of osmotically stressed liposomes. Both electron density profiles and reciprocal space modeling indicate that (1) the incorporation of up to 30 mol % GM1 into EPC bilayers has little effect on bilayer organization and (2) the oligosaccharide portion of the GM1 molecule extends at least 12 A beyond the EPC head group into the fluid space, implying that the GM1 head group is nearly fully extended from the bilayer surface. Pressure-distance relations for EPC:GM1 bilayers in 100 mM ionic strength buffer show that, for large bilayer separations, the interbilayer repulsive pressure decays exponentially with a decay length and magnitude expected for electrostatic repulsion arising from the charged GM1. However, at interbilayer separations of < or = 30 A for 7:3 and 8:2 EPC:GM1 and < or = 22 A for 9:1 EPC:GM1, the pressure-distance curves have distinct upward breaks, with the sharpness of this break depending strongly on the amount of GM1 in the bilayer. For 7:3 EPC:GM1 bilayers, the break is quite sharp so that the distance between bilayers does not decrease below 28 A with further increases in applied pressure. For EPC:GM1 8:2 and 9:1 bilayers, the upward break becomes softer with decreasing GM1 concentration. For uncharged EPC:AGM1 bilayers, the repulsive pressure extends only to an equilibrium fluid separation of about 36 A, but has a similar behavior to the pressure-distance data for EPC:GM1 for separations below 20 A. We argue that the nonelectrostatic repulsive pressures arise primarily from the steric interactions between the hydrated oligosaccharide head groups that protrude from the bilayer surface.

Animals↗
PubMed1994

Colloid osmotic pressure of steer alpha- and beta-crystallins: possible functional roles for lens crystallin distribution and structural diversity.

This study addresses the general mechanisms whereby the major cytoplasmic proteins from the adult bovine lens contribute both to transparency and maintenance of the refractive index gradient across the lens. Colloid osmotic properties and quaternary structure were measured for alpha- and beta-crystallins isolated from the steer lens, including low-molecular-weight crystallins from the cortex (alpha Le and beta L) and nucleus (alpha Ln) and high-molecular-weight crystallins from the nucleus (alpha H and beta H). In electron microscopic images of rotary-shadowed preparations alpha Le appears as spherical particles 16 nm in diameter, alpha Ln appeared as individual spheres or small aggregates of spherical subunits, alpha H contained large irregular aggregates as large as 180 nm, and both beta L and beta H appeared as elliptical particles of 7-9 nm diameter. Secondary osmometry showed that for all these crystallins colloid osmotic pressure increased monotonically in a non-linear fashion with protein concentration. For the alpha-crystallins, osmotic pressure rose more steeply with concentration for alpha Le than for either alpha Ln or alpha H, so that at 0.3 g ml-1 at 0.1 M ionic strength, the colloid osmotic pressure of alpha Le, alpha Ln and alpha H were approximately 2.6 x 10(5) dyn cm-2, 1.6 x 10(5) dyn cm-2 and 1.0 x 10(5) dyn cm-2, respectively. In a similar manner, osmotic pressure rose more steeply with concentration of beta L than for beta H, so that at 0.3 g ml-1 at 0.1 M ionic strength the colloid osmotic pressures of beta L and beta H were 2.6 x 10(5) dyn cm-2 and 1.1 x 10(5) dyn cm-2, respectively. The osmotic pressure of alpha Le dropped as ionic strength was increased from 0.02 to 0.4 M. For beta L and beta H, osmotic pressure dropped as ionic strength was increased from 0.02 to 0.1 M but was nearly the same at 0.1 M and 0.4 M ionic strength. The data for steer alpha Ln and beta H were similar to previous reports for calf cortical alpha L and beta-crystallins, respectively. The osmotic pressure isotherms for alpha Le, beta L and that previously reported for steer cortical extract were nearly identical, whereas the nuclear crystallins (alpha Ln, alpha H or beta H) generated slightly higher pressures than those previously reported for steer nuclear crystallin extracts. In all cases, osmotic pressure rose more steeply with concentration for the cortical crystallins than for the nuclear crystallins.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Increased adhesion between neutral lipid bilayers: interbilayer bridges formed by tannic acid.

Tannic acid (TA) is a naturally occurring polyphenolic compound that aggregates membranes and neutral phosolipid vesicles and precipitates many proteins. This study analyzes TA binding to lipid membranes and the ensuing aggregation. The optical density of dispersions of phosphatidylcholine (PC) vesicles increased upon the addition of TA and electron micrographs showed that TA caused the vesicles to aggregate and form stacks of tightly packed disks. Solution calorimetry showed that TA bound to PC bilayers with a molar binding enthalpy of -8.3 kcal/mol and zeta potential measurements revealed that TA imparted a small negative charge to PC vesicles. Monolayer studies showed that TA bound to PC with a dissociation constant of 1.5 microM and reduced the dipole potential by up to 250 mV. Both the increase in optical density and decrease in dipole potential produced by TA could be reversed by the addition of polyvinylpyrrolidone, a compound that chelates TA by providing H-bond acceptor groups. NMR, micropipette aspiration, and x-ray diffraction experiments showed that TA incorporated into liquid crystalline PC membranes, increasing the area per lipid molecule and decreasing the bilayer thickness by 2 to 4%. 2H-NMR quadrupole splitting measurements also showed that TA associated with a PC molecule for times much less than 10(-4) s. In gel phase bilayers, TA caused the hydrocarbon chains from apposing monolayers to fully interdigitate. X-ray diffraction measurements of both gel and liquid crystalline dispersions showed that TA, at a critical concentration of about 1 mM, reduced the fluid spacing between adjacent bilayers by 8-10 A. These data place severe constraints on how TA can pack between adjacent bilayers and cause vesicles to adhere. We conclude that TA promotes vesicle aggregation by reducing the fluid spacing between bilayers by the formation of transient interbilayer bridges by inserting its digallic acid residues into the interfacial regions of adjacent bilayers and spanning the interbilayer space.

Adsorption↗
PubMed1993

Contributions of hydration and steric (entropic) pressures to the interactions between phosphatidylcholine bilayers: experiments with the subgel phase.

The total repulsive interaction between electrically neutral, fluid bilayer membranes is thought to have a number of components, including a hydration pressure, due to the reorientation of water by the bilayer, and steric (entropic) pressures due to bilayer undulations, head group motion, and molecular protrusions. For fully hydrated, crystalline bilayers these three steric pressures should be relatively small, and the major repulsive pressure present should be the hydration pressure. Therefore, to isolate the contribution of hydration pressure to the total interbilayer interaction, we have measured pressure-distance data by X-ray diffraction analysis of osmotically stressed dipalmitoylphosphatidylcholine (DPPC) multilayers in the subgel phase, where wide-angle and low-angle X-ray data show the bilayers are crystalline. As applied pressure was increased from 0 to 1 x 10(6) dyn/cm2, the interbilayer fluid space (df) decreased less than 1 A from its value at full hydration of 8.4 A. As the pressure was increased from 1 x 10(6) to 3 x 10(7) dyn/cm2, df decreased from about 8 to 4 A. For this range of df, the repulsive pressure decayed exponentially with df with a decay length of 1.4 A. Further increases in applied pressure did not appreciably decrease df, so that there was a sharp upward break in the pressure-distance curve at an interbilayer spacing of about 3 A. In contrast, pressure-distance relations for gel (L beta') phase and liquid-crystalline (L alpha) phase bilayers had much softer upward breaks at df < 5 A and extended to larger df at zero applied pressure. However, the pressure-distance curves for all three phases decayed exponentially with approximately the same decay length for 4 < df < 8 A. We interpret these data to mean the following: (1) the repulsion observed for df < 5 A is primarily a steric pressure whose range depends on head group motion; (2) the steric undulation pressure plays an important role in determining the hydration properties and the range of the total repulsive pressure for fluid membranes; (3) the same underlying mechanisms govern the repulsive pressure for all phases for 4 < df < 8 A; (4) these mechanisms include a pressure due to reorientation of water molecules; and (5) the hydration pressure component extents a maximum of about two water molecules from the bilayer surface for the subgel phase.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Gelation of liposome interior. A novel method for drug encapsulation.

Liposomes can be loaded with weak acids and bases, which exist in solutions in equilibrium with membrane permeable uncharged form, using various gradients across their membranes. Because in some cases the estimated drug concentration in the loaded liposomes exceeds their aqueous solubility we investigated the physical state of the liposome encapsulated anticancer drug Doxorubicin. X-Ray diffraction, electron microscopy, and test tube solubility experiments have shown that upon encapsulation the drug molecules form a gel-like phase.

Doxorubicin↗
PubMed1992

Repulsive interactions and mechanical stability of polymer-grafted lipid membranes.

Liposome membranes containing lipids with covalently attached poly(ethylene glycol) (PEG-lipid) are currently being developed as drug delivery systems. These, so called, 'Stealth' liposomes have a relatively long half life (approximately 1 day) in blood circulation and show an altered biodistribution in vivo. The extended lifetime appears to result from a steric stabilization of the liposome by the grafted polymer. In order to characterize the surface structures that promote steric stability in such polymer-grafted lipid bilayer systems, we have used X-ray diffraction to measure the structural organization and interbilayer repulsion for lipid/cholesterol (2:1) bilayers incorporating 4 mol% of a PEG-lipid in which the molecular weight of the PEG moiety was 1900 g/mol. At this concentration, applied pressure versus interbilayer distance relations showed that the grafted polymer moiety extended approximately 50 A from the lipid surface and gave rise to a strong, slowly decaying repulsive pressure between membranes that opposed their close approach. Also, the pressure vs. distance relations were only modestly altered by changing the ionic strength of the medium (1 mM NaCl and 100 mM NaCl). Therefore, even though the PEG-lipid headgroup bears a negative charge, the long range pressure cannot be due primarily to an electrostatic double layer pressure. Measurements of lipid bilayer elasticity using micropipet manipulation showed that PEG-lipid did not change the cohesive properties of lipid/cholesterol liposomes which was consistent with the X-ray structural data showing that the PEG-lipid did not change the normal structure of the bilayer interior. From these data we conclude that the repulsive barrier properties of lipid-grafted PEG polymer chains originate mainly from a steric pressure and that this simple polymer steric stabilization is the basis for the extended in vivo circulation times observed for polymer-grafted liposomes.

Electric Conductivity↗