The Experts below are selected from a list of 219 Experts worldwide ranked by ideXlab platform
Thomas J. Mcintosh - One of the best experts on this subject based on the ideXlab platform.
-
lipopolysaccharide bilayer structure effect of chemotype core mutations divalent cations and temperature
Biochemistry, 1999Co-Authors: Scott A Snyder, Thomas J. McintoshAbstract:Lipopolysaccharide (LPS), the primary lipid on the surface of Gram-negative bacteria, is thought to act as a protective and permeability barrier. X-ray diffraction analysis of osmotically stressed LPS multilayers was used to determine the structure and interactive properties of LPSs from strains containing the minimum number of sugars necessary for bacterial survival (Re chemotype) to the maximum number of sugars found in rough bacteria (Ra chemotype). At 20 °C in the absence of divalent cations, LPS suspensions gave a sharp wide-angle reflection at 4.23 A and a broad low-angle band centered at 50−68 A depending on the chemotype, indicating the presence of gel phase bilayers separated by large fluid spaces. As osmotic Pressure was applied, the apposing bilayers were squeezed together and lamellar diffraction at 6 A resolution was obtained. At low applied Pressures (<106 dyn/cm2), the total Repulsive Pressure between bilayers could be explained by electrostatic double layer theory. At higher applied pressu...
-
Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.
Biophysical journal, 1999Co-Authors: Thomas J. Mcintosh, Ketan G. Kulkarni, Sidney A. SimonAbstract:It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic Pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the Repulsive Pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation Pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation Pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.
-
Structure and interactive properties of highly fluorinated phospholipid bilayers.
Biophysical journal, 1996Co-Authors: Thomas J. Mcintosh, Sidney A. Simon, P. Vierling, Catherine Santaella, V. RavilyAbstract: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.
-
Experimental Tests for Protrusion and Undulation Pressures in Phospholipid Bilayers
Biochemistry, 1995Co-Authors: Thomas J. Mcintosh, S. Advani, R. E. Burton, Doncho V. Zhelev, David Needham, Sidney A. SimonAbstract: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., & Wennerstrom, 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.
-
Temperature dependence of the Repulsive Pressure between phosphatidylcholine bilayers.
Biophysical journal, 1995Co-Authors: Sidney A. Simon, S. Advani, Thomas J. McintoshAbstract: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.
Sidney A. Simon - One of the best experts on this subject based on the ideXlab platform.
-
Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.
Biophysical journal, 1999Co-Authors: Thomas J. Mcintosh, Ketan G. Kulkarni, Sidney A. SimonAbstract:It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic Pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the Repulsive Pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation Pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation Pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.
-
Structure and interactive properties of highly fluorinated phospholipid bilayers.
Biophysical journal, 1996Co-Authors: Thomas J. Mcintosh, Sidney A. Simon, P. Vierling, Catherine Santaella, V. RavilyAbstract: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.
-
Experimental Tests for Protrusion and Undulation Pressures in Phospholipid Bilayers
Biochemistry, 1995Co-Authors: Thomas J. Mcintosh, S. Advani, R. E. Burton, Doncho V. Zhelev, David Needham, Sidney A. SimonAbstract: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., & Wennerstrom, 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.
-
Temperature dependence of the Repulsive Pressure between phosphatidylcholine bilayers.
Biophysical journal, 1995Co-Authors: Sidney A. Simon, S. Advani, Thomas J. McintoshAbstract: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.
-
LONG- AND SHORT-RANGE INTERACTIONS BETWEEN PHOSPHOLIPID/GANGLIOSIDE GM1 BILAYERS
Biochemistry, 1994Co-Authors: Thomas J. Mcintosh, Sidney A. SimonAbstract: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.
Mauro Antezza - One of the best experts on this subject based on the ideXlab platform.
-
Casimir-Lifshitz force out of thermal equilibrium between dielectric gratings, International Symposium on Nanotechnologies
2015Co-Authors: Brahim Guizal, Antonio Noto, Riccardo Messina, Mauro AntezzaAbstract:We calculate the Casimir-Lifshitz Pressure in a system consisting of two different one-dimensional dielectric lamellar gratings having two different temperatures and immersed in an environment having a third temperature [1]. The calculation of the Pressure is based on the knowledge of the scattering operators, deduced using the Fourier modal method. The behavior of the Pressure is characterized in detail as a function of the three temperatures of the system as well as the geometrical parameters of the two gratings. We show that the interplay between nonequilibrium effects and geometrical periodicity offers a rich scenario for the manipulation of the force. In particular, we find regimes where the force can be strongly reduced for a large range of temperatures. Moreover, a Repulsive Pressure can be obtained, whose features can be tuned by controlling the degrees of freedom of the system. Remarkably, the transition distance between attraction and repulsion can be decreased with respect to the case of two slabs, implying an experimental interest for the observation of repulsion.
-
casimir lifshitz force out of thermal equilibrium between dielectric gratings
Physical Review A, 2014Co-Authors: Antonio Noto, Riccardo Messina, Brahim Guizal, Mauro AntezzaAbstract:We calculate the Casimir-Lifshitz Pressure in a system consisting of two different one-dimensional dielectric lamellar gratings having two different temperatures and immersed in an environment having a third temperature. The calculation of the Pressure is based on the knowledge of the scattering operators, deduced using the Fourier modal method. The behavior of the Pressure is characterized in detail as a function of the three temperatures of the system as well as the geometrical parameters of the two gratings. We show that the interplay between nonequilibrium effects and geometrical periodicity offers a rich scenario for the manipulation of the force. In particular, we find regimes where the force can be strongly reduced for large ranges of temperatures. Moreover, a Repulsive Pressure can be obtained, whose features can be tuned by controlling the degrees of freedom of the system. Remarkably, the transition distance between attraction and repulsion can be decreased with respect to the case of two slabs, implying an experimental interest for the observation of repulsion.
David Needham - One of the best experts on this subject based on the ideXlab platform.
-
hydration potential of lysozyme protein dehydration using a single microparticle technique
Biophysical Journal, 2010Co-Authors: Deborah L Rickard, Brent P Duncan, David NeedhamAbstract:For biological molecules in aqueous solution, the hydration Pressure as a function of distance from the molecular surface represents a very short-range Repulsive Pressure that limits atom-atom contact, opposing the attractive van der Waals Pressure. Whereas the separation distance for molecules that easily arrange into ordered arrays (e.g., lipids, DNA, collagen fibers) can be determined from x-ray diffraction, many globular proteins are not as easily structured. Using a new micropipette technique, spherical, glassified protein microbeads can be made that allow determination of protein hydration as a function of the water activity (aw) in a surrounding medium (decanol). By adjusting aw of the dehydration medium, the final protein concentration of the solid microbead is controlled, and ranges from 700 to 1150 mg/mL. By controlling aw (and thus the osmotic Pressure) around lysozyme, the Repulsive Pressure was determined as a function of distance between each globular, ellipsoid protein. For separation distances, d, between 2.5 and 9 A, the Repulsive decay length was 1.7 A and the Pressure extrapolated to d = 0 was 2.2 × 108 N/m2, indicating that the hydration Pressure for lysozyme is similar to other biological interfaces such as phospholipid bilayers.
-
Experimental Tests for Protrusion and Undulation Pressures in Phospholipid Bilayers
Biochemistry, 1995Co-Authors: Thomas J. Mcintosh, S. Advani, R. E. Burton, Doncho V. Zhelev, David Needham, Sidney A. SimonAbstract: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., & Wennerstrom, 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.
-
Range and magnitude of the steric Pressure between bilayers containing phospholipids with covalently attached poly(ethylene glycol).
Biophysical journal, 1995Co-Authors: Anne K. Kenworthy, Kalina Hristova, David Needham, Thomas J. McintoshAbstract: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.
-
Repulsive interactions and mechanical stability of polymer-grafted lipid membranes.
Biochimica et biophysica acta, 1992Co-Authors: David Needham, Thomas J. Mcintosh, Danilo D. LasicAbstract:Abstract Liposome membranes containing lipids with covalently attached poly(ethylene glycol)(PEG-lipid) are currently being developed as drug delivery systems. These, so called, ‘Stealth 20 ’ liposomes have a relatively long half life (∼ 1day) 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 ∼ 50Afrom 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 concluded 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.
-
Interbilayer interactions between sphingomyelin and sphingomyelin/cholesterol bilayers.
Biochemistry, 1992Co-Authors: Thomas J. Mcintosh, Sidney A. Simon, David Needham, Ching Hsien HuangAbstract:Pressure versus fluid spacing relations have been obtained for sphingomyelin bilayers in the gel phase and equimolar sphingomyelin/cholesterol in the liquid-crystalline phase by the use of X-ray diffraction analysis of osmotically stressed aqueous dispersions and oriented multilayers. For interbilayer separations in the range of 5-20 A, the Repulsive hydration Pressure decays exponentially with increasing fluid spacing. The decay length (lambda) of this Repulsive Pressure is about 2 A for both bovine brain and N-tetracosanoylsphingomyelin, similar to that previously found for phosphatidylcholine bilayers. However, both the magnitude of the hydration Pressure and the magnitude of the dipole potential (V) measured for monolayers in equilibrium with liposomes are considerably smaller for sphingomyelin than for either gel or liquid-crystalline phosphatidylcholine bilayers. Addition of equimolar cholesterol increases both the magnitude of the hydration Pressure and the dipole potential. These data suggest that the magnitude of the hydration Pressure depends on the electric field at the interface as given by (V/lambda)2. For sphingomyelin bilayers, there is a sharp upward break in the Pressure-fluid spacing relation at an interbilayer spacing of about 5 A, indicating the onset of steric hindrance between the head groups of apposing bilayers.
Don M. Zebolsky - One of the best experts on this subject based on the ideXlab platform.
-
Comparisons of Supercritical Properties from an Equation of State with a Hard-Sphere Repulsive Pressure Term and from the Peng−Robinson Equation of State†
Industrial & Engineering Chemistry Research, 2000Co-Authors: Don M. ZebolskyAbstract:An equation of state that adds a Percus−Yevick hard-sphere Repulsive Pressure term to a Morrison−McLinden term for attraction of two adjustable parameters correlates residual enthalpies and solubilities in supercritical carbon dioxide better than the Peng−Robinson cubic equation of state. Its use can be extended to mixtures of inorganic compounds in carbon dioxide. Volumes and enthalpies of mixing in general are correlated and predicted as well as or better by the equation with the hard-sphere repulsion, so there is no reason to continue to use cubic equations of state where the free-volume term is wrong.
-
Excess enthalpies for inorganic liquids with supercritical carbon dioxide: Equation of state correlations
Thermochimica Acta, 1997Co-Authors: Don M. ZebolskyAbstract:Abstract Excess enthalpies along isotherm/isobars for binary mixtures of liquid neopentane, SnCl4, and TiCl4, in supercritical CO2 are correlated with two equations of state. Both equations use a Percus-Yevick hard-sphere Repulsive Pressure term added to an attractive term, and temperature dependent size parameters. One attractive term is empirical, the other is based on a square-well potential. Calculations for pure component enthalpies fit data better than molecular dynamics simulations, but the simulations better predict the excess enthalpies.