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T J McIntosh

Publications and source records attributed to T J McIntosh.

At least 73 records · Page 4Linked to original sources
PubMed1987

Tris buffer causes acyl chain interdigitation in phosphatidylglycerol.

The structure of the gel phase and the properties of the acyl chain disordering transition of dipalmitoyl phosphatidylglycerol (DPPG) have been studied using differential scanning calorimetry, differential scanning dilatometry, and X-ray diffraction. In the presence of small, monovalent cations, DPPG at 22 degrees C exists in a lamellar phase in which the hydrocarbon chains are tilted from the perpendicular to the bilayer surface. Around 34 degrees C, there is a small pretransition (delta H less than 1 kcal/mol) followed by the main transition at 40.4 degrees C (delta H = 8.3 kcal/mol; delta V = 0.0381 ml/g). If DPPG is suspended in Tris-HCl buffer in the absence of other monovalent cations, X-ray diffraction data show that at 22 degrees C, the gel phase consists of interdigitated acyl chains perpendicular to the plane of the bilayer. No pretransition is observed and the main transition occurs at 41.3 degrees C with delta H = 9.1 kcal/mol and delta V = 0.0514 ml/g. If sufficient Na+ or K+ ions are added to the Tris-buffered DPPG, the phase behavior reverts to what is observed in the absence of Tris. Analysis of the energetics of the main transition shows that the increase in van der Waals interaction energy resulting from the larger delta V in Tris can be compensated by the favorable energetics of removing terminal methyl groups from the bilayer surface. The amount of disordering, i.e. formation of gauche rotamers, is likely to be the same in Tris as it is in buffers without amphiphilic cations.

Calorimetry, Differential Scanning↗
PubMed1987

Steric repulsion between phosphatidylcholine bilayers.

The change in pressure needed to bring egg phosphatidylcholine bilayers into contact from their equilibrium separation in excess water has been determined as a function of both distance between the bilayers and water content. A distinct upward break in the pressure-distance relation appears at an interbilayer separation of about 5 A, whereas no such deviation is present in the pressure-water content relation. Thus, this break is not a property of the dehydration process per se, but instead is attributed to steric repulsion between the mobile lipid head groups that extend 2-3 A into the fluid space between bilayers. That is, electron density profiles of these bilayers indicate that the observed break in the pressure-spacing relation occurs at a bilayer separation where extended head groups from apposing bilayers come into steric hindrance. The pressure-spacing data are used to separate steric pressure from the repulsive hydration pressure, as well as to quantitate the range and magnitude of the steric interaction. An appreciable fraction of the measured steric energy can be ascribed to a decrease in configurational entropy due to restricted head-group motion as adjacent bilayers come together.

Kinetics↗
PubMed1987

Determination of the depth of bromine atoms in bilayers formed from bromolipid probes.

X-ray diffraction analysis has been performed on a series of 1-palmitoyl-2-dibromostearoyl-phosphatidylcholines (BRPCs) with bromine atoms at the 6, 7-, the 11, 12-, or the 15, 16-positions on the sn-2 acyl chains. The diffraction patterns indicate that, when hydrated, each of these lipids forms liquid-crystalline bilayers at 20 degrees C. For each lipid, electron density profiles and continuous Fourier transforms were calculated by the use of swelling experiments. In the electron profiles, high-density peaks, due to the bromine atoms, are observed. The separation between these bromine peaks in the profile decreases as the bromine atoms are moved toward the terminal methyl of the acyl chain. For the 6, 7- and 11, 12-bromolipids, experimental Fourier transforms can be approximated by the sum of the transform of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and the transform of two symmetrically placed peaks of electron density (the bromines). For the case of the 15, 16-bromolipids, a better fit is obtained for the transforms of a model bilayer where the thickness of the methylene chain region of the bilayer is 3 A greater than that of POPC. Our analysis indicates the following: for each of these bromolipids, the bromines are well localized in the bilayer; the distance of the bromines from the head-group-hydrocarbon boundary are 3.5, 8.0, and 14 A, for 6, 7-, 11, 12-, and 15, 16-BRPC, respectively; the bilayer thickness and perturbation to bilayer hydrocarbon chain packing caused by the bromine atoms depend on the position of the bromines on the hydrocarbon chain.

Bromine↗
PubMed1986

Area per molecule and distribution of water in fully hydrated dilauroylphosphatidylethanolamine bilayers.

The area per lipid molecule for fully hydrated dilauroylphosphatidylethanolamine (DLPE) has been obtained in both the gel and liquid-crystalline states by combining wide-angle X-ray diffraction, electron density profiles, and previously published dilatometry results [Wilkinson, D. A., & Nagle, J. F. (1981) Biochemistry 20, 187-192]. The molecular area increases from 41.0 +/- 0.2 to 49.1 +/- 1.2 A2 upon melting from the gel to liquid-crystalline phase. The thickness of the bilayer, as measured from the electron density profiles, decreases about 4 A upon melting, from 45.2 +/- 0.3 to 41.0 +/- 0.6 A. A somewhat unexpected result is that the fluid layer between fully hydrated bilayers is the same in both gel and liquid-crystalline phases and is only about 5 A thick. From these data, plus the volume of the anhydrous DLPE molecule, it is possible to determine the number of water molecules per lipid and their approximate distribution relative to the lipid molecule. Our analysis shows that there are about 7 and 9 waters per DLPE molecule in the gel and liquid-crystalline phases, respectively. About half of the water is located in the fluid space between adjacent bilayers, and the remaining waters are intercalated into the bilayer, presumably in the head group region. There are significantly fewer water molecules in the fluid spaces between DLPE bilayers than in the fluid spaces in gel- or liquid-crystalline-phase phosphatidylcholine bilayers. This small fluid space in PE bilayers could arise from interbilayer hydrogen bond formation through the water molecules or electrostatic interactions between the amine and phosphate groups on apposing bilayers.

Gels↗
PubMed1986

Hydration force and bilayer deformation: a reevaluation.

The hydration repulsive force between lipid bilayers and the deformability of both gel and liquid-crystalline bilayers have been quantitated by an X-ray diffraction analysis of osmotically stressed liposomes. Both sampling theorem reconstructions and electron density distributions were calculated from diffraction data obtained from multilayers with applied osmotic pressures of 0-50 atm. The bilayer thickness and area per lipid molecule remain nearly constant (to within about 4%) in this pressure range, as adjacent bilayers move from their equilibrium separation in excess water to within 2-4 A of each other. This analysis indicates that the bilayers are relatively incompressible. This results differs from previously published X-ray diffraction studies of bilayer compressibility but agrees with direct mechanical measurements of the bilayer compressibility modulus. It is also found that the hydration repulsive force decays exponentially with separation between bilayers with a decay constant of 1.4 A for gel-state dipalmitoylphosphatidylcholine and 1.7 A for liquid-crystalline egg phosphatidylcholine bilayers. This implies that the exponential decay constant is not necessarily equal to the diameter of a water molecule, as has been previously suggested on experimental and theoretical grounds.

Dextrans↗
PubMed1986

A subtransition in a phospholipid with a net charge, dipalmitoylphosphatidylglycerol.

Suspensions of dipalmitoylphosphatidylglycerol (DPPG) have been analyzed by differential scanning calorimetry, equilibrium and differential scanning dilatometry, and X-ray diffraction techniques. After the DPPG suspensions are stored several days at 2 degrees C, a new phase transition is observed at a lower temperature than either the main transition or the pretransition. This subtransition has an enthalpy of about 6 kcal/mol and occurs at about 20 degrees C, the exact temperature depending on the buffer used. The lipid partial specific volume increases by 0.035 mL/g upon warming through the subtransition. X-ray diffraction patterns from suspensions in the subgel phase contain orders of a lamellar repeat and several additional sharp and broad wide-angle reflections between 8 and 2 A. As the water content in the specimen is reduced, the lamellar repeat period decreases, whereas the spacings and intensities of these additional wide-angle reflections are unchanged. These data indicate that on incubation at 2 degrees C the lipid molecules crystallize in the plane of each bilayer. X-ray experiments also show that this subgel phase converts to the normal L beta' gel phase above the subtransition.

Calorimetry, Differential Scanning↗
PubMed1986

Structure of the crystalline bilayer in the subgel phase of dipalmitoylphosphatidylglycerol.

The structure of the subgel phase of dipalmitoylphosphatidylglycerol (DPPG) has been analyzed by X-ray diffraction techniques. Diffraction recorded from highly oriented DPPG specimens in the subgel phase extends to 2-A resolution. There are sharp lamellar reflections on the meridian, and other reflections lie on a series of wide-angle lattice lines parallel to the meridian and crossing the equator in the range of 8-2 A. The wide-angle lattice lines consist of radially sharp reflections centered on the equator of the X-ray film and also a series of broader, off-equatorial maxima. The lattice lines indicate that the DPPG molecules in each bilayer crystallize in a two-dimensional oblique lattice with dimensions a = 5.50 A, b = 7.96 A, and gamma = 100.5 degrees. These oblique lattices are not regularly aligned from bilayer to bilayer. Analysis of the lamellar diffraction shows that the bilayer has about the same thickness in the subgel and gel (L beta') phases. In the direction normal to the hydrocarbon chains, the chains are significantly closer together in the subgel phase as compared to the normal L beta' gel phase but have about the same separation as the chains in polyethylene and the crystalline n-alkanes. The bilayer thickness, area per lipid molecule, and intensity distribution along the lattice lines all indicate that in the subgel phase the hydrocarbon chains are tilted between 30 and 35 degrees from the normal to the bilayer plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization↗
PubMed1985

Effects of phenylamide herbicides on the physical properties of phosphatidylcholine membranes.

A number of phenylamide herbicides are observed to uncouple electron transport in isolated chloroplasts and mitochondria and alter the H+ permeability of artificial liposomes. Several of these phenylamides were incorporated into phosphatidylcholine multilamellar and small unilamellar vesicles to measure their effects on the physical properties of membranes. X-ray diffraction analysis of the multilamellar vesicles revealed that the herbicides partitioned into the hydrocarbon chain region of the bilayer, but caused only minimal perturbations on hydrocarbon chain packing. 31P-NMR spectroscopy of these multilamellar vesicles showed both a broadening and lowering of the phase transition temperature of the bilayer lipids upon addition of the herbicides. 13C-NMR spectroscopy of small, unilamellar phosphatidylcholine vesicles was performed to measure the effects of the phenylamides on the chemical shifts and the spin-lattice relaxation times of the individual phosphatidylcholine carbon atoms. None of the added compounds had any measurable effect on the 13C-NMR chemical shifts of the phosphatidylcholine. However, the herbicides significantly modified spin-lattice relaxation times of certain of the lipid carbon atoms. These results generally indicate that the herbicides orient in the lipid bilayers such that the hydrocarbon chains of the phenylamides associate with the hydrocarbon chains of the lipid, whereas the phenyl moiety resides in the polar region of the bilayer.

Electron Transport↗
PubMed1985

Membrane specializations in mammalian lens fiber cells: distribution of square arrays.

Fragments of intact rat, pig and bovine lenses and isolated membranes from bovine lenses were examined by freeze-fracture-etch electron microscopy employing samples ultra-rapidly frozen in the absence of fixatives or cryoprotectants. Complementary replicas of the bovine outer cortex clearly display small patches of square arrays (6.6 nm repeat) as structures distinct from gap junctions in the same membranes. In the outer cortex square arrays appear in single membranes where the extracellular space is not thinned, whereas gap junctions, as in other tissues, only occur where the extracellular space is greatly attenuated. Square arrays in the middle cortex appear as patches of varying size surrounded by smooth membrane fracture faces. Within the undulating membranes of tongue-and-groove interdigitations in the inner cortex and nucleus, the square arrays are extensive and are located specifically on the regions which have convex curvature toward the cytoplasm. Square crystalline regions alternate with non-crystalline regions within each membrane of an undulating pair. Across the extracellular space, crystalline regions are matched with non-crystalline regions, thus making it unlikely that the square array participates in intercellular communication. Because the undulations occur in isolated membranes following urea washing to remove cytoplasmic and extrinsic membrane proteins, the square arrays probably play a crucial role in the formation of the undulations and in the maintenance of membrane curvature. X-ray diffraction experiments show reflections from the crystalline square array for all twenty of our preparations of isolated membranes. These x-ray experiments indicate that the square arrays observed by freeze-fracture are abundant in lens fiber cell membranes.

Animals↗
PubMed1984

On the localization of ubiquinone in phosphatidylcholine bilayers.

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05-0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [13C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the 13C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (P beta') phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.

Freeze Fracturing↗
PubMed1984

New structural model for mixed-chain phosphatidylcholine bilayers.

Multilamellar suspensions of a mixed-chain saturated phosphatidylcholine with 18 carbon atoms in the sn-1 chain and 10 carbon atoms in the sn-2 chain have been analyzed by X-ray diffraction techniques. The structural parameters for this lipid in the gel state are quite different than usual phosphatidylcholine bilayer phases. A symmetric and sharp wide-angle reflection at 4.11 A indicates that the hydrocarbon chains in hydrated C(18):C(10)PC bilayers are more tightly packed than in usual gel-state phosphatidylcholine bilayers and that there is no hydrocarbon chain tilt. The lipid thickness is about 12 A smaller than would be expected in a normal bilayer phase, and the area per molecule is 3 times the area per hydrocarbon chain. In addition, the bilayer thickness increases upon melting to the liquid-crystalline state, whereas normal bilayer phases decrease in thickness upon melting. On the basis of these data, we propose a new lipid packing model for gel-state C(18):C(10)PC bilayers in which the long C(18) chain spans the entire width of the hydrocarbon region of the bilayer and the short C(10) chain aligns or abuts with the C(10) chain from the apposing molecule. This model is novel in that there are three hydrocarbon chains per head group at the lipid-water interface. Calculations show that this phase is energetically favorable for mixed-chain lipids provided the long acyl chain is nearly twice the length of the shorter chain. In the liquid-crystalline state C(18):C(10)PC forms a normal fluid bilayer, with two chains per head group.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena↗
PubMed1984

Interdigitated hydrocarbon chain packing causes the biphasic transition behavior in lipid/alcohol suspensions.

It has been shown recently by Rowe ((1983) Biochemistry 22, 3299-3305) that ethanol has a 'biphasic' effect on the transition temperature (Tm) of phosphatidylcholine bilayers, reducing Tm at low concentrations but increasing Tm at high concentrations. Our X-ray diffraction data show that this reversal of Tm is a consequence of the induction of an unusual gel phase, where the lipid hydrocarbon chains from apposing monolayers fully interpenetrate or interdigitate. The properties of this interdigitated phase also explain the lipid chain length dependence of the reversal in the Tm versus ethanol concentration curves and the narrow width of the transition at high ethanol concentrations, as well as spectroscopic and calorimetric data from lipid suspensions containing other drugs such as methanol, benzyl alcohol, phenyl ethanol, and chlorpromazine.

Lipid Bilayers↗
PubMed1982

Interaction of benzene with bilayers. Thermal and structural studies.

The thermal and structural properties of saturated phosphatidylcholine liposomes are significantly altered by benzene. Upon the addition of benzene, the liposomes first swell and then disperse into small multilamellar vesicles. At 20 degrees C these vesicles contain striations or ripples in the plane of the bilayer. Major changes in the thermal behavior of DSPC-benzene liposomes occur near mole ratios of 2:1 and 1:1. At a 2:1 mole ratio, the area under the main endothermic peak, delta Hm, essentially disappears; however, the total heat absorbed, delta Hf, remains approximately equal to that of the control. This occurs because for benzene mole fractions 0.12 less than x less than 0.50, benzene increases the apparent molar heat capacity, Cp, of the gel phase to about 1.2 kcal/(mol . deg). We interpret this increase in heat capacity to be due to an increase in the concentration of defects (or disorder) in the gel phase. At mole fractions of benzene between 0.5 and 0.9, the transition temperature decreases by 20-30 degrees C, and broad, multiple transitions are observed. From 0.5 less than or equal to x less than or equal to 0.9, the apparent molar heat capacity of the liquid-crystal phase increases to that of the defected rippled gel phase. The value of delta Hf approaches the heat of fusion for 2 mol of n-octadecane, suggesting that benzene uncouples the liquid-crystalline acyl chains. The lipids affected by benzene or "boundary lipids" have higher heat capacity than nonperturbed lipids. The apparent molar specific heat, Cp, of 1,2-distearoyl-sn-glycero-3-phosphorylcholine (and 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine) multilamellar vesicles is 0.20 +/- 0.05 kcal/(mol. deg) in the L beta', P beta, and L alpha phases. Cp fluctuates about this value in all three phases upon repeated phase transitions in the same sample. However, the value of Cp in the P beta (rippled) phase exhibits much greater fluctuations in Cp than that in the L alpha phase. We attribute these fluctuations to crystal packing defects.

Benzene↗
PubMed1982

Influence of cholesterol on water penetration into bilayers.

X-ray diffraction and capacitance measurements have been used to calculate the depth to which water penetrates in fully hydrated bacterial phosphatidylethanolamine bilayers in the presence and absence of cholesterol. The data indicate that cholesterol decreases in depth of water penetration by about 2.5 angstroms.

Cholesterol↗
PubMed1982

Alamethicin-induced changes in lipid bilayer morphology.

We have found that alamethicin, in the absence of an electric field, modifies both the hydrophilic surface and hydrophobic core of lipid bilayers. As shown by freeze-fracture and X-ray diffraction experiments with multiwalled vesicles, alamethicin increases the fluid space between bilayers by as much as 50 nm, and at the same time perturbs the hydrocarbon regions of the bilayers. For suspensions of gel-state lipid treated with alamethicin, uniformly spaced rows of particles cover the fracture faces and corresponding linear arrays of stain-collecting depressions cover the hydrophilic surfaces. In the liquid-crystalline state, alamethicin induces an irregular granular texture on the fracture faces.

Alamethicin↗
PubMed1982

The structure of junctions between lens fiber cells.

Junctions which have been isolated from bovine lens may correspond to the tongue-and groove interdigitations observed between intact cells. These junctions have an overall thickness of 13-14 nm, a striking undulating surface topology, and contain protein subunits of congruent to 27 000 daltons which are arranged in a 6.6-6.7-nm square lattice in the plane of each membrane. A similar square-lattice structure has been observed in junctions from fragments of intact rate lens. Junctions from both sources are structurally different from "typical' gap junctions.

Animals↗
PubMed1982

Morphology of the intermediate stages in the lamellar to hexagonal lipid phase transition.

The addition of calcium to suspensions of egg phosphatidylcholine and cardiolipin converts multiwalled liposomes to the hexagonal (HII) phase (Rand, R.P., Sengupta, S. (1972) Biochim. Biophys. Acta 255:484-492). We have studied this lamellar to hexagonal phase transition by freeze-fracture, thin-section electron microscopy, and X-ray diffraction and have morphologically characterized the intermediate stages. The first step in the transition involves the invagination and fusion of bilayers, marked by the appearance of lipidic intramembrane particles and "crater-like" indentations, as the large liposomes are converted to smaller flattened and elongated vesicles. The next step is the formation of tightly packed hexagonal arrays of tubules, each tubule being about 11 to 15 nm in diameter. These tubules are filled with fluid and a lipid bilayer forms the wall of each cylinder. Finally this tubular bilayer phase is converted to the hexagonal (HII) phase, where the distance between tubes is 5.5 to 7.5 nm.

Cardiolipins↗