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P Yager

Publications and source records attributed to P Yager.

At least 37 records · Page 2Linked to original sources
PubMed1995

Modeling success and failure of Langmuir-Blodgett transfer of phospholipid bilayers to silicon dioxide.

Formation of planar phospholipid bilayers on solid and porous substrates by Langmuir-Blodgett transfer of monolayers from the air-water interface could be of much greater utility if the process were not irreproducible and poorly understood. To that end the energetics of transferring two phospholipid monolayers to a hydrophilic surface has been examined. An approximate mathematical relationship is formulated that relates the surface pressure of the precursor monolayers to the tension within the bilayer created. Data are presented that demonstrate that bilayer transfer can be carried out reproducibly even with refractory phospholipids such as phosphatidylcholine, but only over a very narrow range of precursor monolayer surface pressures. This range is related to the lysis tension of the bilayer. The morphology of films formed within and below the successful range of surface pressures are examined by fluorescence microscopy, and the observed features are discussed in terms of the relationship above. These results provide practical guidelines for successful formation of lipid bilayers on hydrophilic surfaces; these guidelines should prove useful for research into the properties of biomembranes and for development of bilayer-based biosensors.

Biophysical Phenomena↗
PubMed1993

Perturbation of the chain melting transition of DPPC by galactose, agarose and Laurdan as determined by differential scanning calorimetry.

Differential scanning calorimetry was used to determine the effects of agarose hydrogel, galactose and the fluorophore Laurdan on the thermal behavior of multilamellar liposomes of DPPC. Although the effect of agarose on the phase transition was found to be negligible at low concentrations (< or = 2.5%), higher concentrations result in an endotherm that broadens and splits into two calorimetric events, one of which is at a higher temperature than that of hydrated DPPC. Equal weight fractions of galactose produce similar effects, although both fractions have raised melting temperatures. The higher melting components may be produced by osmotically-driven dehydration of the inner liposomal monolayers, a physical interaction between the carbohydrates and lipid headgroups, or a combination of both. Laurdan has little effect on the phase transition of DPPC vesicles at the concentration used in the sensor (0.67 mol%); concentrations up to 5.4 mol% only slightly lowered the melting temperature.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Microstructural polymorphism in bovine brain galactocerebroside and its two major subfractions.

Aqueous suspensions of either brain galactocerebrosides or its subfraction consisting of alpha-hydroxyacyl galactocerebrosides are mainly composed of vesicles or granular lipid with occasional multilamellar sheets. In aqueous media the other subfraction consisting of non-hydroxyacyl galactocerebrosides forms some helical structures, but most of the lipid remains as granules or vesicles. It is demonstrated that thermal cycling of non-hydroxyacyl galactocerebrosides in polar nonaqueous solvents can greatly enhance the degree of conversion to helical ribbons about 100 nm in diameter. These structures appear to be a stable dehydrated crystalline form of this lipid and are morphologically similar to helical microstructures produced by a few synthetic lipids. On the other hand, similar treatment of unfractionated bovine brain cerebroside and its alpha-hydroxy fatty acyl subfraction quantitatively produces straight needles that appear to be cochleate cylinders. While their dimensions depend on formation conditions, a typical suspension has uniform particles with diameters close to 100 nm and lengths variable from one to a few hundred micrometers. This is the first report demonstrating the quantitative formation of crystalline high axial ratio microstructures from complex mixtures of natural lipids. The different microstructures formed by the two components appear related to the various forms of lipid deposits occurring in lipid storage diseases. The similarity of these "synthetic" microstructures to biological structures in which they are found (such as myelin and intestinal brush border microvilli) strengthens the possibility that galactocerebrosides have a role in stabilizing cylindrical biological structures.

Animals↗
PubMed1991

Electrolyte effects on bilayer tubule formation by a diacetylenic phospholipid.

A general effect by dissolved electrolytes to destabilize the curvature of bilayer tubules prepared from the diacetylenic phospholipid, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine is not found. This observation discounts the role of an electrostatic interaction between polarization charges on the edges of a ferroelectric bilayer as a means by which the cylindrical curvature may be stabilized in these structures (de Gennes, P. G. 1987. C. R. Acad. Sci. Paris. 304:259-263). The solution-mediated ionic interactions of electrolytes with this phospholipid appear not to influence significantly the relative stability of the crystalline state of the tubule, but at high levels of a few salts, may affect the nucleation and growth of the crystalline bilayer. Curvature of the bilayer in these tubular structures apparently derives from an interaction that is not very sensitive to the presence of electrolytes. Cylindrical curvature may alternatively arise from a bending force within the bilayer that is intrinsic to the anisotropic packing of the lipid molecules (Helfrich, W., and J. Prost. 1988. Phys. Rev. A38:3065-3068; Chappell, J. S., and P. Yager. 1991. Chem. Phys. Lipids. In press), and may therefore be largely determined by the packing interactions within the hydrophobic region of the tubular bilayer.

Calorimetry↗
PubMed1990

Optical method for monitoring the concentration of general anesthetics and other small organic molecules. An example of phase transition sensing.

As an example of chemical sensing based on perturbations of thermal phase transitions, we have shown that phospholipids labeled with a fluorescent dye may be used to measure the concentration of general anesthetics and other small organic molecules. The emission maximum of the hydrophobic fluorescent probe Laurdan in phospholipid bilayers shifts from a wavelength of 445 nm below the main phase transition of the lipid to 480 nm above it, with an isosbestic point at approximately 475 nm. The greatest changes in intensity at the transition occur at 440 and 500 nm, so the ratio of the intensities at these two points was used as an "order parameter". The effects of variation of the liposomal preparation method on the order parameter were explored, and it was found that in mixed lipids the parameter varied nearly linearly over the physiological temperature range. Fluorometry detected changes in the order of bilayers caused by solubilization of the anesthetic isoflurane (Forane) and of ethanol. At a defined temperature, the intensity ratio measured in the presence of anesthetic decreases in a concentration-dependent manner. Immobilizing the liposomes in a hydrogel did not perturb the response of the system. This work demonstrates the potential for using lipid phase transitions in an optical sensor for monitoring anesthetics and other small nonpolar molecules.

Anesthetics↗
PubMed1988

Development of membrane-based biosensors: measurement of current from photocycling bacteriorhodopsin on patch clamp electrodes.

Our initial work toward developing membrane protein-based biosensors has involved use of bacteriorhodopsin (BR) as a model membrane protein. BR was incorporated into liposomes of a polymerizable lecithin, and was shown to pump protons in response to illumination both before and after polymerization of the lipids. In the work described in this paper, BR was first reincorporated in liposomes of asolectin by consonication with purple membrane. The liposomes, which sustained the function of the protein, were used to form a monolayer at the air-water interface. This monolayer was transferred as a bilayer onto patch electrode. When illuminated with a pulse of 514.5-nm light the lipid/protein patch produced a current spike into the pipette corresponding to events no later than the generation of the 412-nm intermediate, probably caused by pumping of protons across the patch membrane. The experiment demonstrates not only the extreme sensitivity of amperometric detection, but also a small tendency for membrane proteins to preferentially orient in this configuration.

Bacteriorhodopsins↗
PubMed1988

Structure of polymerizable lipid bilayers. I--1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, a tubule-forming phosphatidylcholine.

This report presents the first X-ray diffraction data on diacetylenic phospholipids. The tubule-forming polymerizable lipid, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC), was studied by low angle X-ray diffraction from partially dehydrated oriented multibilayers in both polymerized and unpolymerized form. Bilayers of this material were found to be highly ordered, yielding as many as 16 orders of lamellar diffraction, in both the polymerized and unpolymerized states. The unit cell dimension was very small for a lipid of this size. In addition to the features usually observed in the electron density profile structure of phospholipid bilayers, the electron-dense diacetylenic portions of the fatty acyl chain produced electron density maxima at two well-defined levels on each side of the bilayer approximately 15 A and 9 A from the bilayer midplane. A model molecular conformation deduced from the one-dimensional electron density map features all-trans acyl chains tilted at approximately 28 degrees from the bilayer normal that are interdigitated with chains of the opposing monolayer by approximately two carbons at the bilayer center. The linear diacetylene moieties on beta- and gamma-chains appear at different positions along the bilayer normal axis and are roughly parallel to the bilayer surface. This model is discussed in terms of a polymerization mechanism.

Diynes↗
PubMed1988

Fabrication of key components of a receptor-based biosensor.

A three-pronged approach was taken to the development of receptor-based bisensors. First, asymmetric bilayer membranes were developed with one monolayer adaptable to the particular receptor of interest and the other monolayer polymerized to enhance membrane stability. Second, alamethicin and calcium channel complexes were introduced into the stabilized membrane and tested for ion-channel function. Third, a porous support for the receptor-containing membrane was fabricated, which is compatible with silicon technology. Preliminary devices incorporating these components were constructed.

Alamethicin↗
PubMed1987

Orientation of lipid tubules by a magnetic field.

Lipid tubules, which are straight hollow cylinders consisting of lipid bilayers, are shown to orient in strong magnetic fields. Birefringence measurements were made of dilute samples of tubules of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC23PC) in magnetic fields of up to 4 T. The tubules were found to orient with their long axes parallel to the field direction, with saturated orientation [P2 (cos theta] approximately greater than 0.95) found at approximately 2 T. From known distributions of lengths and the number of bilayers in the walls, a value delta chi = (-7 +/- 1) X 10(-9) erg cm-3 G-2 was calculated for the tubules, which compares well with some previously reported values for phosphatidylcholines. Magnetic alignment will permit more sophisticated structural studies of monomeric and polymeric tubules, and provide a method of orienting macromolecules in the tubule walls or interior.

Birefringence↗
PubMed1987

Rabies preexposure prophylaxis with human diploid cell rabies vaccine: a dose-response study.

Recent studies suggest that human diploid cell rabies vaccine (HDCV) may not always produce acceptable titers after intradermal (id) preexposure prophylaxis. To stimulate accidental deviation from the recommended route of administration and to determine the immunogenicity of smaller-than-recommended doses of HDCV, we injected each of 154 persons either intramuscularly (im) with 100%, 10%, or 3% of the standard im dose of vaccine or id with 10%, 3% or 1% of the standard im dose. Seroconversion (titers of antibody greater than or equal to 1:11) was found in all subjects at 49 and 90 days after vaccination. Titers were higher for subjects receiving 100% of the recommended dose im than for those receiving 10% of this dose id (P less than .01); these titers in turn were higher than those from persons receiving smaller doses (P less than .05). Persons receiving 10% or 3% of the standard im dose had lower titers on day 49 than did those receiving the same dose id (P less than .05).

Antibodies, Viral↗
PubMed1983

Encapsulation of hemoglobin in phospholipid vesicles.

Hemoglobin has been encapsulated in phospholipid vesicles by extrusion of hemoglobin/lipid mixtures through polycarbonate membranes. This technique avoids the use of organic solvents, sonication, and detergents which have proven deleterious to hemoglobin. The vesicles are homogeneous, with a mean size of 2400 A as determined by photon correlation spectroscopy. The encapsulated hemoglobin binds oxygen reversibly and the vesicles are impermeable to ionic compounds. Hemoglobin encapsulated in egg phosphatidylcholine vesicles converts to methemoglobin within 2 days at 4 degrees C. By contrast, when a mixture of dimyristoyl phosphatidylcholine, cholesterol and dicetyl phosphate is used there is no acceleration in methemoglobin formation, and the preparation is stable for at least 14 days at 4 degrees C.

Animals↗
PubMed1982

Changes in size and shape of liposomes undergoing chain melting transitions as studied by optical microscopy.

Changes in the shape and size of dipalmitoylphosphatidylcholine liposomes at the phase transition at 41.5 degrees C have been monitored by light microscopy. All liposomes change size or shape at the transition and those with simple topologies such as spheres and cylinders can be readily measured. The surface area of these is some 24% greater above the transition than below. This surface area change is virtually identical to that predicted by crystallographic measurements on this system. Also, the rate of transition from one state to another is seen to proceed more rapidly in the smaller liposomes. Optical microscopic observation provides a rapid simple method for monitoring the dependence of the lipid bilayer area on temperature.

Lipid Bilayers↗
PubMed1982

The kinetics of the main phase transition of aqueous dispersions of phospholipids induced by pressure jump and monitored by Raman spectroscopy.

The sensitivity of the melting transition temperature of aqueous dispersions of dipalmitoyl- and distearoylphosphatidylcholine to hydrostatic pressure is used to allow measurement of the rates of isothermal freezing and melting of the lipids by rapidly changing the pressure. The degree of order of the lipids is measured by monitoring a ratio of two points in the Raman spectrum of the lipids which changes sharply at the melting temperature. Use of this Raman order ratio allows correlation between the order of the sample and the rates of transition in a manner which is impossible by monitoring only turbidity. Our longest relaxation times range upwards from a few seconds for both compounds. The freezing rates are slowest when the samples are initially fully melted, and the melting rates are slowest when the samples are initially frozen. These results imply that nucleation of the growing phase dominates the kinetics of both freezing and melting.

Chemical Phenomena↗
PubMed1980

Statistical mechanical analysis of Raman spectroscopic order parameter changes in pressure-induced lipid bilayer phase transitions.

The statistical mechanical cluster theory of Fisher as applied by Kanehisa and Tsong to phospholipid bilayers is modified to describe the effects of hydrostatic pressure on the state of an aqueous dispersion of the phospholipid dipalmitoyl phosphatidylcholine. A high pressure Raman scattering cell has been built to obtain the Raman spectra of aqueous dispersions of phospholipids as a function of the applied hydrostatic pressure from 0 to 100 atmospheres. Predicted thermal and pressure-induced phase transitions are compared with an experimentally obtained Raman order parameter derived from the ratio of two bands in the C-H stretching region of the Raman spectrum of the sample. The parameters of the theory are adjusted to obtain a satisfactory fit of the Raman order parameter versus temperature. The theory is then found to give an excellent prediction of the observed pressure dependence of the Raman order parameter with no changes in the adjustable parameters. The implications of the success of the theoretical fit is discussed. Particularly of interest is the rather high value of the critical temperature, Tc, for lipid bilayers which is predicted by the model.

Hydrostatic Pressure↗
PubMed1978

Conformational nonequivalence of chains 1 and 2 of dipalmitoyl phosphatidylcholine as observed by Raman spectroscopy.

Raman spectroscopic data indicate that the conformations of the two hydrocarbon chains of dipalmitoyl phosphatidylcholine in aqueous dispersions of the lipid differ signficantly. The compounds 1-palmitoyl, 2-palmitoyl-d31-3-sn-phosphatidylcholine and 1-palmitoyl-d31, 2-palmitoyl-3-sn-phosphatidylcholine were synthesized. Aqueous dispersions of these phospholipids display very similar phase behavior, with both premelting and melting transitions at nearly identical temperatures, midway between the comparable transition temperatures of undeuterated and completely deuterated dipalmitoyl phosphatidylcholine. We have monitored the state of chains 1 and 2 of these molecules simultaneously and independently by Raman spectroscopy. Raman difference spectra taken between samples of the two compounds under identical conditions show significant features. We attribute these spectral differences to nonequivalent conformations of the fatty acyl chains attached at positions 1 and 2 on the glycerol backbone. Below the pretransition the conformation of chain 2 is, on average, slightly less all-trans than is the chain at position 1. There is some evidence that the conformations of the terminal methyl group of the two chains are significantly different at low temperatures.

Calorimetry, Differential Scanning↗
PubMed1978

Deuterated phospholipids as nonperturbing components for Raman studies of biomembranes.

The deuterated phospholipid, 1,2-dipalmitoyl-d62-phosphatidylcholine is shown by Raman spectroscopic measurements to be useful for obtaining information concerning phospholipid conformation in complex phospholipid and lipidprotein mixtures. The Raman bands of the deuterated phospholipid are assigned, and the sensitivity of these vibrational modes to conformational changes in the bilayer is demonstrated. Deuteration of the alkyl chains reveals the CH vibrations of the head group. A change in these bands is observed at the melting temperature and is assigned to alteration of the glycerol backbone conformation upon melting.

Calorimetry, Differential Scanning↗
PubMed1978

Interpretation of biomembrane structure by Raman difference spectroscopy. Nature of the endothermic transitions in phosphatidylcholines.

Raman difference spectroscopy has been applied to aqueous dispersions of dipalmitoyl phosphatidylcholine (DPPC). Difference spectra have been created by computer subtraction of absolute Raman spectra taken in each of three different temperature ranges: below the endothermic pretransition at 34 +/- 2 degrees C; between this temperature and the melting transition at 42 degrees C; and above the melting temperature. The resultant difference spectra are both quantitatively and qualitatively different, indicating that a distinct phospholipid conformation occurs in each of the three temperature ranges. Furthermore, the difference spectra show details of Raman spectral changes with greater clarity than is possible with conventional Raman techniques. A description of the lateral interchain order and the longitudinal chain order is given for each of the three temperature ranges. In addition to obtaining a more precise quantitative measurement of the changes in the Raman spectra, we observed some significant and previously unreported changes. It is suggested that distortion in the hexagonal lattice below the pretransition temperature previously reported by X-ray diffraction techniques may be responsible for interchain interactions which give rise to a Raman band observed only in the triclinic lattice of even-numbered n-alkanes.

Liposomes↗