Search PubMed⌕ Search

Biomedical subjects

A Blume

Publications and source records attributed to A Blume.

At least 55 records · Page 3Linked to original sources
PubMed1992

Thermodynamics of ion binding to phosphatidic acid bilayers. Titration calorimetry of the heat of dissociation of DMPA.

The heat of dissociation of the second proton of 1,2-dimyristoylphosphatidic acid (DMPA) was studied as a function of temperature using titration calorimetry. The dissociation of the second proton of DMPA was induced by addition of NaOH. From the calorimetric titration experiment, the intrinsic pK0 for the dissociation reaction could be determined by applying the Gouy-Chapman theory. pK0 decreases with temperature from ca. 6.2 at 11 degrees C to 5.4 at 54 degrees C. From the total heat of reaction, the dissociation enthalpy, delta Hdiss, was determined by subtracting the heat of neutralization of water and the heat of dilution of NaOH. In the temperature range between 2 and 23 degrees C, delta Hdiss is endothermic with an average value of ca. 2.5 kcal.mol-1 and shows no clear-cut temperature dependence. In the temperature range between 23 and 52 degrees C, delta Hdiss calculated after subtraction of the heat of neutralization and dilution is not the true dissociation enthalpy but includes contributions from the phase transition enthalpy, delta Htrans, as the pH jump induces a transition from the gel to the liquid-crystalline phase. The delta Cp for the reaction enthalpy observed in this temperature range is positive. Above 53 degrees C, the pH jump induces again only the dissociation of the second proton, and the bilayers stay in the liquid-crystalline phase. In this temperature range, delta Hdiss seems to decrease with temperature. The thermodynamic data from titration calorimetry and differential scanning calorimetry as a function of pH can be combined to construct a complete enthalpy-temperature diagram of DMPA in its two ionization states.

Calorimetry, Differential Scanning↗
PubMed1992

Acyl chain conformational ordering in 1,2 dipalmitoylphosphatidylethanolamine. Integration of FT-IR and 2H NMR results.

The extent of trans-gauche isomerization at the 4 and 4' positions of the acyl chains of fully hydrated 4,4,4',4'-d4 1,2-dipalmitoylphosphatidylethanolamine (4-d4 DPPE) bilayers was quantitatively evaluated from the infrared (IR) intensity of the CD2 rocking modes. About 20% gauche conformers were observed at 72 degrees C (above Tm), while at 23 degrees C, well below Tm, about 4% were noted. The order parameter SC-D was determined from 2H nuclear magnetic resonance (NMR) quadrupolar splittings. SC-D is the product of a segmental order parameter (S gamma), which depends on conformational order, and a chain order parameter (S alpha) which depends on slower motions such as chain wobble. The IR-determined percentage of gauche forms was converted into a segmental order parameter and factored out of the measured value for SC-D to yield an estimate of S alpha = 0.59 for L alpha phase DPPE. A comparison with S alpha for 1,2-dipalmitoylphosphatidylcholine (DPPC) suggests that increased wobble is responsible for enhanced motional averaging of the quadrupolar splittings in the latter at a similar reduced temperature. The extent of conformational disordering [at the 4(4') position] is essentially unchanged between the two molecules. The current study demonstrates the advantage of integrating quantitative IR with 2H NMR data, for elucidation of the contributions of the individual motions that average the NMR quadrupolar splittings.

Deuterium↗
PubMed1991

Phase transitions of polymerizable phospholipids.

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.

Lipid Bilayers↗
PubMed1990

Epidermal growth factor inhibits transiently the progression from G2-phase to mitosis: a receptor-mediated phenomenon in various cells.

An immediate effect of epidermal growth factor (EGF) on the cell cycle is described. EGF, when given to replicating cells such as HeLa, A431, and D HER 14, very rapidly inhibits the transition from G2 phase to mitosis (M) in a transient fashion. The influence of EGF (10(-10) to 10(-7) M concentrations) on the G2-M transition of individual cells has been analyzed by time-lapse photography in cell lines carrying intact, mutated, or no EGF receptor. The G2-M transition of cells devoid of EGF receptor or carrying an EGF receptor devoid of most of the cytoplasmic domain was not influenced by EGF. In cells carrying intact EGF receptor, EGF caused a transient and dose-dependent delay in G2 phase which could last for greater than 2 h. Cells were inhibited in G2 within less than 10-20 min prior to prophase. A parasynchronous recovery from G2 inhibition was observed at large EGF concentrations; the G2-M transition rate exceeded that of the controls. The system described may represent a model for the mechanistic analysis of a ligand-induced transient restriction of the cell cycle, particularly at the G2-M border.

Animals↗
PubMed1988

Fourier transform infrared spectroscopy of 13C = O-labeled phospholipids hydrogen bonding to carbonyl groups.

Fourier transform infrared spectroscopy has been used to characterize the carbonyl stretching vibration of DMPC, DMPE, DMPG, and DMPA, all labeled with 13C at the carbonyl group of the sn-2 chain. Due to the vibrational isotope effect, the 13C = O and the 12C = O vibrational bands are separated by ca. 40-43 cm-1. This frequency difference does not change when the labeling is reversed with the 13C = O group at the sn-1 chain. For lipids in organic solvents possible conformational differences between the sn-1 and sn-2 ester groups have no effect on the vibrational frequency of the C = O groups. In aqueous dispersion unlabeled phospholipids always show a superposition of two bands for the C = O vibration located at ca. 1740 and 1727 cm-1. These two bands have previously been assigned to the sn-1 and sn-2 C = O groups. FT-IR spectra of 13C-labeled phospholipids show that the vibrational bands of both, the sn-1 as well as the sn-2 C = O group, are clearly superpositions of at least two underlying components of different frequency and intensity. Band frequencies were determined by Fourier self-deconvolution and second-derivative spectroscopy. The difference between the component bands is ca. 11-17 cm-1. Again, the conformational effect as shown by reversed labeling is negligible with only 1-2 cm-1. The splitting of the C = O vibrational bands in H2O and D2O is caused by hydrogen bonding of water molecules to both C = O groups as shown by a comparison with spectra of model ester compounds in different solvents. To extract quantitative information about changes in hydration, band profiles were stimulated with Gaussian-Lorentzian functions. The chemical nature of the head group and its electronic charge have distinctive effects on the extent of hydration of the carbonyl groups. In the gel and liquid-crystalline phase of DMPC the sn-2 C = O group is more hydrated than the sn-1 C = O. This is accord with the conformation determined by X-ray analysis. In DMPG the sn-1 C = O group seems to be more accessible to water, indicating a different conformation of the glycerol backbone.

Carbon Isotopes↗
PubMed1988

Nuclear magnetic resonance and calorimetric study of the structure, dynamics, and phase behavior of uranyl ion/dipalmitoylphosphatidylcholine complexes.

The interaction of UO2(2+) with dipalmitoylphosphatidylcholine (DPPC) has been studied as a function of temperature and composition using nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), and monolayer studies. Computer simulations of the 31P-NMR powder spectra of DPPC dispersions in the presence of various concentrations of UO2(2+) are consistent with the binding stoichiometry of [UO2(2+)]/[DPPC] = 1:4 at [UO2(2+)]/[DPPC] less than 0.3. This complex undergoes a phase transition to the liquid crystalline phase at T'm = 50 +/- 3 degrees C with a breadth delta T'm = 7 +/- 3 degrees C. This broad transition gradually disappears at higher UO2(2+) concentrations, suggesting the presence of yet another UO2(2+)/DPPC complex (or complexes) whose NMR spectra are indistinguishable from those of the 1:4 UO2(2+)/DPPC species. The temperature-dependent 13C powder spectra of 2(1-13C) DPPC dispersions in the presence of 1.2 mol ratio of UO2(2+) show that this higher order complex (complexes) also undergoes a phase transition to the liquid crystalline state at T'm +/- = 58 +/- 3 degrees C with a breadth delta T"m = 15 +/- 5 degrees C. The NMR spectra indicate that exchange among these various UO2(2+)/DPPC complexes is slow. In addition, computer simulations of the 31P-, 13C-, and 2H-NMR powder spectra show that axial diffusion of the DPPC molecules about their long axes is quenched by addition of UO2(2+) and acyl chain isomerization is the dominant motional mode. The isomerization is best described as two-site hopping of the greater than C-D bond at a rate of approximately 10(6) s-1, a motional mode which is expected for a kink diffusion.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1986

Dimyristoylphosphatidic acid/cholesterol bilayers. Thermodynamic properties and kinetics of the phase transition as studied by the pressure jump relaxation technique.

Lipid bilayers and monolayers composed of dimyristoylphosphatidic acid (DMPA) and cholesterol were characterized by differential scanning calorimetry and film balance measurements. Increasing cholesterol content decreases the bilayer phase transition temperature and enthalpy in a manner similar to that observed before for other lipid/cholesterol systems. In monomolecular films at the air-water interface cholesterol exhibits the well known condensing effect in the liquid-expanded phase, while the liquid-condensed phase is less affected. As with the bilayer phase transition, the transition temperature and change in area at the liquid-condensed to liquid-expanded phase transition, as measured from isobars at 25 dynes/cm, decreases with increasing cholesterol content. The kinetics of the phase transition of DMPA/cholesterol bilayers were measured using the pressure jump relaxation technique with optical detection. Three relaxation times were observed. The relaxation times and amplitudes pass through maximum values at the transition midpoint. With increasing cholesterol content the maximum values of the relaxation times decrease but not in a linear fashion. The time constants display an intermediate maximum at ca. 10% to 12 mol% cholesterol. This observation is discussed in terms of a possible change in the nature of the phase transition from first-order with phase separation to a continuous second-order transition. The dependence of the relaxation amplitudes on cholesterol content gave evidence for nucleation being the rate limiting step for the transition in this particular system.

Calorimetry, Differential Scanning↗
PubMed1983

Effect of the lipid phase transition on the kinetics of H+/OH- diffusion across phosphatidic acid bilayers.

The kinetics of H+/OH- diffusion across dimyristoyl phosphatidic acid bilayer membranes was measured by following the absorbance of the pH-sensitive indicator Cresol red (o-cresolsulfonphthalein) entrapped in single lamellar vesicles after rapidly changing the external pH in a stopped-flow apparatus. The H+/OH- permeability coefficient was found to be in the 10(-5) to 10(-3) cm . s-1 range. The lipid phase transition has a strong influence on the permeation kinetics as the permeability coefficients in the liquid-crystalline phase are drastically higher. The permeability shows no maximum at the phase transition temperature as is the case for other ions, but displays a similar temperature dependence as water permeation. This is also reflected in the high activation energy of approx. 20 kcal/mol and supports the hypothesis (Nichols, J.W. and Deamer, D.W. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 2038-2042) of H+/OH- permeation via hydrogen bonded water molecules. A second slower kinetic phase is also observed, where the permeation is obviously controlled by counterion diffusion. The temperature dependence of this slow process displays the for ion diffusion characteristic maximum in the permeability at the phase-transition temperature.

Diffusion↗
PubMed1982

Molecular dynamics and conformation in the gel and liquid-crystalline phases of phosphatidylethanolamine bilayers.

Solid-state deuterium and carbon-13 nuclear magnetic resonance (NMR) spectra have been used to study the molecular dynamics and conformation of dipalmitoylphosphatidylethanolamine (DPPE) in both the gel (L beta) and liquid-crystalline (L alpha) phases. For this purpose DPPE was labeled with 13C in the carbonyl group of the sn-2 chain and with 2H at three different positions--4, 8, and 12--of the sn-2 chain, at the 2 position of the glycerol backbone, and at the 1 position of the ethanolamine head group. The 13C carbonyl and 2H chain spectra indicate that in the gel phase the DPPE molecules are diffusing about their long axes at rates of 10(5)-10(6) s-1 and the acyl chains are in an approximately all-trans conformation. The glycerol backbone spectra suggest that the backbone is in a gauche conformation in the gel state, rather than a trans conformation such as found in single crystals. The head group spectra in the gel phase are broad, featureless lines of about 20-kHz width. At the L beta leads to L alpha phase transition several changes take place. As is well-known, the chains disorder, and fast long-axis rotational diffusion begins, which results in the sharp, axially symmetric L alpha phase 2H spectra, which are a factor of 2 narrower than those observed in the L beta phase. The head group spectra also sharpen substantially at the transition, although their total width remains approximately constant. The invariance of the spectral width suggests that the average head group conformation is similar in both phases. However, the sharper spectra seen in the L alpha phase indicate that the rates of the head group motions in this phase are at least 3 orders of magnitude faster than those in the L beta phase. Thirdly, the 2H spectra of the glycerol backbone labeled DPPE narrow by a factor of about 4, and we believe this is due to a conformational change in this region of the molecule. Consistent with this interpretation is the fact that the powder pattern exhibited by the sn-2 13C = O in the L beta phase collapses to an isotropic-like line at the phase transition.

Gels↗
PubMed1982

Carbon-13 and deuterium nuclear magnetic resonance study of the interaction of cholesterol with phosphatidylethanolamine.

Mixtures of dipalmitoylphosphatidylethanolamine (DPPE) and cholesterol (CHOL) have been studied with solid-state 13C and 2H nuclear magnetic resonance (NMR) techniques. DPPE was 13C labeled at the carbonyl group of the sn-2 chain, and 2H was introduced at the 4 position of the sn-2 chain and the 1 position of the ethanolamine head group. The 13C and 2H spectra of each labeled lipid were studied as a function of temperature and CHOL concentration, and the results indicate three distinguishable temperature-composition regions. In region I, which occurs at low temperatures and CHOL concentrations, the 13C and 2H spectra are similar to those observed for pure DPPE in its gel phase. In region II, which occurs at higher temperatures or CHOL concentrations, the sn-2 13C = O spectra of DPPE/CHOL mixtures display two components, indicating the coexistence of two conformationally and dynamically inequivalent DPPE molecules. One of these is similar to gel-state DPPE, while the second "fluid" fraction displays some liquid-crystalline character. The two-component 13C spectra can be simulated quantitatively with a two-parameter chemical exchange model that permits the fraction of each form and the exchange rate to be determined as a function of temperature and composition. The 2H spectra observed in region II do not exhibit two components in an obvious way. Nevertheless, with some reasonable assumptions, the 2H spectra obtained from chain-labeled DPPE can also be simulated with a two-component model with the exchange rates and fractional populations obtained from the 13C results. The calculations predict not only the line shapes but also the losses in spectral intensity arising from use of the quadrupole echo technique. Furthermore, the 2H spectra show that with increasing temperature the fluid fraction observed in region II undergoes a transition to a higher degree of disorder, and should therefore not be labeled "liquid crystalline". In region III, which occurs at high temperatures and CHOL concentrations, both the 13C and 2H spectra are those expected of liquid-crystalline lipid. The NMR results are compared to, and found to be different from, those obtained with calorimetric investigations. It is suggested that these differences are due to the small domains present in DPPE/CHOL mixtures that lead to phase transitions of low cooperativity. Some metastability of the DPPE/CHOL system was observed at high CHOL concentrations and low temperatures.

Carbon Isotopes↗
PubMed1982

Phase equilibria, molecular conformation, and dynamics in phosphatidylcholine/phosphatidylethanolamine bilayers.

Solid-state 13C and 2H NMR experiments have been used to examine the phase equilibria and dynamic structure of binary mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). The experiments rely on changes in the 13C and 2H spectra of sn-2 13C = O labeled and 2H chain and head group labeled lipids at phase boundaries. In particular, broad powder patterns are observed in the gel state, and these patterns narrow dramatically in the liquid-crystalline phase. In the two-phase region a superposition of the gel-phase and liquid-crystalline-phase spectra is observed, which results from the presence of large domains in the DPPC/DPPE system. The appearance of the liquid-crystalline component and the disappearance of the gel component permit the solidus and liquidus curves of the phase diagram to be located accurately. Furthermore, a comparison of the temperature dependence of the 13C and 2H spectra shows the phase transition mechanism to be a function of composition. This result, together with other evidence, supports the hypothesis that the gel-state lattice changes from the L beta or P beta to the L beta configuration with increasing DPPE content. Finally, a detailed examination of the composition dependence of the spectra provides evidence for nonideal mixing.

Carbon Isotopes↗
PubMed1982

Incorporation kinetics of lysolecithin into lecithin vesicles. Kinetics of lysolecithin-induced vesicle fusion.

The incorporation kinetics of L-palmitoylphosphatidylcholine (lysolecithin) into dimyristoyl- and dipalmitoylphosphatidylcholine vesicles and the subsequent aggregation and fusion of the vesicles into larger aggregates were studied by using stopped-flow rapid-mixing techniques. The half-times for the lysolecithin incorporation vary between 50 and 500 ms. The incorporation rate has a maximum in the temperature range of the vesicle phase transition. This process is not diffusion controlled. The rate-limiting step is the incorporation of the lysolecithin monomer into the liquid bilayer. After this fast process, a slow reaction in the 10-50-min time range is observed. The large irreversible increase in turbidity indicates aggregation and fusion of the vesicles. The initial step is a second-order reaction with respect to the vesicle concentration, indicating aggregation or fusion of two vesicles. The aggregation rate passes through a maximum at the phase transition temperature.

Dimyristoylphosphatidylcholine↗
PubMed1982

Structure and dynamics of phospholipid membranes: an electron spin resonance study employing biradical probes.

The large zero-field splitting of rigid biradicals makes them important candidates for spin probes of phospholipid membranes. Here we develop an electron spin resonance line-shape model for such probes on the basis of the stochastic Liouville equation. Particular emphasis is given to the slow-diffusional regime, characteristic of bilayers in the gel phase. The theory is employed to study the line shapes of bis(verdazyl) biradicals, incorporated into oriented multibilayers of dimyristoylphosphatidylcholine. Computer simulations of the angular-dependent spectra provide the orientational distribution functions and rotational correlation times of the spin probes. They occupy two different sites in bilayer membrane. The orientational distribution of the spin probes is related to the structure of the phospholipid phases. In the L beta' phase the hydrocarbon chains are uniformly tilted by delta = 23 degrees with respect to the bilayer normal. For the P beta' phase we observe a random distribution of tilt angles from delta = 0 degree to delta = 19 degrees, indicating that the chains orient perpendicular to the local (rippled) bilayer surfaces. This structure has not been established previously. In agreement with other studies we find no tilt for the L alpha phase. The order parameters of the hydrocarbon chains increase with decreasing temperature, jumping from S less than or equal to 0.6 to S greater than or equal to 0.8 at the main transition. From the rotational correlation times of the spin probes, intrinsic bilayer viscosities of 0.08 P less than or equal to eta less than or equal to 20 P (50 degrees C greater than or equal to T greater than or equal to 1 degree C) are determined. An Arrhenius plot provides activation energies of the viscous flow. The values increase from Evisc approximately 10 kcal/mol in the L alpha phase to Evisc approximately 18 kcal/mol in the L beta' phase.

Dimyristoylphosphatidylcholine↗
PubMed1981

Calorimetric and fluorescence depolarization studies on the lipid phase transition of bacteriorhodopsin--dimyristoylphosphatidylcholine vesicles.

The thermotropic lipid phase transition of dimyristoylphosphatidylcholine vesicles reconstituted with bacteriorhodopsin was investigated as a function of the lipid to protein ratio by means of differential scanning calorimetry and fluorescence depolarization of the embedded probe 1,6-diphenyl-1,3,5-hexatriene. Two attractive features of this system are that the lipid phase transition induces lipid-protein segregation and that the state of aggregation of the protein is known. Above the lipid phase transition and above molar lipid to protein ratios of about 100, bacteriorhodopsin is monomeric. Well below the phase transition, bacteriorhodopsin is aggregated in a hexagonal protein lattice. With increasing amounts of incorporated bacteriorhodopsin, the calorimetric transition broadens, and a second component develops at a temperature which is lower than that of the unperturbed transition. The latter transition was assigned to the disaggregation of the bacteriorhodpsin lattice which occurs 6-7 degrees C below the phase transition of the protein-free lipids according to previous measurements. The van't Hoff enthalpy of the phase transition, as determined from the fluorescence depolarization of diphenylhexatriene, is in surprisingly good agreement with that obtained from differential scanning calorimetry over a wide range of lipid to protein ratios. The differential scanning calorimetry data can be simulated on the basis of a model which takes the protein segregation and crystallization specifically into account. The essential feature of this model is that calorimetrically detectable lipid melting occurs in the temperature region of the protein crystallization, since for the disaggregation of the close packed bacteriorhodopsin lattice additional lipids of an intermediate chain conformation are required.

Bacteriorhodopsins↗
PubMed1981

A calorimetric study of the thermotropic behaviour of 1,2-dipentadecylmethylidene phospholipids.

Differential scanning calorimetry was used to study the thermotropic behaviour of 1,2-dipentadecylmethylidene phospholipids with various head groups. The structural variation in the glycerol backbone region leads to a strong restriction of conformational freedom for the first two methylene segments of the chains, so that dipentadecylmethylidene phospholipids show lower transition temperatures, lower enthalpies and lower cooperativity of the transition from the gel to the liquid crystalline phase. The extreme chemical stability of these lipids in the alkaline pH region enables investigations of phosphatidylethanolamine and phosphatidic acid dispersions at high pH values. Both phospholipids show a decrease in the transition temperature and in the transition enthalpy as they become singly and doubly charged, respectively. A complex behaviour of the transition enthalpy of doubly charged 1,2-dipentadecylmethylidene phosphatidic acid was observed when the NaCl concentration of the dispersion was increased.

Calorimetry, Differential Scanning↗
PubMed1980

Thermotropic behavior of phosphatidylethanolamine-cholesterol and phosphatidylethanolamine-phosphatidylcholine-cholesterol mixtures.

The thermotropic behavior of aqueous dispersions of phosphatidylethanolamine-cholesterol and phosphatidyl-ethanolamine-phosphatidylcholine-cholesterol mixtures has been studied by high-sensitivity differential scanning calorimetry. The gel to liquid-crystalline phase transition of phosphatidylethanolamines is broadened and shifted to lower temperature when cholesterol is incorporated into the bilayer. When the cholesterol content is below 25 mol %, the calorimetric endotherms seem to consist of two components, a broad one at considerably lower temperature than the original transition and another component at only slightly lower temperature. This thermotropic behavior can be explained by the assumption of a homogenous distribution of cholesterol in phosphatidylethanolamine bilayers. Scanning calorimetry of equimolar mixtures of phosphatidylethanolamines with phosphatidylcholines, which show either ideal or nonideal mixing properties, reveals that when cholesterol is added to these mixtures it shows no preferential affinity for either of the phospholipids.

Calorimetry, Differential Scanning↗