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PubMed1989

Reversibility of the thermal transitions of chloroplast thylakoid membranes.

Thylakoid membranes from cucumbers and peas have been examined by high-sensitivity differential scanning calorimetry. Data was collected during both heating and subsequent cooling scans in order to observe reversibility. Cucumber thylakoids exhibited almost no reversibility; a very small reversible exothermic peak was observed at approximately 12 degrees C in cooling scans. However, thylakoids from peas had reversible transitions at 50 and 68 degrees C, as well as other transitions which were visible as shoulders in a second heating scan. When pea grana thylakoids were unstacked, the high temperature transitions were sharpened and their reversibility was enhanced. This is the first report of chloroplast thylakoid membranes exhibiting reversible high temperature transitions. The results indicate that considerable variation can occur in the calorimetric profiles of thylakoids from different plants.

Calorimetry, Differential Scanning↗
PubMed2001

Pressure-temperature phase transition diagram for wheat starch.

Wheat starch suspensions in water (5% dry matter) were subjected to various pressures (0.1-600 MPa) and temperatures (-20 to 96 degrees C) for 15 min. The gelatinization rate was measured after treatment by using microscopic measurements of the loss of birefringence of the granules. This method was previously calibrated by differential scanning calorimetry. Curves of isogelatinization were found to be quite similar to a pressure-temperature (P-T) diagram of unfolding proteins. Results were first analyzed by considering the thermodynamic aspects related to the dT/dP curve shifts. On the basis of equations already shown for proteins, the P-T gelatinization diagram of wheat starch would show different kinds of thermal contributions, suggesting endothermic, athermic, or exothermic melting reactions. Second, as a practical consequence, these previous P-T areas corresponded to specific gelatinization conditions as confirmed by hydration evaluation measured by starch swelling index. Depending on the pressure-temperature conditions, gelatinization would involve hydration. Lowering the pressure and temperature resulted in a complete gelatinization with less hydration in comparison with a thermal treatment at atmospheric pressure. A hydration model based on an energetic approach was proposed.

Birefringence↗
PubMed2005

Decoupled phase transitions and grain-boundary melting in supported phospholipid bilayers.

Two separate liquid-solid phase transitions are detected in the two monolayers of a mica-supported phospholipid bilayer by atomic force microscopy. The phase transitions of the two monolayers are decoupled by the stronger interaction between the lipid headgroups of the proximal monolayer and the mica support. The transition temperature of the proximal monolayer is increased and this transition occurs over a narrower temperature range. Both transitions occur via grain-boundary melting and the variation of the width of the interfacial zone with temperature is consistent with mean-field theory.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1998

A pragmatic test of a simple calorimetric method for determining the fragility of some amorphous pharmaceutical materials.

PURPOSE: To evaluate a simple calorimetric method for estimating the fragility of amorphous pharmaceutical materials from the width of the glass transition region. METHODS: The glass transition temperature regions of eleven amorphous pharmaceutical materials were characterized at six different heating and cooling rates by differential scanning calorimetry (DSC). RESULTS: Activation energies for structural relaxation (which are directly related to glass fragility) were estimated from the scan rate dependence of the glass transition temperature, and correlations between the glass transition widths and the activation energies were examined. The expected correlations were observed, and the exact nature of the relationship varied according to the type of material under consideration. CONCLUSIONS: The proposed method of determining the fragility of amorphous materials from the results of simple DSC experiments has some utility, although "calibration" of the method for each type of materials is necessary. Further work is required to establish the nature of the relationships for a broad range of amorphous pharmaceutical materials.

Calorimetry↗
PubMed2001

Effect of silica aerosil dispersions on the dielectric properties of a nematic liquid crystal.

The static and dynamic aspects of (4-n-heptyl-4'-cyanobiphenyl) (7CB) perturbed by the dispersion of 70-A-diameter hydrophilic silica aerosil spheres have been investigated using dielectric spectroscopy. Results on five mixtures of 7CB plus silica aerosil are presented in order to probe systematically the disorder introduced by the silica aerosil network on the 7CB molecules. Measurements on homeotropically aligned samples have been made from 75 kHz to 30 MHz in the temperature range 30-60 degrees C. It was found for the mixtures that the nematic-isotropic transition temperature is lower than that of the bulk 7CB. Bulklike relaxation processes due to the rotation of the molecules around the short axis have been analyzed, and were found to follow Arrhenius-type behavior in the nematic phase except in the vicinity of the nematic-isotropic transition temperature. These processes are slightly faster in the mixture than in the free phase. A dielectric process in the low frequency range, absent in the bulk, has been observed in samples with higher silica concentration. All the observed relaxation processes in the mixtures are of non-Debye type.

Journal Article↗
PubMed1977

Pyrrolidone carboxylyl peptidase in Streptococcus cremoris: dependence on an interaction with membrane components.

A study of the distribution of pyrrolidone carboxylyl peptidase (PCP) activity among cell fractions of Streptococcus cremoris HP revealed that this enzyme is associated with a particulate fraction, which mainly consists of membrane material. This location could only be established using a gentle nonmechanical method for the disruption of spheroplasts under the conditions of which intracellular marker enzymes are released. The effect of monovalent anions and treatments, which do not destroy covalent binding, suggests an association of the enzyme with surrounding structures determined by both hydrophobic and electrostatic interactions. The activity of PCP associated with cells harvested from different growth phases and in the solubilized state was studied as a function of the temperature in the absence and in the presence of the membrane-interfering agent n-butanol. A decrease in the apparent activation energy, inherent to the solubilized enzyme, is induced in situ at a lower transition temperature. Only with logarithmic-phase cells is this transition followed (mid-logarithmic cells) or accompanied (late logarithmic cells) by a secondary decrease in the energy of activation. n-Butanol appeared to decrease the lower transition temperature of the enzyme activity in situ, and additionally it exerted an effect on the manifestation of the secondary transition. Thecorganization of membrane components, mainly the lipids.

Aminopeptidases↗
PubMed2007

Relationship of solvent to the photopolymerization process, properties, and structure in model dentin adhesives.

The ratio of the double-bond content of monomer to polymer, i.e. degree of conversion (DC) has been used frequently as a convenient means of comparing the behavior and properties of dental composites and adhesives. The purpose of this investigation was to study the relationship of photopolymerization processes, bulk properties, and structure using model dentin adhesives cured in the presence of different ethanol content as an example. There was little difference in the DC of model BisGMA-based adhesives cured in the presence of ethanol concentrations ranging from 0 to 40 wt %, but there were substantial differences in the mechanical properties. Ultimate tensile strength (UTS) and modulus of elasticity decreased with an increase in ethanol content. Polymer structure was revealed by thermal behavior in the glass transition temperature (Tg) region; these measurements were obtained by modulated temperature differential scanning calorimetry (MTDSC) technology, which removes the competing irreversible effects associated with release of volatiles and residual curing. Glass transition temperature of model adhesives decreased substantially with an increase in ethanol content. The DC based on the quantity of remaining double bond has been used extensively to characterize and provide a relative assessment of the quality of dentin adhesives and dental composites. Since polymers differing in linearity, and therefore crosslink density, may have a similar degree of conversion, the measurement of monomer/polymer conversion does not necessarily provide complete representation of the quality or durability of the polymer structure.

Dentin-Bonding Agents↗
PubMed2000

Viscous solutions, networks and the glass transition in high sugar galactomannan and kappa-carrageenan mixtures.

Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and kappa-carrageenan on the vitrification of glucose syrup at a total level of solids of 83%. The method of reduced variables allowed construction of composite curves covering the glass transition and glassy state (from 10(5) to 10(9.5) Pa) over a wide frequency range (up to 15 orders of magnitude). The combined WLF/free volume framework was employed to determine the rheological glass transition temperature (T(g)), fractional free volume and thermal expansion coefficient of the samples. It was found that the WLF-predicted glass transition temperature matched the cross over of experimental modulus traces in the passage from the glass transition (GG') to the glassy state (GG"). This coincides with the mechanistic transformation from free volume effects to the Arrhenius-type phenomena, thus ascribing physical significance to the rheological T(g). The T(g) value of 83% glucose syrup at a scan rate of 2 degrees C min(-1) was -25.3 degrees C. Replacing, for example, 1% glucose syrup with guar gum shifted the T(g) of the mixture to -19.7 degrees C. Network formation via the K(+)-supported junction zones of the kappa-carrageenan chains further increased the T(g) to about -1 degrees C. It appears that the low rates of relaxation processes and diffusion mobility in the presence of a polysaccharide network accelerate the collapse of the free volume thus inducing vitrification of the high sugar/polysaccharide mixture at high temperatures.

Carbohydrates↗
PubMed1987

Binding of lipophilic cations to the liposomal membrane: thermodynamic analysis.

Lipophilic ions are widely used as probes for measuring membrane potentials. Since binding of the probes to the membrane interferes with the accurate estimation of the membrane potential, it is necessary to clarify the characteristics of probe binding to membranes. The present paper deals with the binding of lipophilic cations to liposomes. The results can be summarized as follows: (1) The binding of triphenylmethylphosphonium, its homologues and tetraphenylphosphonium to liposomes of dipalmitoylphosphatidylcholine followed the Langmuir adsorption isotherm. (2) Spin-labeled lipophilic cations were synthesized and the binding to liposomes of egg phosphatidylcholine was examined. The binding also followed the Langmuir adsorption isotherm. The dissociation constant (the concentration giving half-maximal binding), K, was independent of the temperature, indicating that the binding is entropy-driven. (3) The binding was influenced by the fluidity of the membrane. Except in the case of triphenylmethylphosphonium (TPMP+), K and A (maximum amounts of binding) increased above the transition temperature. In other words, above the phase transition temperature the binding affinity is decreased, while maximum amounts of binding are increased for all phosphoniums used except TPMP+.

Adsorption↗
PubMedDate not supplied

Designing novel polymers with targeted properties using the signature molecular descriptor.

A method for solving the inverse quantitative structure-property relationship (QSPR) problem is presented which facilitates the design of novel polymers with targeted properties. Here, we demonstrate the efficacy of the approach using the targeted design of polymers exhibiting a desired glass transition temperature, heat capacity, and density. We present novel QSPRs based on the signature molecular descriptor capable of predicting glass transition temperature, heat capacity, density, molar volume, and cohesive energies of linear homopolymers with cross-validation squared correlation coefficients ranging between 0.81 and 0.95. Using these QSPRs, we show how the inverse problem can be solved to design poly(N-methyl hexamethylene sebacamide) despite the fact that the polymer was used not used in the training of this model.

Algorithms↗
PubMed1986

Correlation of side chain mobility with cholesterol retention by phospholipid vesicles.

Multilamellar vesicles were prepared from choline phospholipids with various fatty acyl chains, singly and in mixtures, with and without cholesterol. Mobility of acyl side chains for each type of vesicle was measured by fluorescence polarization with diphenylhexatriene, and the amounts of cholesterol and phospholipid retained by them after extraction with a nonpolar solvent were determined. The data suggest that structures of acyl chains determine the extractability of cholesterol. Phosphatidylcholines with unsaturated or short saturated side chains above transition temperature retain less cholesterol upon extraction with petroleum ether than phosphatidylcholines with saturated side chains below transition temperature. Correlation of cholesterol retention with side chain mobility showed that cholesterol is more easily removed from vesicles with mobile acyl side chains than from vesicles with rigid side chains. The presence of cholesterol also alters extractability of phospholipids from vesicles and suggests that sterol affects the polarity rather than spacing of headgroups on vesicle surfaces.

Carbon Radioisotopes↗
PubMed2001

A macroscopic description of lipid bilayer phase transitions of mixed-chain phosphatidylcholines: chain-length and chain-asymmetry dependence.

A macroscopic model is presented to quantitatively describe lipid bilayer gel to fluid phase transitions. In this model, the Gibbs potential of the lipid bilayer is expressed in terms of a single order parameter q, the average chain orientational order parameter. The Gibbs potential is based on molecular mean-field and statistical mechanical calculations of inter and intrachain interactions. Chain-length and chain-asymmetry are incorporated into the Gibbs potential so that one equation provides an accurate description of mixed-chain phosphatidylcholines of a single class. Two general classes of lipids are studied in this work: lipid bilayers of partially or noninterdigitated gel phases, and bilayers of mixed interdigitated gel phases. The model parameters are obtained by fitting the transition temperature and enthalpy data of phosphatidylcholines to the model. The proposed model provides estimates for the transition temperature and enthalpy, van der Waals energy, number of gauche bonds, chain orientational order parameter, and bond rotational and excluded volume entropies, achieving excellent agreement with existing data obtained with various techniques.

Biophysical Phenomena↗
PubMed1983

Effect of prostaglandin F2 alpha on Na+-K+-ATPase activity in luteal membranes.

Slices of rat corpora lutea (CL) incubated with prostaglandin F2 alpha (PGF2 alpha) in Krebs-Hensenleit (K-H) Ringer solution showed a decrease in Na+-K+-ATPase activity after 60 min of incubation. However, PGF2 alpha in vitro did not alter Na+-K+-ATPase activity of isolated luteal membrane fractions. Following PGF2 alpha-induced in vivo luteal regression, reduction of Vmax and elevation of the activation energy above transition temperature of the lipid phase of the membrane occurred without changes in Km, optimum pH and transition temperature. These results suggest that reduction of Na+-K+-ATPase activity after PGF2 alpha treatment may be due to reduction in the number of enzyme molecules or to masking of the active site of the enzyme without any change in enzyme characteristics. In addition, a change in membrane-bound enzyme activity may be an early step in PGF2 alpha-induced luteolysis.

Animals↗
PubMed1975

Assembly of complex III into newly developing mitochondrial membranes.

Yeast cells grown anaerobically in 0.02% linoleic acid were transferred to air in the presence of 0.02% elaidic acid. At varying times Arrhenius plots were made of QH2-cytochrome c reductase activities in isolated mitochondria. A transition temperature of 8.2 degrees C at 0.5 h was characteristic of linoleate; at 3 h the transition temperature was increased to 24 degrees C characteristic of elaidate. At early times the enzyme was associated with anaerobic promitochondrial membranes; at later states the newly synthesized enzyme was associated with newly developed elaidate membranes.

Aerobiosis↗
PubMed1988

Effects of gossypol acetic acid on the absorptive and digestive functions of rat intestine.

Oral administration of Gossypol acetic acid (10 mg/kg body wt./day, daily for 15 days), an experimental antifertility agent to male rats, caused significant reduction in the uptake of glucose, alanine, leucine and calcium in the small intestinal segments. Gossypol also caused significant decrease in the intestinal brush border membrane--associated enzymes, sucrase, lactase, maltase and alkaline phosphatase. Kinetic analysis indicated that Gossypol decreased the apparent velocity of the disaccharidases while the Km was not altered. It also caused a shift in the transition temperature in these enzymes and predictably changed the energy of activation both below and above the transition temperature, although the Arrhenius expressions of the temperature dependence still showed proximity and were parallel to the control group.

Administration, Oral↗
PubMed1981

[Viscoelastic behaviour of inlay waxes. (Part 3) Dynamic viscoelastic properties of ternary and quaternary mixtures of waxes (author's transl)].

The viscoelastic properties of ternary and quaternary mixtures added carnauba wax, dammar and beeswax to paraffin were investigated by measurement of dynamic viscoelasticity. The effects of the composition of waxes and the crystal transition temperature of paraffin to the viscoelastic behaviour of these waxes were discussed. The viscoelastic properties of ternary mixtures added carnauba wax and dammar to paraffin were similar to these of binary mixtures for paraffin and dammar in the region of lower temperatures, and similar to these of binary mixtures for paraffin and carnauba wax in the region of higher temperatures. In quaternary mixtures added carnauba wax, dammar and beeswax, tan delta was larger than that of ternary mixtures, so the effect of additions of dammar was disappeared in the region of lower temperatures. Additions of carnauba wax, like to the case of ternary mixtures, had an effect on the viscoelastic properties of quaternary mixtures, but the temperature dependence of G' and tan delta of these waxes were slightly broader than that of ternary mixtures in the region of higher temperature. For a same composition in ternary and quaternary mixtures, even if the paraffins with different transition temperature were used, the shapes of curves of G' and tan delta were so similar to one another that were approximately superposable.

Elasticity↗
PubMed1997

Structural organization of lipid phase and protein-lipid interface in apolipoprotein-phospholipid recombinants: influence of cholesterol.

The complexes of individual human plasma apolipoproteins (apo) A-I, E and A-II with dipalmitoylphosphatidylcholine (DPPC) in the absence or in the presence of cholesterol (Chol) were prepared with initial DPPC/Chol/protein weight ratio as 3:0.15:1. ApoA-I/DPPC/Chol complexes with different protein content (initial DPPC/apoA-I weight ratios were changed from 10.5:1 to 2.6:1) but with a fixed initial DPPC/Chol weight ratio of 20:1 were also prepared. The complexes were isolated by gel-filtration and characterized by size and composition. ApoA-I- and apoA-II-complexes had the same size (80-84 A) and the complexes became more heterogeneous upon Chol inclusion; apoE-complexes were larger (97-100 A) and more homogeneous and Chol addition had no effect on their hydrodynamic properties. Chol seems to be excluded partially in the following manner for isolated complexes with different apo's: A-II > E > A-I. The possible existence of two lipid regions in the complexes differing in lipid dynamics - the lipid shell in the vicinity of apolipoprotein (boundary lipid) opposite to the remaining part of the lipid bilayer - has been studied by absorbance and fluorescence spectroscopy with cis-parinaric acid (cis-PA) and trans-parinaric acid (trans-PA) embedded into the complexes. Their application is based on a strong preference of trans-PA for solid lipid while cis-PA distributes more equally between co-existing fluid and solid lipid regions (Sklar et al. (1979) Biochemistry 18, 1707-1716). (1) For apoA-I-complexes, the partition of cis-PA between water and lipid phase at temperatures below and above the transition temperature of DPPC (T(t)) was insensitive to Chol and temperature, while partition of trans-PA into the lipid phase of Chol-containing complex was increased at high temperature and decreased at low temperature. These results seem to be related to trans-PA redistribution between Chol-rich and protein-rich lipid domains, the latter being more disordered at T < T(t) and more immobilized at T > T(t) compared to the bulk bilayer; cis-PA localizes preferentially in boundary lipid. This hypothesis was directly confirmed by measurements of energy transfer between apoA-I tryptophanyls and probe molecules. (2) The relative response of trans-PA fluorescence intensity to temperature-induced phase transition of DPPC in apoA-I/DPPC/Chol complexes was decreased as a function of apolipoprotein content in a non-monotonic fashion with a transition midpoint at a mol ratio DPPC/A-I of 250:1, probably indicating two different modes of apolipoprotein/DPPC interaction in different sized complexes. (3) The comparative study of lipid dynamics in apoA-I-, apoE- and apoA-II-containing complexes with temperature response to phospholipid phase transition with fluorescence parameters such as intensity and anisotropy of cis-PA and trans-PA revealed the presence of boundary lipid in all three complexes without Chol. In contrast to apoA-I-containing complexes, in apoA-II/DPPC/Chol complexes, trans-PA seems to move preferentially into boundary lipid and cis-PA to distribute between two different regions probably as a result of more ordering action induced by apoA-II compared to apoA-I on the nearest phospholipid molecules in Chol-containing complexes; the apoE action on trans-PA and cis-PA distribution could be intermediate. Based on these results, the degree of Chol exclusion from the boundary lipid region for complexes with different apo's increasing in the order A-II > E > A-I can be suggested. Different Chol distributions between two lipid regions in the complexes seems not to be a function of complex size, but rather is an inherent property of the particular apolipoprotein molecule.

1,2-Dipalmitoylphosphatidylcholine↗