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At least 37 records · Page 2Linked to original sources
PubMed2001

The effect of ethanol on the phase transition temperature and the phase structure of monounsaturated phosphatidylcholines.

Previous studies from our laboratories have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines, C(X):C(Y)PC, and the effect of ethanol concentration, [EtOH], on the main phase transition temperature, T(m), and the phase structure of the lipid bilayer composed of C(X):C(Y)PC using differential scanning calorimetry and X-ray diffraction techniques [Huang and McIntosh, Biophys. J. 72 (1997) 2702--2709]. In the present work, we have extended these studies to characterize the effect of [EtOH] on the T(m) and the phase structure of the lipid bilayer composed of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines with various positions of the cis double bond. Specifically, five positional isomers of 1-eicosanoyl-2-eicosenoyl-sn-glycero-3-phosphocholines, C(20):C(20:1 Delta(n))PC with n=5, 8, 11, 13 and 17, were synthesized and studied. For C(20):C(20:1 Delta(n))PC with n=5 and 8, results from the calorimetric experiments showed that in response to various concentrations of ethanol, the change in T(m) of the lipid bilayer composed of monounsaturated lipids was characterized by a sigmoidal or biphasic profile in the plot of T(m) versus [EtOH]. In contrast, a continuous depression of the T(m) by ethanol was observed calorimetrically for C(20):C(20:1 Delta(n))PC with n> or =11. The X-ray diffraction experiments further demonstrated that C(20):C(20:1 Delta(5))PC and C(20):C(20:1 Delta(8))PC can undergo the ethanol-induced gel-to-fully interdigitated phase transition at T<T(m). Such a transition, however, was not observed for C(20):C(20:1 Delta(13))PC even at a very high ethanol concentration of 100 mg/ml. These distinct different effects of [EtOH] on the phase transition temperature and the phase structure can be attributed to various positions of the cis double bond in these monounsaturated phosphatidylcholines. And the different effects of ethanol can, in fact, be explained based on the molecular structures of these monounsaturated lipids packed in the gel-state bilayer as generated by molecular mechanics simulations. To the best of our knowledge, this is the first time that the ethanol-induced fully interdigitated bilayers are observed at T<T(m) for unsaturated phospholipids with well defined double bond positions in their sn-2 acyl chains.

Calorimetry, Differential Scanning↗
PubMed2001

Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions. II. Report of an IUCr Working Group on Phase Transition Nomenclature.

A general nomenclature applicable to the phases that form in any sequence of transitions in the solid state has been recommended by an IUCr Working Group [Acta Cryst. (1998). A54, 1028-1033]. The six-field notation of the first Report, hereafter I, was applied to the case of structural phase transitions, i.e. to transformations resulting from temperature and/or pressure changes between two crystalline (strictly periodic) phases involving modifications to the atomic arrangement. Extensive examples that illustrate the recommendations were provided. This second Report considers, within the framework of a similar six-field notation, the more complex nomenclature of transitions involving magnetic phases, incommensurate phases and transitions that occur as a function of composition change. Extension of the nomenclature to the case of phases with less clearly established relevance to standard schemes of transition in equilibrium systems, namely polytype phases, radiation-induced and other transient phases, quasicrystalline phases and their transitions is recommended more tentatively. A uniform notation for the translational periodicity, propagation vector or wavevector for magnetic and/or incommensurate substances is specified. The notation adopted for incommensurate phases, relying partly on the existence of an average structure, is also consistent with that for commensurate phases in a sequence. The sixth field of the nomenclature is used to emphasize the special features of polytypes and transient phases. As in I, illustrative examples are provided for each category of phase sequence.

Journal Article↗
PubMed1983

The influence of water on the phase transition of sheep lung surfactant. A possible mechanism for surfactant phase transitions in vivo.

The effect of water on the thermal properties of sheep lung surfactant lipids was determined by differential scanning calorimetry. Dry surfactant exhibited a phase transition with an upper limit of about 54 degrees C, whereas that of the fully hydrated surfactant was about 30 degrees C. The effect of water was confined to a range of hydration values from 0 to 25%. The results indicate that pulmonary surfactant lipids are capable of undergoing both thermotropic and lyotropic mesomorphism in vitro. The degree of hydration of the surfactant could influence its in vivo biophysical role in alveolar dynamics. Indeed, small changes in the surfactant to water ratio induced by regional differences in the surfactant concentration at the alveolar surface during alveolar expansion and contraction could be sufficient to trigger isothermal phase transitions in the surfactant lipids. This would allow changes to occur in the equilibrium between solidus and fluidus surfactant during the respiratory cycle.

Animals↗
PubMed2000

Crossover between a displacive and an order-disorder phase transition

The phase transition in a three-dimensional array of classical anharmonic oscillators with harmonic nearest-neighbor coupling (discrete straight phi(4) model) is studied by Monte Carlo (MC) simulations and by analytical methods. The model allows us to choose a single dimensionless parameter a determining completely the behavior of the system. Changing a from 0 to +infinity allows to go continuously from the displacive to the order-disorder limit. We calculate the transition temperature T(c) and the temperature dependence of the order parameter down to T=0 for a wide range of the parameter a. The T(c) from MC calculations shows an excellent agreement with the known asymptotic values for small and large a. The obtained MC results are further compared with predictions of the mean-field and independent-mode approximations as well as with predictions of our own approximation scheme. In this approximation, we introduce an auxiliary system, which yields approximately the same temperature behavior of the order parameter, but allows the decoupling of the phonon modes. Our approximation gives the value of T(c) within an error of 5% and satisfactorily describes the temperature dependence of the order parameter for all values of a.

Journal Article↗
PubMed1993

Phase transitions of reversed-phase stationary phases. Cause and effects in the mechanism of retention.

We provide a somewhat historical perspective on the work that has been accomplished that supports the suggestion of phase transitions in reversed-phase liquid chromatographic bonded phases. In addition to this general review, we consider from both a practical and theoretical viewpoint the implications of such transitions in the rational design of improved reversed-phase separations. Analytical theory and molecular simulation approaches to chromatographic stationary phases and related systems are reviewed. Initial results are presented on the temperature, density and chain length dependences of stationary phase properties. The role of computer simulation in analyzing transition behavior and retention processes is discussed.

Chemical Phenomena↗
PubMed2000

Yang-Lee theory for a nonequilibrium phase transition

To analyze phase transitions in a nonequilibrium system, we study its grand canonical partition function as a function of complex fugacity. Real and positive roots of the partition function mark phase transitions. This behavior, first found by Yang and Lee under general conditions for equilibrium systems, can also be applied to nonequilibrium phase transitions. We consider a one-dimensional diffusion model with periodic boundary conditions. Depending on the diffusion rates, we find real and positive roots and can distinguish two regions of analyticity, which can be identified with two different phases. In a region of the parameter space, both of these phases coexist. The condensation point can be computed with high accuracy.

Journal Article↗
PubMed2001

Influence of confinement on the orientational phase transitions in the lamellar phase of a block-copolymer melt under shear flow.

In this paper, we incorporate some real-system effects into the theory of orientational phase transitions under shear flow [M. E. Cates and S. T. Milner, Phys. Rev. Lett. 62 1856 (1989) and G. H. Fredrickson, J. Rheol. 38, 1045 (1994)]. In particular, we study the influence of the shear-cell boundaries on the orientation of the lamellar phase. We predict that at low shear rates, the parallel orientation appears to be stable. We show that there is a critical value of the shear rate at which the parallel orientation loses its stability and the perpendicular one appears immediately below the spinodal. We associate this transition with a crossover from the fluctuation to the mean-field behavior. At lower temperatures, the stability of the parallel orientation is restored. We find that the region of stability of the perpendicular orientation rapidly decreases as shear rate increases. This behavior might be misinterpreted as an additional perpendicular to parallel transition recently discussed in literature.

Journal Article↗
PubMedDate not supplied

[Non-isothermal pattern of phase transition in vesicle membranes].

Phase transition of lipid molecules between liquid and crystalline states in the vesicle membrane has a non-isothermal pattern; it is spread in some temperature range, which depends on the vesicle size. With a change of phase transition the area which falls within a lipid molecule is changed. As a result elastic tensions appear in the membrane. They may account for the non-isothermal pattern of phase transition. The number of molecules in each state is calculated in relation to temperature, and the temperature range is calculated within which the phase transition is realized. The smaller is the vesicle radius, the higher is the temperature range value. Temperature relationship of the membrane heat capacity at non-isothermal phase transition is calculated. The obtained results well agree with the number of experimental data.

Crystallization↗
PubMed2002

Theory of bent-core liquid-crystal phases and phase transitions.

We study phases and phase transitions that can take place in the recently discovered bow-shaped or bent-core liquid-crystal molecules. We show that to completely characterize phases exhibited by such bent-core molecules a third-rank tensor T(ijk) order parameter is necessary in addition to the vector and the nematic (second-rank) tensor order parameters. We present an exhaustive list of possible liquid phases, characterizing them by their space-symmetry group and order parameters, and catalog the universality classes of the corresponding phase transitions that we expect to take place in such bent-core molecular liquid crystals. In addition to the conventional liquid-crystal phases such as the nematic phase, we predict the existence of other liquid phases, including the spontaneously chiral nematic (N(T)+2)(*) and chiral polar (V(T)+2)(*) phases, the orientationally ordered but optically isotropic tetrahedratic T phase, and a nematic N(T) phase with D(2d) symmetry that is neither uniaxial nor biaxial. Interestingly, the isotropic-tetrahedratic transition is continuous in mean-field theory, but is likely driven first order by thermal fluctuations. We conclude with a discussion of smectic analogs of these phases and their experimental signatures.

Journal Article↗
PubMed1981

On kinetics of phase transitions in cell membranes.

Phase transitions in a bicomponent lipid membrane are considered. It is shown that in this case metastable states practically do not arise and phase transitions are smooth and hysteresisless. An elastic frame on the surface of the membrane changes the character of phase transitions: they become sharp and hysteretic. The role of membrane phase transitions for regulation of cell processes is considered.

Cell Membrane↗
PubMed1986

Dynamic approach to thermotropic and inotropic phase transitions.

Thermotropic and inotropic phase transitions have been analysed with a dynamic theory on a self-organization. An equation of motion of a molecular assembly with strong interactions may be approximately described as: dQ/dt' congruent to -K1Q-K3Q3, where Q is a displacement from the equilibrium point Q0(identical to 0) in a vibrational state, K1 is a transition parameter. When the parameter K1 concerned with an internal driving force (partial system) changes from positive to negative through the potential bifurcation, the system transfers to a new stable state breaking down the symmetry. Such a sign change of K1 serves as a trigger to a phase transition. Using Weiss' approximation, we have evaluated the change of K1 by a function of temperature, kappa (T-TC), and have obtained the critical temperature TC of thermotropic phase transition. We have furthermore treated inotropic phase transition caused by the binding of divalent cations like Ca2+ using the function kappa (T-beta TC), where beta is a shift parameter of the critical temperature.

Calcium↗
PubMed1991

The study of lipid-protein interactions: effect of melittin on phase transition of phosphatidylethanolamine and sensitivity of phospholipases to phase state.

The effects of melittin on the bilayer-to-inverted hexagonal (HII) phase transition of egg phosphatidylethanolamine (EPE) and the influence of the phase state of membrane matrix on hydrolysis of EPE by phospholipases have been studied. The phase transitions were measured using the fluorescent probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (N-NBD-PE) and differential scanning calorimetry. In the presence of melittin at a lipid-to-melittin molar ratio (R1) of 200, 100, and 20, the phase transition of EPE disappeared, indicating that melittin stabilizes the bilayer structure. In the presence of 10 mol% of cholesterol, the phase transition temperature (TH) decreased and TH was observed even in the presence of melittin at R1 of 200 and 100. The fluorescence intensity of the tryptophan residue of melittin is sensitive to the phase transition and the wavelength of emission maxima shift from 352 to 337 nm upon addition of EPE and EPE-cholesterol (10 mol%) at R1 of 200. Kinetic parameters for phospholipase-catalyzed hydrolysis of EPE in bilayer and HII phases showed that HII phase of EPE is a poorer substrate for phospholipases and that cholesterol decreases the susceptibility of EPE to phospholipases.

Enzyme Stability↗