Low-Temperature Dynamical Transition in Lipid Bilayers Detected by Spin-Label ESE Spectroscopy

Data on neutron scattering in biological systems show low-temperature dynamical transition between 170 and 230 K manifesting itself as a drastic increase of the atomic mean-squared displacement, 〈x²〉, detected for hydrogen atoms in the nano- to picosecond time scale. For spin-labeled systems, electron spin echo (ESE) spectroscopy—a pulsed version of electron paramagnetic resonance—is also capable of detection of dynamical transition. A two-pulse ESE decay in frozen matrixes is induced by spin relaxation arising from stochastic molecular librations, and allows to obtain the 〈α²〉τ_c parameter, where 〈α²〉 is a mean-squared angular amplitude of the motion and τ_c is the correlation time lying in the sub- and nanosecond time ranges. In this work, the ESE technique was applied to spin-labeled amphiphilic molecules of three different kinds embedded in bilayers of fully saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mono-unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. Two-pulse ESE data revealed the appearance of stochastic librations above 130 K, with the parameter 〈α²〉τ_c obeying the Arrhenius type of temperature dependence and increasing remarkably above 170–180 K. A comparison with a dry sample suggests that onset of motions is not related with lipid internal motions. Three-pulse ESE experiments (resulting in stimulated echos) in DPPC bilayers showed the appearance of slow molecular rotations above 170–180 K. For D₂O-hydrated bilayers, ESE envelope modulation experiments indicate that isotropic water molecular motions in the nearest hydration shell of the bilayer appear with a rate of ~?10⁵ s^?1 in the narrow temperature range between 175 and 179 K. The similarity of the experimental data found for three different spin-labeled compounds suggests a cooperative character for the ESE-detected molecular motions. The data were interpreted within a model suggesting that dynamical transition is related with overcoming barriers, of 10–20 kJ/mol height, existing in the system for the molecular reorientations.     

Magnetic Skyrmions and Phase Transitions in Antiferromagnetic/Ferroelectric Bilayers

JETP Letters - We study in this paper the ground state and the properties of a skyrmions in magnetoelectric films, namely antiferromagnetic/ferroelectric superlattices in a support by steepest...     

Direct Observation of Lipid Domains in Free-Standing Bilayers Using Two-Photon Excitation Fluorescence Microscopy

The direct observation of temperature-dependent lipid phase equilibria, using two-photon excitation fluorescence microscopy on giant unilamellar vesicles (GUVs) composed of different lipid mixtures, provides novel information about the physical characteristics of lipid domain coexistence. Physical characteristics such as the shape, size, and time evolution of different lipid domains are not directly accessible from the traditional experimental approaches that employ either small and large unilamellar vesicles or multilamellar vesicles. In this review article, we address the most relevant findings reported from our laboratory regarding the direct observation of lipid domain coexistence at the level of single vesicles in artificial and natural lipid mixtures. In addition, key points concerning our experimental approach will be discussed. The unique advantages of the fluorescent probe 6-dodecanoyl-2-dimethylaminonaphthalene (LAURDAN) under two-photon excitation fluorescence microscopy is particularly addressed, especially, the possibility of obtaining information on the phase state of different lipid domains directly from the fluorescent images.     

Influence of Cholesterol on Molecular Motions in Spin-Labeled Lipid Bilayers Observed by Stimulated ESE

Electron spin echo (ESE) study was performed for spin-labeled lipids 1-palmitoyl-2-stearoyl-(5-d)-sn-glycero-3-phosphocholine in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phospholipid bilayer. Recently (Isaev and Dzuba in J Phys Chem B 112:13285–13291, 2008), three-pulse stimulated ESE was shown to be sensitive to two types of orientational motion of spin labels in phospholipid bilayers at low temperatures (~100–150 K). The first one is fast stochastic libration, with correlation time on the nanosecond time scale. The second one is slow rotational motion, developing on the accessible for measurements microsecond time scale in a small range of reorientation angles, ~0.1°–1°. These two types of motions may be easily discriminated by dependences of the echo decay rates on the time delays between the pulses. The presence of cholesterol in lipid bilayers is found to suppress remarkably rotational motions, while on the contrary stochastic librations seem to become somewhat enhanced. These results evidence that cholesterol increases the long-time stability of lipid orientations in the bilayer, with simultaneous increase of fast fluctuations of these orientations. The former may be related to the known condensing effect of cholesterol and to raft formations, while the latter to the ordering effect.     

Statistical-Mechanical Foundations for a Generalized Thermodynamics of Dissipative Processes

A statistical-mechanical formalism for nonequilibrium systems, namely the nonequilibrium statistical operator method, provides microscopic foundations for a generalized thermodynamics of dissipative processes. This formalism is based on a unifying variational approach that is considered to be encompassed in Jaynes' Predictive Statistical Mechanics and principle of maximization of the statistical-informational entropy. Within the framework of the statistical thermodynamics that follows from the method, we demonstrate the existence of generalized forms of the theorem of minimum (informational) entropy production, the criterion for evolution, and the thermodynamic (in)stability criterion. The formalism is not restricted to local equilibrium but, in principle, to general conditions (its complete domain of validity is not yet fully determined). A H-theorem associated to the formalism is presented in the form of an increase of the informational entropy along the evolution of the system. Some of the results are illustrated in an application to the study of a model for a photoexcited direct-gap semiconductor.     

X- and Q-Band Electron Spin Echo Study of Stochastic Molecular Librations of Spin Labels in Lipid Bilayers

Electron spin echo (ESE) of nitroxide spin labels allows detecting fast nanosecond stochastic restricted rotations (stochastic molecular librations), which is a common property of molecules in disordered media including biological systems. Under the typical experimental conditions, the anisotropic electron paramagnetic resonance (EPR) spectrum of a nitroxide is only partly excited by microwave pulses, which allows selecting an anisotropic contribution to the transverse spin relaxation by comparing echo decays at different spectral positions. On the other hand, for low-amplitude orientational motion, the excitation bandwidth is large enough to cover the range of spectral diffusion occurring during the echo formation. To verify that the two-pulse echo decay is indeed related to fast motions, the stimulated electron spin echo can be used. In addition, theory predicts an increase of the relaxation rates at higher microwave resonance frequency. To check this prediction, in the present work we performed a comparative study of ESE decays at microwave X- and Q-bands, for spin-labeled lipids in the gel phase of a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. A good agreement found between experimental data and computer simulation provides additional justification for the model of fast stochastic molecular librations.     

多组分磷脂巨囊泡的相结构和胆固醇对微畴成长的影响

巨囊泡作为细胞的简化模型,其相分离与出芽动力学规律已引起许多领域科学家的关注.本实验采用DPPC/DOPC/Chol的三组分形成的巨囊泡作为模型,借助荧光显微镜观察该三组分体系侧向分离的相结构图,并对微畴的成长过程作了系统的观察研究和理论分析.实验发现:从高温的均相区域淬灭到低温的分相区域,膜表面发生侧向分离形成微畴.体系内胆固醇的掺入量的多少会影响磷脂膜的相结构和膜内微畴的成长,固定DOPC/DPPC为1:1的前提下,微畴尺寸随着胆固醇掺入量的增加而变大.     

多组分磷脂巨囊泡的相结构和胆固醇对微畴成长的影响

摘要：巨囊泡作为细胞的简化模型,其分相与出芽机理及动力学规律已引起许多领域科学家的关注。在富含胆固醇的典型生物膜体系如二棕榈酰磷脂酰胆碱DPPC(2-dihexadecanoyl-rac-glycero-3phosphocholine)/二油酰磷脂酰胆碱DOPC(dioleoyl-phosphatidylcholine)/胆固醇(Chol)的三组分形成的巨囊泡作为模型,从高温退火至低温会发生相分离,形成微畴。实验中借助荧光显微镜观察生物膜体系侧向分离的相结构图。实验发现,体系各组分的不同会影响磷脂膜的相结构和膜内微畴的成长,固定 DOPC/DPPC为1:1的前提下,微畴尺寸随着胆固醇参入量的增加而变大。最后运用理论进一步分析了微畴的成长机理。     

Exciton Luminescence of Wse2 Bilayers

Russian Physics Journal - WSe2 films with thickness of two monolayers are deposited on SiO2/Si substrates using the top-down technology. The thickness and the composition of the films are confirmed...     

Ripple damping on water surface covered by a spreading film: Theory and experiment

Summary The results of point-by-point laboratory dampingvs. frequency measurements using mechanically generated ripples on a water surface covered by monomolecular film of oleic alcohol are discussed in terms of a theory that takes into account coupling of Laplace and Marangoni modes.

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Riassunto Una teoria delle onde gravito-capillari alla superficie di acqua ricoperta da un film monomolecolare di spandimento, che tien conto dei modi di Laplace e di Marangoni, e del loro accoppiamento, è usata per guistificare delle misure di smorzamento ondoso effettuate in vasca con film di alcool oleico.

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Резюме В рамках теории, которая учитывает связь мод Лапласа в Марангони, обсуждаются результаты лабораторных измерений затухания в зависимотси от частоты. При измерения использовалась механически возбужденная рябь на поверхности воды, покрытой мономолекулярной пленкой олеинового спирта.

     

Dynamics in Diether Lipid Bilayers and Interdigitated Bilayer Structures Studied by Time-Resolved Emission Spectra, Decay Time and Anisotropy Profiles

We present a comparative fluorescence spectroscopic investigation of diacyl and diether phosphatidylcholine vesicles using different probes with well-defined localization within either the hydrophilic headgroup region or the hydrophobic part of the bilayer. Time-resolved emission spectra have been used to characterize the solvent relaxation behavior in both symmetric and asymmetric diether and diacyl phosphatidylcholines. It is shown that time-resolved emission spectra of Prodan (6-propionyl-2-(dimethylamino)-naphthalene) and its long-alkyl chain derivative Patman (6-palmitoyl-2-[[trimethylammoniumethyl]methylamino]-naphthalene chloride) are a sensitive tool for the detection of differences in the micropolarities and viscosities at the hydrophobic/hydrophilic membrane interface of diether and diacyl lipids, respectively. Moreover, a new approach for the detection of interdigitated bilayers is discussed. It relies on the construction of anisotropy and decay time profiles for the set of n-anthroyloxy fatty acids and is compared with an older fluorescence assay based on intensity measurements only. The shape of plots of the fluorescence steady-state anisotropy versus the position of the chromophore (anthracene-9-carboxylic acid) combined with fluorescence lifetime measurements can be used to differentiate among non-fully, and mixed interdigitated gel phase structures and to predict structures for new lipid species.     

Derivation of the Geometrical Phase from the Evans Phase Law of Generally Covariant Unified Field Theory

The phase law of generally covariant electrodynamics is used to explain straightforwardly the origin of the geometrical and Berry phase effects, exemplified by the Tomita-Chiao effect. Both effects are described by a phase factor that is constructed from the generally covariant Stokes formula of differential geometry, a phase factor in which the contour integral over the potential field A ⁽³⁾ is equated to the area integral over the gauge invariant field B ⁽³⁾, the Evans-Vigier field. The latter is the fundamental spin Casimir invariant of the Einstein group of general relativity applied to electrodynamics. General relativity as extended in the Evans unified field theory is needed for a correct understanding of all phase effects in physics, an understanding that is forged through the Evans phase law, the origin both of the Berry phase and the geometrical phase of electrodynamics observed in the Sagnac and Tomita-Chiao effects.     

Theory for Difference Between Photoinduced Phase and Thermally Excited Phase

A possible difference between the photoinduced phase and the thermally excited one is studied by using a two-dimensional extended Peierls-Hubbard model, which includes a strong electron-phonon coupling and a on-site interelectron repulsion, as well as an anharmonic lattice potential. Because of this anharmonicity, the system undergoes a first order phase transition from an insulating CDW state to a metallic one at a high temperature. Although some sign of an SDW order is expected to appear due to this repulsion, it is always hidden in any equilibrium phase of the present system. In fact, it is hidden, not only in the CDW ground state, but also in this metallic one, since the high temperature itself destroys the SDW order, far before the CDW-metal transition occurs, while a photo-excitation at low enough temperature is shown to generate a local metastable SDW domain. Therefore, to observe the presence of such Coulomb interaction and the resultant broken symmetry, a nonequilibrium photoinduced phase is shown to be most straightforward. Thus, the photoinduce phase transition can make an interaction appear as a broken symmetry only in this phase, even though this interaction is almost completely hidden in all the equilibrium phases from low temperature to high ones.     

Fluorescence Spectroscopic Studies on Phase Heterogeneity in Lipid Bilayer Membranes

There is a growing interest in functional membrane heterogeneity on the mesoscopic (several tens to hundreds of molecular dimensions) scale. However, the physical-chemical basis for this sort of heterogeneity in membranes is not entirely clear. Unambiguous methods to demonstrate that the cell plasma membrane and other cellular membranes are in fact heterogeneous on the mesoscopic level are also not generally available. Fluorescence techniques do, however, provide excellent tools for this purpose. In particular, the emerging techniques of scanning near-field optical microscopy and single-molecule fluorescence microscopy hold a great deal of promise for the near-future. All these methods require the use of fluorescent probes (lipids and/or proteins) and a clear definition of how these probes partition between domains of coexisting membrane phases. The development of the concept of membrane heterogeneity over the years since the first proposal of the fluid mosaic model is reviewed briefly. The use of lipid-binding proteins in experimental protocols for the labeling of membranes with fluorescent lipid amphiphiles as monomers in aqueous solutions at concentrations well above their critical aggregation concentrations is discussed. The methods of fluorescence spectroscopy available to the cell biologist for determining probe partition coefficients for partitioning between coexisting membrane phases are reviewed in some detail, as is the relevant theoretical and experimental work reported in the literature.     

Statistical-Mechanical Entropies of Schwarzschild Black Hole due to Arbitrary Spin Fields in Different Coordinates

The statistical-mechanical entropies of the Schwarzschild black hole arising from the scalar, Weyl neutrino, electromagnetic, Rarita-Schwinger and gravitational fields are investigated in the Painlevg and Lemaitre coordinates. Although the metrics in the Painlevg and the Lemaitre coordinates do not obviously possess the singularity as that in the Schwarzschild coordinate, we find that the entropies of the arbitrary spin fields in both the Painlevd and Lemaitre coordinates are exactly equivalent to that in the Schwarzschild coordinate.     

Europium Coordination Complexes as Potential Anticancer Drugs: Their Partitioning and Permeation Into Lipid Bilayers as Revealed by Pyrene Fluorescence Quenching

The present study was undertaken to evaluate the membrane-associating properties of a series of novel antitumor agents, Eu(III) coordination complexes (EC), using the pyrene fluorescence quenching as an analytical instrument. Analysis of EC-induced decrease in pyrene fluorescence intensity in terms of partition and solubility-diffusion models allowed us to evaluate the partition and permeation coefficients of the examined compounds into the lipid vesicles prepared from zwitterionic lipid phosphatidylcholine (PC) and its mixtures with cholesterol (Chol) and anionic lipid cardiolipin (CL). The drug-lipid interactions were found to have the complex nature determined by both EC structure and lipid bilayer composition. High values of the obtained partition and permeation coefficients create the background for the development of EC liposomal formulations.     

Theory of Phase Separation in Finite-Lifetime Systems

We study the phase separation dynamics in finite lifetime systems. Particles created by the external fields are unstable and decay within the finite lifetime. Effects of the particle lifetime and the interparticle interaction for the phase separation dynamics are discussed. In our theory, the formulation is based on the coarse-grained lattice gas model. As a result, effects of the finite lifetime are that the phase separation is restrained and that the finite size of domains remain finally at t = X .     

Liquid Model of Phase Transition from Quark-Gluon Phase to Hadron Phase on Analogy of BCS Theory

A phenomenological model of the transition from quark-gluon phase to hadron phase is presented on the analogy of BCS theory. The massive current-quarks constitute the quark Cooper-pair, i.e., the hadron at T_c = 150~200 MeV, The order parameter of qq-pair takes a value in the range from 0.4 to 0.2 GeV. An experimental verification method of the present model in the heavy-ion collision is proposed.