Rotational Order Parameter Research Articles

Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars.

Simple carbohydrates are associated with the enhanced risk of cardiovascular disease and adverse changes in lipoproteins in the organism. Conversely, sugars are known to exert a stabilizing effect on biological membranes, and this effect is widely exploited in medicine and industry for cryopreservation of tissues and materials. In view of elucidating molecular mechanisms involved in the interaction of mono- and disaccharides with biomimetic lipid systems, we study the alteration of dielectric properties, the degree of hydration, and the rotational order parameter and dipole potential of lipid bilayers in the presence of sugars. Frequency-dependent deformation of cell-size unilamellar lipid vesicles in alternating electric fields and fast Fourier transform electrochemical impedance spectroscopy are applied to measure the specific capacitance of phosphatidylcholine lipid bilayers in sucrose, glucose and fructose aqueous solutions. Alteration of membrane specific capacitance is reported in sucrose solutions, while preservation of membrane dielectric properties is established in the presence of glucose and fructose. We address the effect of sugars on the hydration and the rotational order parameter for 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC) and 1-stearoyl-2-oleoyl-sn-glycero-3- phosphocholine (SOPC). An increased degree of lipid packing is reported in sucrose solutions. The obtained results provide evidence that some small carbohydrates are able to change membrane dielectric properties, structure, and order related to membrane homeostasis. The reported data are also relevant to future developments based on the response of lipid bilayers to external physical stimuli such as electric fields and temperature changes.

Behavior-dependent critical dynamics in collective states of active particles

We experimentally demonstrate critical collective behavior in a system of active colloidal particles (APs), achieved through variations of their mutual interactions using feedback control. At the transition between a swarm and a swirl we observe an explicit bifurcation dynamics of the rotational order parameter and a critical slowing down, i.e., a growth of the relaxation time by almost one order of magnitude. Additional signatures of critical dynamics, including hysteresis in presence of symmetry-breaking particle interactions, and a maximum of the susceptibility, are measured and characterized in terms of a theoretical model which is based purely on the time-reversal symmetry of the order parameter.

NMR investigations on binding and dynamics of imidazolium-based ionic liquids with HEWL.

Molecular level insights on protein-ionic liquid (P-IL) interactions are beneficial for assessing protein stability, binding and dynamics. In the present work, interactions of ILs, namely, 1-butyl 3-methylimidazolium methyl sulfate (IL1), 1-butyl 3-methylimidazolium octyl sulfate (IL2) and 1-butyl 3-methylimidazolium chloride (IL3) with hen egg white lysozyme (HEWL) protein were investigated using solution-state nuclear magnetic resonance (NMR) spectroscopy. To ascertain the binding and dynamics from the perspective of both protein and IL, various ligand based NMR approaches such as selective and non-selective nuclear spin-relaxation (R1SEL and R1NS), saturation transfer difference (STD), difference of inversion recovery rate with and without target irradiation (DIRECTION), 35Cl line-shape and spin-relaxation, and protein back bone amide chemical shift perturbations (CSPs) from 1H-15N HSQC were utilized. Among the ILs investigated, IL2 experiences significant interaction relative to those of IL1 and IL3, as revealed by the combined R1SEL and R1NS analysis, which is further supported by STD NMR. CSP analyses of 1H-15N HSQC spectra of aqueous P-IL mixtures enabled to identify the potential binding sites of ILs with HEWL. Whereas, 15N longitudinal (R1) and transverse (R2) spin-relaxation rates and 15N{1H} heteronuclear nuclear Overhauser effect (hetNOE) data subjected to the model free analysis for IL2 yielded the rotational correlation times and order parameters of various residues of HEWL. Furthermore, the results could discern the nature of interactions between studied ILs and HEWL in terms of specific and non-specific interactions.

Temperature Behavior of the Rotational Order Parameters in a La0.25Ca0.75MnO3 Solid Solution

The behavior of the crystal structure of a La0.25Ca0.75MnO3 solid solution in the temperature range of 80–725 K is studied. It is established that rotational structural distortions predominate during the transition to a charge-ordered state

Structural transformations and phenomenological description of the formation of phase states in elpasolites Cs2RbDyF6 and Rb2KB′F6 (B′ = Ho, Dy, Tb)

The structures of the cubic and monoclinic phases of an Rb2KHoF6 crystal from a series of elpasolites Rb2KB′F6 (B′ = Ho, Dy, Tb), which undergoes a trigger phase transition, have been investigated using X-ray powder diffraction. The critical and noncritical displacements of atoms in the Rb2KHoF6 structure have been determined from the group-theoretical analysis of the complete condensate of order parameters. It has been reliably established that a change in the symmetry due to the phase transition in this crystal can be represented in the form \(Fm\bar 3m\frac{{11 - 9(\Gamma _4^ + ) \oplus 10 - 3(X_3^ + )}} {{(\phi ,\phi ,\psi )}}P12_1 /n1 \). A phenomenological analysis of the conditions responsible for the formation of phase states upon the phase transitions in elpasolites Cs2RbDyF6 and Rb2KB′F6 (B′ = Ho, Dy, Tb) has been carried out. Taking into account the structural data and using the phenomenological theory, it has been found that the main factor determining the formation of phase states in these and related crystals is the instability of the elpasolite structure with respect to rotational distortions of two types. It has been shown that an important role in the formation of a sequence of structural transformations in these crystals is played by the interaction of rotational order parameters and displacements of the cation from the center of a cuboctahedral hole.

Over Half a Century of Research in Oxides

An attempt will be made to recall some of the highlights of the research in oxides achieved by the author and various collaborators over a period of 58 years. These were the observation of the paramagnetic resonance of transition metal ions of high valency like Fe3+, Fe4+, Fe5+ and Co4+ and their photochromic behaviour; then the detection of Mn4+ in BaTiO3 and the proof for the presence of order–disorder character in these ferroelectric transitions, and then the temperature dependence of the rotational order parameter of the structural phase transitions in LaAlO3 and SrTiO3. Near the transition, the observation of critical behaviour compatible with renormalisation group theory and the presence of a Potts transition line, a critical end point and a Lifshitz point under uniaxial stress in SrTiO3 and CaGdF3. This will be followed by the determination of the stabilisation energy of the Jahn–Teller effect with octahedral oxygen neighbours to various ions; leading over to the concept of Jahn–Teller polarons as quasi particles for the generation of superconductivity, and the experimental observation of Jahn–Teller bipolarons and their stabilisation energy determined in one of the HTS copper oxides.

Comparing lipid membranes in different environments.

When engineering lipid membranes for applications, it is essential to characterize them to avoid artifacts introduced by manipulation and the experimental environment. Wide-angle X-ray scattering is a powerful structural characterization tool for well-ordered lipid systems. It reveals remarkable differences in rotational order parameters for samples prepared in different ways. New data and perspectives are presented here for multilamellar systems that support and extend the characterization work on unilamellar systems that is reported by Watkins et al. in this issue of ACS Nano.

Coxeter groups as automorphism groups of solid transitive 3-simplex tilings

In the papers of I.K. Zhuk, then more completely of E. Moln?r, I. Prok, J. Szirmai all simplicial 3-tilings have been classified, where a symmetry group acts transitively on the simplex tiles. The involved spaces depends on some rotational order parameters. When a vertex of a such simplex lies out of the absolute, e.g. in hyperbolic space H3, then truncation with its polar plane gives a truncated simplex or simply, trunc-simplex. Looking for symmetries of these tilings by simplex or trunc-simplex domains, with their side face pairings, it is possible to find all their group extensions, especially Coxeter?s reflection groups, if they exist. So here, connections between isometry groups and their supergroups is given by expressing the generators and the corresponding parameters. There are investigated simplices in families F3, F4, F6 and appropriate series of trunc-simplices. In all cases the Coxeter groups are the maximal ones.

Biocompatible nanodispersions as delivery systems of food additives: A structural study

Nanodispersions based on food grade biocompatible materials were developed and structurally characterized to be used as carriers of bioactive compounds with specific nutritional value. The main idea was to formulate concentrated solutions of specific food components at the nanoscale to be consumed either on their own or as integrating parts of classic foods, upon aqueous dilution. For this purpose microemulsions consisting of (R)-(+)-limonene/ethanol/Tween 40/water/propylene glycol were formulated in the presence and in the absence of squalene, gallic acid and octyl gallate. The limits of the single-phase region as described by pseudo-ternary phase diagrams were related to the nature of the food additive. The more extended monophasic region was obtained when octyl gallate was added in the system. Interfacial properties of the microemulsions were studied by electron paramagnetic resonance (EPR) spectroscopy employing the nitroxide spin probe 5-doxylstearic acid (5-DSA). In general guest molecules decreased the flexibility of the surfactant monolayer as manifested from the calculation of rotational correlation time (τR) and order parameter S of 5-DSA. Particle size measurements were performed using dynamic light scattering (DLS) and oil droplet diameters in the range of 11.7 to 17.4nm were observed. The addition of squalene resulted in the formulation of larger oily droplets whereas octyl gallate formed smaller ones. Finally SAXS experiments provided qualitative information of o/w microemulsions showing squalene solubilization in the dispersed oily phase, octyl gallate localization on the membrane and gallic acid solubilization in the continuous aqueous phase.

Development and characterization of a digestion model based on olive oil microemulsions

AbstractOlive oil microemulsions were used as biomimetic model to simulate aspects of the digestion process in the small intestine. Hydrolytic activity of trypsin and alkaline phosphatase in microemulsions in the presence and in the absence of squalene and phenolic acids at pH 8.5 and 37°C was examined. In the case of trypsin kinetic constants were calculated and found Km = 0.080 ± 0.002 mM and Vmax = 3.8 × 10−5 ± 0.06 × 10−5 mM/s. When gallic acid and squalene were added, hydrolytic activity of trypsin was reduced to 77 ± 2% and 69 ± 3%, respectively. In the presence of o‐, m‐, and p‐coumaric acid, protocatechouic acid, and caffeic acid, enzymatic activity was increased up to 142 ± 6%. Hydrolytic activity of alkaline phosphatase was inhibited in the presence of squalene and phenolic acids. To evaluate the size of the dispersed aqueous droplets in the presence and in the absence of enzymes, substrates, and antioxidants, dynamic light scattering (DLS) measurements were carried out and diameters in the range of 8.9–13.4 nm were observed. Membrane flexibility and micropolarity were studied by electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation time (τR), order parameter (S), and hyperfine splitting constant (A0) of doxyl stearic acid in the microemulsions were calculated and found 2.06 ns, 0.17, and 14.30 G, respectively. These values were slightly affected in the presence of additives.Practical applications: The main aim of the present study is to enhance our understanding on the digestibility of proteins and lipids in the presence of food antioxidants suggesting an in vitro digestion model for hydrolases based on virgin olive oil microemulsions. The information generated in this study could potentially facilitate the design and development of novel functional food formulations based on food‐grade colloidal delivery systems. Since modification or fortification of different food products focusing on different areas of health concern are of growing interest among the various sectors of food industry, the proposed research results could be of particular interest.

Computational Insights into Dynamics of Protein Aggregation and Enzyme-Substrate Interactions.

In this Perspective, the roles of protein dynamics have been discussed in the aggregation of amyloid beta (Aβ) peptides and formation of enzyme-substrate complexes of beta-secretase (BACE1) and insulin-degrading enzyme (IDE). The studies regarding the influence of individual amino acid residues and specific regions on the structures and oligomerization of early Aβ aggregates and computations of their translational and rotational diffusion coefficients and order parameters exhibited that even the short-time-scale molecular dynamics simulations can reproduce certain experimental parameters with reasonable accuracy. The simulations elucidating the enzyme-substrate interactions of BACE1 and IDE successfully showed that the chemical nature and length of the substrates influence the dynamics and plasticity of both the enzyme and substrate. An atomic-level understanding of these processes will advance our efforts to develop therapeutic strategies for several deadly diseases through the design of small molecules with antiaggregation properties and substrate-specific "designer" forms of enzymes.

Using spin-label W-band EPR to study membrane fluidity profiles in samples of small volume

Conventional and saturation-recovery (SR) EPR at W-band (94GHz) using phosphatidylcholine spin labels (labeled at the alkyl chain [n-PC] and headgroup [T-PC]) to obtain profiles of membrane fluidity has been demonstrated. Dimyristoylphosphatidylcholine (DMPC) membranes with and without 50mol% cholesterol have been studied, and the results have been compared with similar studies at X-band (9.4GHz) (L. Mainali, J.B. Feix, J.S. Hyde, W.K. Subczynski, J. Magn. Reson. 212 (2011) 418–425). Profiles of the spin-lattice relaxation rate (T1-1) obtained from SR EPR measurements for n-PCs and T-PC were used as a convenient quantitative measure of membrane fluidity. Additionally, spectral analysis using Freed’s MOMD (microscopic-order macroscopic-disorder) model (E. Meirovitch, J.H. Freed J. Phys. Chem. 88 (1984) 4995–5004) provided rotational diffusion coefficients (R⊥ and R||) and order parameters (S0). Spectral analysis at X-band provided one rotational diffusion coefficient, R⊥. T1-1, R⊥, and R|| profiles reflect local membrane dynamics of the lipid alkyl chain, while the order parameter shows only the amplitude of the wobbling motion of the lipid alkyl chain. Using these dynamic parameters, namely T1-1, R⊥, and R||, one can discriminate the different effects of cholesterol at different depths, showing that cholesterol has a rigidifying effect on alkyl chains to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. The nondynamic parameter, S0, shows that cholesterol has an ordering effect on alkyl chains at all depths. Conventional and SR EPR measurements with T-PC indicate that cholesterol has a fluidizing effect on phospholipid headgroups. EPR at W-band provides more detailed information about the depth-dependent dynamic organization of the membrane compared with information obtained at X-band. EPR at W-band has the potential to be a powerful tool for studying membrane fluidity in samples of small volume, ∼30nL, compared with a representative sample volume of ∼3μL at X-band.

Dynamics and ordering of lipid spin-labels along the coexistence curve of two membrane phases: An ESR study

An analysis of electron spin resonance (ESR) spectra from compositions along the liquid-ordered (Lo) and liquid-disordered (Ld) coexistence curve from the brain-sphingomyelin/dioleoylphosphatidylcholine/cholesterol (SPM/DOPC/Chol) model lipid system was performed to characterize the dynamic structure on a molecular level of these coexisting phases. We obtained 200 continuous-wave ESR spectra from glycerophospholipid spin-labels labeled at the 5, 7, 10, 12, 14, and 16 carbon positions of the 2nd acyl chain, a sphingomyelin spin-label labeled at the 14 carbon position of the amide-linked acyl chain, a headgroup-labeled glycerophospholipid, a headgroup-labeled sphingomyelin, and the cholesterol analogue spin-label cholestane all within multi-lamellar vesicle suspensions at room temperature. The spectra were analyzed using the MOMD (microscopic-order macroscopic-disorder) model to provide the rotational diffusion rates and order parameters which characterize the local molecular dynamics in these phases. The analysis also incorporated the known critical point and invariant points of the neighboring three-phase triangle along the coexistence curve. The variation in the molecular dynamic structures of coexisting Lo and Ld compositions as one moves toward the critical point is discussed. Based on these results, a molecular model of the Lo phase is proposed incorporating the “condensing effect” of cholesterol on the phospholipid acyl chain dynamics and ordering and the “umbrella model” of the phospholipid headgroup dynamics and ordering.

Spin-Hamiltonian parameters and local rotational order parameters for Gd 3+ ion in the tetragonal phase of SrTiO 3 crystal

Using a diagonalization method, the spin-Hamiltonian parameters ( g factors g ||, g ⊥ and zero-field splittings b 2 0, b 4 0, b 2 4, b 6 0, b 6 4) for Gd 3+ ion at the 12-fold coordinated Sr 2+ site in the tetragonal phase of SrTiO 3 at T≈4.2 and 77 K are calculated in a unified way. The calculated results are in reasonable agreement with the experimental values. The local rotational order parameters ϕ l of Gd 3+ center at the two temperatures are also obtained from the calculations. It is found that the local rotational order parameters ϕ l of Gd 3+ impurity center at both temperatures are different from the corresponding parameters ϕ h in the host SrTiO 3 crystal and the changes of spin-Hamiltonian parameters with temperature are caused by the change of local rotational order parameter ϕ l with temperature. The results are discussed.

Synthesis of spin labelled silica and EPR molecular dynamics study at silica–polymer interfaces

AbstractInformation concerning the interface structure in filler/polymer composites is of key importance for the rationalization of reaction mechanisms in mechano‐chemical (extrusion, blending, etc.), thermal or radiation induced free radical processes and for elucidating the factors underlying the reinforcing mechanism. The analysis of the chain dynamics is a suitable tool for undertaking such investigations because any reactivity parameter (rate constants, collisional frequencies, activation energies) and bonding interactions are strictly related to the mobility of the interacting centres. EPR spectroscopy coupled with specific spin labelling at the filler/polymer interface is a tool for making such novel perspective available. In this work, a spin labelling study of the molecular motion at the filler–rubber interface in a silica–SBR blend is reported. Spin labels of different length, spanning a 9–11 Å depth and linked to the surface of silica particles, were prepared and used for determining the rotational diffusion tensors, the T50 and order parameter in silica/SBR interfaces. The measurements carried out as a function of the temperature in comparison with unbound spin probes dispersed in the rubber matrix have afforded information consistent with the structure of the interfaces predicted by molecular–level theoretical models. Copyright © 2010 John Wiley & Sons, Ltd.

Response of Small-Scale, Methyl Rotors to Protein−Ligand Association: A Simulation Analysis of Calmodulin−Peptide Binding

Changes in the free energy barrier (DeltaE), entropy, and motional parameters associated with the rotation of methyl groups in a protein (calmodulin (CaM)) on binding a ligand (the calmodulin-binding domain of smooth-muscle myosin (smMLCKp)) are investigated using molecular dynamics simulation. In both the bound and uncomplexed forms of CaM, the methyl rotational free energy barriers follow skewed-Gaussian distributions that are not altered significantly upon ligand binding. However, site-specific perturbations are found. Around 11% of the methyl groups in CaM exhibit changes in DeltaE greater than 0.7 kcal/mol on binding. The rotational entropies of the methyl groups exhibit a nonlinear dependence on DeltaE. The relations are examined between motional parameters (the methyl rotational NMR order parameter and the relaxation time) and DeltaE. Low-barrier methyl group rotational order parameters deviate from ideal tetrahedrality by up to approximately 20%. There is a correlation between rotational barrier changes and proximity to the protein-peptide binding interface. Methyl groups that exhibit large changes in DeltaE are found to report on elements in the protein undergoing structural change on binding.

The Motional Dynamics in Bolaamphiphilic Nanofibers and Micellar Aggregates: An ESR Spin Probe Study

The aggregation behavior of the bolaamphiphilic hydrogelators dotriacontane-1,32-diyl-bis[2-(trimethylammonio)ethylphosphate] (PC-C32-PC) and the pH-sensitive dotriacontane-1,32-diyl-bis[2-(dimethylammonio)-ethylphosphate] (Me2PE-C32-Me2PE) was characterized before using different methods. Dependent on the temperature, pH, and concentration, these bolaamphiphiles self-asssemble into long nanofibers or other aggregates, such as short rods or micelles. In order to obtain information about the motional dynamics and microscopic order inside of these aggregates, we carried out a systematic ESR spin probe study. The spectra obtained with the spin probes 5-, 12- and 16-doxyl stearic acid (n-DSA) were found to be highly sensitive to changes in the bolaamphiphilic arrangement. Rotational correlation times and order parameters were obtained from full ESR line shape simulations with the programs EasySpin and EPRSIM. The maximum hyperfine splitting, 2Amax, was used to determine the transition temperatures, which are in agreement with the DSCdata. By comparison of 5-DSA and 12-DSA, which are residing at different positions in the alkyl chain region of the aggregates, it was found that trans-gauche isomerization is predominantly occurring in the outer region of the aggregates. For Me2PE-C32-Me2PE at pH 10, our ESR data indicate that micelles are short rods rather than being spherical in shape. Therefore, increasing the concentration from 1 to 10 mg/mL leads only to a one-dimensional growth of these micelles.

EPR Investigations of Spin‐Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound

AbstractDynamics of various spin probes in aqueous dispersions of nonionic amphiphilic compound, [poly(oxyethylene) hydrogenated castor oil, HCO], were investigated by EPR (electron paramagnetic resonance) and saturation recovery (SR) spectroscopies. Partitioning, rotational correlation time (τR), rotational diffusion coefficient, and electron spin‐lattice relaxation time (T1e) in dispersions of the HCO membrane were obtained. The partitioning of water soluble spin probes, DTBN and TEMPO, in the aqueous and vesicle phases was determined by an EPR linewidth simulation as a function of temperature. The results suggest that DTBN and TEMPO have a similar partitioning in the vesicle phase throughout the temperatures studied. The longer τR and shorter T1e (~0.33 μs) values of DTBN in the vesicle phase were obtained, and could be attributed to the probe environment in the membrane. The simulation results for fast tumbling probes were quite different from those of conventional intensity analysis (spectral parameter, f). Thus, the simulation and T1e analyses have provided a quantitative understanding of the probe dynamics in both phases. Aliphatic spin probes, doxylstearic acids (DSAs) and 3β‐doxyl‐5α‐cholestane (CHL), were used for monitor of various membrane motions. The EPR spectra were quantitatively analyzed by a slow tumbling simulation. The rotational diffusion coefficients and order parameter were obtained by the simulation. In addition, the direct observations of the behavior of the probes were measured by SR method. The results were consistent with T1e obtained for spin probes. Thus, the quantitative results regarding EPR, SR method, various simulation analyses have provided detailed information regarding physicochemical properties of the various moieties of the probe region in the amphiphilic compound.

Rotational Dynamics of Phospholamban Determined by Multifrequency Electron Paramagnetic Resonance

We have used multifrequency electron paramagnetic resonance to define the multistate structural dynamics of an integral membrane protein, phospholamban (PLB), in a lipid bilayer. PLB is a key regulator of cardiac calcium transport, and its function requires transitions between distinct states of intramolecular dynamics. Monomeric PLB was synthesized with the TOAC spin label at positions 11 (in the cytoplasmic domain) and 46 (in the transmembrane domain) and reconstituted into lipid bilayers. Unlike other protein spin labels, TOAC reports directly the motion of the peptide backbone, so quantitative analysis of its dynamics is worthwhile. Electron paramagnetic resonance spectra at 9.4 GHz (X-band) and 94 GHz (W-band) were analyzed in terms of anisotropic rotational diffusion of the two domains. Motion of the transmembrane domain is highly restricted, while the cytoplasmic domain exhibits two distinct conformations, a major one with moderately restricted nanosecond dynamics ( T) and another with nearly unrestricted subnanosecond motion ( R). The global analysis of spectra at two frequencies yielded values for the rotational correlation times and order parameters that were much more precisely determined than at either frequency alone. Multifrequency EPR is a powerful approach for analysis of complex rotational dynamics of proteins.

Effect of temperature on liposome structures studied using EPR spectroscopy

The effect of temperature on liposome structures has been investigated by means of electron paramagnetic resonance spectroscopy with the use of the spin labelling technique. The EPR spectra were recorded on a Bruker EMX spectrometer at the X band in the temperature range 300–340 K. Liposomes were prepared from L?a?phosphatidylcholine dipalmitoyl (1,2?dihexadecanoyl?sn?glycerol?3?phosphocholine) 99% (DPPC), DL?a?phosphatidylcholine dimyristoyl (1,2?ditetradecanoyl?rac?glycerol?3?phosphocholine) 99% (DMPC) and cholesterol (5?cholesten?3ß?ol) 99+% to constitute the membrane. The spin marker, 2?(3carboxypropyl)?4,4?dimethyl?2?tridecyl?3?oxazolidinyloxyl free radical (5?DOXYL), placed in a liposome membrane, allows for observation of structural changes in liposomes with temperature increase. The changes of rotational correlation time and order parameter values with increasing temperature result from the motion rise of the spin probe. The intensity of the EPR signal of 5?DOXYL gives us information about membrane fluidity.