Ripple Phase Research Articles

SCFT Study of Microphase Separation in Mixed Polymer Brushes Grafted on Cylindrical Surface

We report on microphase separation behaviors of mixed polymer brushes grafted onto an infinitely long cylindri- cal rod by performing polymer self-consistent field theory (SCFT) calculation with "masking" technique. The "masking" technique is especially suitable to deal with systems of confined polymers grafted onto curved surfaces. We have developed a method to solve the morphology of block copolymers confined into complicated topographic surfaces with SCFT. In this paper, this unique technique is extended to solve the SCFT for nanorod grafted by mixed polymer brushes. Furthermore, the use of simple Cartesian grids in a cubic computational cell with periodic boundary conditions makes it possible to solve dif- fusion equations in SCFT by utilizing an efficient and highly accurate pseudo-spectral method involving fast Fourier trans- form. Both parallel rippled phase and ring-shaped phase are predicted. We have investigated the influences of the cylinder radius, grafting density and interaction between the two incompatible grafting polymers on the stability of the two typical phases. Our results show that the system prefers the ring-shaped phase with the increase of the cylinder radius, grafting den- sity and interaction between the two grafting polymers. Phase diagrams involving these parameters are constructed, and we explain the reason of the transition between the parallel rippled phase and ring-shaped phase in terms of the degree of phase segregation. Again, the degree of phase segregation is higher with larger cylinder radius, grafting density and interaction between the two grafting polymers. By comparing the degree of phase segregation and free energy of the parallel rippled and ring-shaped phases at the same condition, we found that the ring-shaped phase favors the entropic part of the free energy while the parallel rippled phase significantly reduces the enthalpy. Therefore, when the degree of phase segregation is low, the free energy of the system is dominated by the enthalpy, leading to the parallel rippled phase; when the degree of phase segregation is high, the free energy of the system is dominated by the entropic part and the ring-shaped phase is stable. We also found that the domain numbers of parallel rippled phase and the period of alternating ring-shaped phase vary with the radius of cylinder. These predictions are expected to be helpful in rational design and fabrication of such novel polymer brushes.

Ripple formation in unilamellar-supported lipid bilayer revealed by FRAPP

The mechanisms of formation and conditions of the existence of the ripple phase are fundamental thermodynamic questions with practical implications for medicine and pharmaceuticals. We reveal a new case of ripple formation occurring in unilamellar-supported bilayers in water, which results solely from the bilayer/support interaction, without using lipid mixtures or specific ions. This ripple phase is detected by FRAPP using diffusion coefficient measurements as a function of temperature: a diffusivity plateau is observed. It occurs in the same temperature range where ripple phase existence has been observed using other methods. When AFM experiments are performed in the appropriate temperature range the ripple phase is confirmed.

Hearing sensitivity to shifts of rippled-spectrum patterns

The sensitivity of human hearing to shifts in rippled spectrum patterns of sound was investigated. The test signal was band-limited rippled noise with spectrum ripples of various frequency spacing and bandwidth; this type of sound may be considered as a quantitatively controlled imitation of complex natural sounds. The listener was required to detect a shift in the spectrum ripple phase while keeping the other parameters of the noise constant. For cosine-shaped ripples, the lowest threshold (1.1%) was found at a ripple frequency of 3.5 ripples per octave (rpo), which corresponds to a ripple spacing of 20% of the center frequency. The threshold increased for both lower and higher ripple densities. Qualitatively similar patterns of threshold dependence on ripple density were observed for center frequencies from 1 to 4 kHz. Making the ripples narrower than cosine decreased the thresholds to 0.7%-0.75% for ripple densities of 2-5 rpo. Keeping the ripple width constant at 3.5%-7.5% of the frequency resulted in a monotonic threshold dependence on ripple density: The threshold decreased with decreasing density (down to 0.7%). An excitation-pattern model explains qualitatively the observed dependence of the ripple-phase shift threshold on ripple pattern parameters.

Direct Visualization of Three-Dimensional Morphology in Hierarchically Self-Assembled Mixed Poly(tert-butyl acrylate)/Polystyrene Brush-Grafted Silica Nanoparticles

Three-dimensional (3D) morphology of hierarchically self-assembled mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles cast from chloroform on a carbon-coated transmission electron microscopy (TEM) grid was directly visualized using electron tomography (i.e., 3D TEM). The number-average molecular weights for PtBA and PS were 22.2 and 23.4 kDa, and their grafting densities were 0.51 and 0.34 chains/nm2, respectively. For a dense monolayer of mixed brush-grafted silica particles, a rippled phase structure was observed, and the monolayer consisted of protruded top portions from large nanoparticles and a bottom continuous film. In the top protruded part of large nanoparticles, isolated cylindrical PS microdomains were formed in the PtBA matrix due to the low PS volume fraction (i.e., 37%). The morphology in the lower continuous film was strongly influenced by interparticle interactions via mixed polymer brushes. As a result, bicontinuous nanostructures instead of i...

Monolayer curvature stabilizes nanoscale raft domains in mixed lipid bilayers

According to the lipid raft hypothesis, biological lipid membranes are laterally heterogeneous and filled with nanoscale ordered "raft" domains, which are believed to play an important role for the organization of proteins in membranes. However, the mechanisms stabilizing such small rafts are not clear, and even their existence is sometimes questioned. Here, we report the observation of raft-like structures in a coarse-grained molecular model for multicomponent lipid bilayers. On small scales, our membranes demix into a liquid ordered (lo) phase and a liquid disordered (ld) phase. On large scales, phase separation is suppressed and gives way to a microemulsion-type state that contains nanometer-sized lo domains in an ld environment. Furthermore, we introduce a mechanism that generates rafts of finite size by a coupling between monolayer curvature and local composition. We show that mismatch between the spontaneous curvatures of monolayers in the lo and ld phases induces elastic interactions, which reduce the line tension between the lo and ld phases and can stabilize raft domains with a characteristic size of the order of a few nanometers. Our findings suggest that rafts in multicomponent bilayers might be closely related to the modulated ripple phase in one-component bilayers.

Investigation of Torque Ripples in Permanent Magnet Synchronous Machines With Skewing

This paper investigates the influence of skewing on torque ripples, including the electromagnetic (EM) torque ripple, and the cogging torque, in permanent magnet (PM) machines. It is found that the effectiveness of skewing largely depends on the axial variation of a torque ripple phase but less on its magnitude under skewing. It is further found that, in both linear and nonlinear cases, the EM and the on-load torque ripple cannot be fully eliminated by skewing one on-load torque ripple period or any other angles, except 360° electrical, which is impractical. This is due to the fact that the EM torque ripple, especially its phase, of each slice is different due to the axial variation of the equivalent current phase advance angle introduced by skewing. In nonlinear cases, the EM torque ripple variation is further aggravated by the axial variation of magnetic saturation. The EM torque ripple as well as the on-load torque ripple may be even increased by skewing, especially when the cogging torque is low and electric loading is high.

Biophysical characterization of the drug–membrane interactions: The case of propranolol and acebutolol

The interaction of propranolol and acebutolol with biological membranes was assessed in the present work by using a range of biophysical techniques and liposomes, as membrane mimetic models. Liposomes were made of zwitterionic phosphatidylcholines and experiments were performed at physiologic pH and at various membrane physical states (gel, ripple and fluid phases). Fluorescence techniques were used to study the partition coefficient of β-blockers, the influence of drugs on membrane fluidity and the drugs–membrane binding. Moreover, small and wide angle X-ray scattering techniques were used to evaluate the β-blockers effect on long range bilayer order and hydrocarbon chain packing. The gathered results highlighted the importance of electrostatic interactions between propranolol and acebutolol with membranes. Furthermore, both β-blockers exhibited a membrane-fluidizing effect and the capacity to disturb the membrane organization. In general, propranolol unveiled a more pronounced effect on membrane fluidity and structure than acebutolol. In the current study, the obtained results were also correlated with the cardioprotective properties of the β-blockers studied.

Vesicle fusion to planar membranes is enhanced by cholesterol and low temperature

Lipid composition and properties play an important role in many cellular properties such as fusion of vesicles to cell membranes, an essential process for exocytosis. Using a model system composed of artificial vesicles (liposomes) and artificial membranes (planar lipid bilayers), we observed that fusion is significantly affected by the lipid phase of the planar membrane. To determine the effect of lipid phases on fusion rates, we utilized the nystatin/ergosterol fusion assay and stimulated fusion with an osmotic gradient. Phase of the planar membrane was altered by changing cholesterol or temperature while the vesicular lipids were held constant. Liquid disordered (Ld or Lα) planar membranes were formed from phosphatidylethanolamine and phosphatidylcholine with unsaturated acyl chains. Addition of cholesterol shifts these membranes to the liquid ordered (Lo) phase and increases liposome fusion. Planar membranes in the Lα phase were also made from dipalmitoylphoshatidylcholine (DPPC) above the transition temperature (Tm) of 41.5°C. Decreasing the temperature below Tm shifts these membranes into the ripple phase (Pβ′) and also increases liposome fusion. The cholesterol and temperature data are consistent with the hypothesis that fusion is promoted in membranes that have greater exposure of their lipid tails or in membranes which can form leaflet domains with negative curvature. The data are not consistent with the hypothesis that lipid mismatch drives fusion.

Interaction of Celecoxib with Membranes: The Role of Membrane Biophysics on its Therapeutic and Toxic Effects

The present work provides a biophysical characterization of the interaction of celecoxib, a cyclo-oxigenase-2 selective nonsteroidal anti-inflammatory drug, with membranes using liposomes, constituted by phosphatidylcholines, as membrane model systems. In order to mimic biological conditions, the experiments were performed at physiological pH (7.4); at an acidic pH to mimic the conditions of the inflamed cells (5.0); and at different membrane physical states (gel, ripple, and fluid phase). Important information regarding the celecoxib-membrane interactions was gathered by the complementary biophysical techniques: derivative spectrophotometry was used to determine liposome/water partition coefficient of celecoxib; dynamic light scattering (DLS) measurements were performed to study the influence of celecoxib on lipid main phase transition temperature; fluorescence binding measurements were made to assess the location of celecoxib within the membrane; and small-angle and wide-angle X-ray scattering (SAXS and WAXS) were used to assess the changes in the structure and order of phosphatidylcholine bilayers caused by the presence of celecoxib. The overall results obtained indicate that celecoxib greatly interacts with membranes. Briefly, celecoxib exhibits a high liposome/water partition coefficient that is non-pH-dependent, but the location of celecoxib within the membrane is pH-dependent. In fact, celecoxib is more deeply located inside the membrane at pH 5.0, while it locates closer to the surface at pH 7.4. DLS, SAXS, and WAXS results have shown a high membrane fluidization in the presence of celecoxib, especially at pH 7.4. Overall, the current study can contribute to a biophysical characterization of the celecoxib-membrane interaction. The relevance of the gathered results will be discussed in terms of the reported celecoxib therapeutic and toxic effects.

Modulated phases of phospholipid bilayers induced by tocopherols

The influence of α-, γ‐ and δ-tocopherols on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) bilayers has been determined from X-ray diffraction studies on oriented multilayers. In all the three cases the main-transition temperature (Tm) of DPPC was found to decrease with increasing tocopherol concentration up to around 25mol%. Beyond this the main transition is suppressed in the case of γ‐tocopherol, whereas Tm becomes insensitive to composition in the other two cases. The pre-transition is found to be suppressed over a narrow tocopherol concentration range between 7.5 and 10mol% in DPPC‐γ-tocopherol and DPPC-δ‐tocopherol bilayers, and the ripple phase occurs down to the lowest temperature studied. In all the three cases a modulated phase is observed above a tocopherol concentration of about 10mol%, which is similar to the Pβ phase reported in DPPC–cholesterol bilayers. This phase is found to occur even in excess water conditions at lower tocopherol concentrations, and consists of bilayers with periodic height modulation. These results indicate the ability of tocopherols to induce local curvature in membranes, which could be important for some of their biological functions.

Phase behavior of two-component lipid membranes: Theory and experiments

The structure of the ripple phase of phospholipid membranes remains poorly understood in spite of a large number of theoretical studies, with many experimentally established structural features of this phase unaccounted for. In this article we present a phenomenological theory of phase transitions in single- and two-component achiral lipid membranes in terms of two coupled order parameters: a scalar order parameter describing lipid chain melting, and a vector order parameter describing the tilt of the hydrocarbon chains below the chain-melting transition. This model reproduces all the salient structural features of the ripple phase, providing a unified description of the phase diagram and microstructure. In addition, it predicts a variant of this phase that does not seem to have been experimentally observed. Using this model we have calculated generic phase diagrams of two-component membranes. We have also determined the phase diagram of a two-component lipid membrane from x-ray diffraction studies on aligned multilayers. This phase diagram is found to be in good agreement with that calculated from the model.

Thermal behavior of 1,2-dipalmitoyl-sn-3-phosphoglycerocholine bi- and multi-layers, deposited with physical vapor deposition under ellipsometric growth control

1,2-dipalmitoyl-sn-3-phosphoglycerocholine membranes were deposited onto a silicon substrate (Si/SiO(2)) using physical vapor deposition with in situ ellipsometric thickness control. Along several heating cycles it was possible to identify well-defined boundaries for gel, ripple, liquid crystalline, and fluid-disordered phases. Particularly, the second order transition between gel and ripple phase was clearly identified in the range of ~28-34 °C using Raman spectroscopy. Atomic force microscopy and imaging ellipsometry (IE) were used to observe and characterize the ripple phase undulations of period λ = 20.8 nm and average height h = 19.95 nm along the temperature interval of ~34 to 40 °C. Clusters/agglomerations heights of more than twice the membrane thickness were observed with IE, induced by heating cycles.

Ripple Phase Behavior in Mixtures of DPPC/POPC lipids: SAXS and SANS Studies

Multilayer membranes of DPPC (l,2–dipalmitoyl–sn–glycero–3–phosphatidylcholine) and mixture DPPC/POPC (1–palmitoyl–2–oleoyl–sn–glycero–3–phosphatidylcholine) in excess of water were investigated by small angle X–ray scattering and neutron small angle scattering. Structural parameters of lipid bilayers were determined in a wide temperature range. It was shown that addition of the POPC to the DPPC/H2O does not change the temperature of the DPPC main phase transition Tm. The observed phase separation occurs for the system DPPC/POPC/H2O up to the pre-transition temperature Tr (gel phase – ripple phase) of the DPPC multilayers.

Design of Large Bandwidth Phase Shifters Using Common Mode All-Pass Networks

The design of digital phase shifters with bandwidths from 2:1 up to 20:1 using common mode all-pass networks (APNs) is presented. Because of the inherently large return loss value of the APNs, the phase shift response is not affected by the return loss and therefore the phase shift can be treated separately. The common mode APNs enable the use of standard switches and low cost fabrication. By properly designing the common mode APN phase shifter, an equal ripple phase shift response over a very large bandwidth can be realized. As an example, a 3 b 360° phase shifter covering a bandwidth of one decade (10:1) is designed using discrete components on PCB. An RMS phase error less than 10°, a return loss larger than 10 dB, and an amplitude imbalance less than 1.5 dB are measured from 100 MHz to 940 MHz.

Vesicle-Membrane Fusion is Enhanced by Cholesterol and Low Temperature

Lipid composition and properties play an important role in fusion of vesicles to cell membranes, an essential process for exocytosis. Using a model system composed of artificial vesicles and planar lipid membranes, we observed that fusion is significantly affected by the lipid phase of the planar membrane. To specifically determine the effect of lipid phases on fusion rates, we utilized the nystatin/ergosterol fusion assay and stimulated fusion with an osmotic gradient. The lipid phase of the planar membrane was altered by changing cholesterol or temperature, while the vesicular lipids were held constant. Liquid disordered (Lα) planar membranes were formed from PE and PC. Addition of cholesterol shifts these membranes to the liquid ordered (Lo) phase and increased vesicle fusion. Planar membranes in the Lα phase were also made from DPPC lipids above the transition temperature (41.5°C); decreasing the temperature shifts these membranes into the ripple phase (Pβ') and again increased vesicle fusion. These data are consistent with the hypothesis that fusion is promoted in membranes with some negative curvature.

Bicellar systems to modify the phase behaviour of skin stratum corneum lipids

Bicellar systems are a fascinating category of versatile lipid assemblies that comprise bilayered disk-shaped nanoaggregates formed in water by long and short alkyl chain phospholipids. Bicelles bridge the gap between micelles and lipid vesicles by combining the attractive properties of both systems. These structures have recently been proposed in dermatological, cosmetic and pharmaceutical applications. Two new binary bicellar systems composed of cholesterol sulphate (SCHOL) and long-chain phospholipids (dimyristoyl-phosphatidylcholine, DMPC, or dipalmitoyl-phosphatidylcholine, DPPC) are characterised herein by differential scanning calorimetry, fluorescence spectroscopy, X-ray scattering and microscopy. Additionally, a comparative study on skin treated with the new SCHOL systems (DMPC/SCHOL and DPPC/SCHOL) and classic DHPC systems (DMPC/DHPC and DPPC/DHPC) was performed. These studies were conducted to determinate how deeply bicelles penetrate into the skin and the extension of their effect on the phase behaviour of stratum corneum (SC) lipids using attenuated total reflectance-Fourier transform infrared spectroscopy and two-photon excitation fluorescence microscopy. Our results show that SCHOL modified the typical discoidal morphology and the phase behaviour of the systems, inducing coexistence of two phases, liquid-ordered and ripple phases. The effect of the systems on SC lipids depends on their composition and is related to the fluidity of the SC lipid alkyl chains. Thus, systems with DMPC induced more disorder in SC lipids than systems with DPPC, and SCHOL did not modify the lipid arrangement. Perdeuterated systems in the infrared spectroscopy technique supported a different distribution in the tissue for every system. DMPC systems were primarily at the first layers of the SC, whereas DPPC systems were more widely distributed. Systems with SCHOL had enhanced distribution and penetration of bicellar systems throughout the SC.

Density, DSC, X-ray and NMR measurements through the gel and lamellar phase transitions of 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC): observation of slow relaxation processes and mechanisms of phase transitions

Dialkyl lecithin dispersions in water exhibit two phase transitions upon cooling from the lamellar phase (L(α)). At the main transition (T(M)) the L(α) phase changes to a ripple (gel) phase (P(β')) which then transforms to a second gel phase (L(β')) at the "pretransition" (T(P)). We have made accurate density measurements through the various phases for two lecithins having unequal chains: 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC). The measurements were carried out over five heat/cool cycles from 5 to 55 °C, followed by cooling back to 5 °C. The samples were then held at 50 °C for 24 hours, followed by a further three cool/heat cycles. For SMPC we observe an increase in density of the gel phases over the first 5 cycles, followed by much smaller changes after incubation at 50 °C. The lamellar phase also shows an increase in density, albeit much smaller. This parallels the behaviour of 1,2-di-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-di-myristoyl-sn-glycero-3-phosphatidylcholine (DMPC) reported earlier (Jones et al., Liquid Crystals 32, 1465 (2005)). For MSPC we observe a decrease in density within the gel phases while T(P) almost disappears after the first cycle. The lamellar phase shows little evidence of any change with each cycle. Within the lamellar phases there is a marked reduction in density on approaching T(M), which is attributed to the formation of transitory gel phase domains. Additional measurements by DSC and X-ray diffraction show that the changes in densities are not accompanied by large changes in transition enthalpies or phase structures. NMR data indicate that the pretransitional event within the L(α) phase is accompanied by ordering of the alkyl chains. The results indicate that the exact nature of the lipid alkyl chains could play a key role in the formation of gel phase patches within membrane bilayers. Their detailed chemical structures merit more attention than by simply assuming a uniform "bending energy" to describe the behaviour.

Partitioning of ethanol into lipid membranes and its effect on fluidity and permeability as seen by X-ray and neutron scattering

We present a combined neutron and X-ray scattering investigation to study the effect of ethanol on the molecular structure and dynamics of lipid membranes. 1,2-Dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) powder hydrated with a 5 wt% ethanol solution (corresponding to 2 mol% of ethanol) was used in this study. From high-resolution X-ray experiments the position and partitioning of the ethanol molecules in the phospholipid bilayers was determined in their gel and fluid phases. We find that the ethanol molecules reside in the head group region of the bilayers, with 1.6 ethanol molecules per lipid molecule in the gel phase and 1.2 ethanol molecules per lipid molecule in the fluid phase. We find evidence for enhanced permeability in both fluid and gel phases of the phospholipid bilayers in the presence of ethanol molecules. Elastic and quasi-elastic neutron scattering data, obtained using a neutron backscattering spectrometer, was used to study slow, nanosecond molecular dynamics on length scales corresponding to lipid diffusion, acyl chain dynamics and solvent dynamics. While the presence of ethanol molecules had no observable effect on these types of dynamics in the fluid (Lα) phase, the membranes appeared to have a higher degree of order in gel (Lβ) and ripple (Pβ′) phases. In particular, lipid diffusion was found to be slower by a factor of two in the more rigid gel phase when ethanol was present.

Phase behavior of phospholipid–phytosterol membranes

We have studied the influence of some major phytosterols on the structure and phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) membranes. Phase diagrams of the binary systems have been determined from X-ray diffraction data. All DPPC–phytosterol membranes studied exhibit a sterol-induced modulated phase (Pβ) below the main transition, first reported in DPPC–cholesterol membranes. Structural features of this phase have been deduced from electron density maps calculated from the diffraction data. In addition, a re-entrant ripple phase is observed in DPPC–stigmasterol membranes as a function of temperature. The increase in the bilayer thickness in the fluid phase, due to the ordering of the lipid chains by the sterol molecules, has also been determined from the electron density profiles of the bilayers. All the phytosterols studied are found to increase the bilayer thickness, but to a lower extent compared to cholesterol. Some of the phytosterols phase separate from POPE even at very low concentrations, whereas the others have a similar effect on the phase behavior of POPE, with the temperature range over which the fluid lamellar phase occurs first decreasing and then increasing with increasing phytosterol concentration.

Correlation between the ripple phase and stripe domains in membranes

We investigate the relationship between stripe domains and the ripple phase in membranes. These have previously been observed separately without being linked explicitly. Past results have demonstrated that solid and ripple phases exhibit rich textural patterns related to the orientational order of tilted lipids and the orientation of ripple corrugations. Here we reveal a highly complex network pattern of ripple and solid domains in DLPC, DPPC bilayers with structures covering length scales from 10nm to 100μm. Using spincoated double supported membranes we investigate domains by correlated AFM and fluorescence microscopy. Cooling experiments demonstrate the mode of nucleation and growth of stripe domains enriched in the fluorescent probe. Concurrent AFM imaging reveals that these stripe domains have a one-to-one correspondence with a rippled morphology running parallel to the stripe direction. Both thin and thick stripe domains are observed having ripple periods of 13.5±0.2nm and 27.4±0.6nm respectively. These are equivalent to previously observed asymmetric/equilibrium and symmetric/metastable ripple phases, respectively. Thin stripes grow from small solid domains and grow predominantly in length with a speed of ~3 times that of the thick stripes. Thick stripes grow by templating on the sides of thinner stripes or can emerge directly from the fluid phase. Bending and branching angles of stripes are in accordance with an underlying six fold lattice. We discuss mechanisms for the nucleation and growth of ripples and discuss a generic phase diagram that may partly rationalize the coexistence of metastable and stable phases.