Lia Q. Amaral

Study of a magnetically oriented lyotropic mesophase

A study of a magnetically oriented lyomesophase formed by a quaternary system (Na decyl sulfate/water/decanol/Na sulfate) is reported. Small angle x‐ray diffraction measurements have been performed on unoriented samples and samples previously subjected to the action of magnetic fields (H). Diffraction patterns show a diffuse inner halo at 80–140 A and a sharp outer ring at 38 A. They reduce to spots in the equator for oriented samples with the x‐ray beam parallel to H; no diffraction figure is produced for the x‐ray beam perpendicular to H. NMR spectra indicate that the director of the phase orients perpendicular to H. These results show that this lyomesophase has a structure not previously encountered. A model of finite planar micelles surrounded by water is proposed. In this model the micelles consist of a bilayer in the form of platelets, probably disk shaped. These platelets align in presence of magnetic fields, with their plane parallel to H, and further orientational restrictions are imposed by the container; they tend to be parallel and some periodicity appears in the director of the direction of the phase, perpendicular to the plane of polar heads. The mesophase satisfies the definition for nematics taking into account periodicities induced in nematic phases by magnetic fields.

Thermal transitions of DMPG bilayers in aqueous solution: SAXS structural studies

Dimyristoylphosphatidylglycerol (DMPG) has been extensively studied as a model for biological membranes, since phosphatidylglycerol is the most abundant anionic phospholipid in prokaryotic cells. At low ionic strengths, this lipid presents a peculiar thermal behavior, with two sharp changes in the light scattering profile, at temperatures named here T(on)(m) and T(off)(m). Structural changes involved in the DMPG thermal transitions are here investigated by small angle X-ray scattering (SAXS), and compared to the results yielded by differential scanning calorimetry (DSC) and electron spin resonance (ESR). The SAXS results show a broad peak, indicating that DMPG is organized in single bilayers, for the range of temperature studied (10-45 degrees C). SAXS intensity shows an unusual effect, starting to decrease at T(on)(m), and presenting a sharp increase at T(off)(m). The bilayer electron density profiles, obtained from modeling the SAXS curves, show a gradual decrease in electron density contrast (attributed to separation between charged head groups) and in bilayer thickness between T(on)(m) and T(off)(m). Results yielded by SAXS, DSC and ESR indicate that a chain melting process starts at T(on)(m), but a complete fluid phase exists only for temperatures above T(off)(m), with structural changes occurring at the bilayer level in the intermediate region.

Micelles forming biaxial lyotropic nematic phases

The paper by Yu and Saupe on the first biaxial nematic phase created excitement for a number of reasons. Some theories of biaxial phases already existed, but experimental observation was still lacking. The phase was discovered in a lyotropic system with three components, which in theory is difficult. Lyotropic liquid crystals are composed of supramolecular assemblies of amphiphilic molecules, which may change shape and size as a function of concentration and temperature. The experimental phase diagram of the lyotropic biaxial phase was rather complex, with the biaxial region inserted between nematic cylindrical and nematic discotic phases via second-order transitions. In addition, re-entrant behaviour was evident. Saupe investigated further systems experimentally, observing that the biaxial phase might be absent in cases where a direct transition between the cylindrical and discotic phases occurred. He provided a range of theoretical and experimental contributions on the properties of these lyotropics, but was very cautious regarding the detailed amphiphilic assemblies involved. The present paper reviews this area, focusing on proposals for the structure of the micellar assemblies. Emphasis is placed on recent papers which indicate a transformation of the two uniaxial shapes, in mixing conditions, both from the theoretical and the experimental point of view, and to questions still requiring further study.

Extensive bilayer perforation coupled with the phase transition region of an anionic phospholipid.

At low ionic strength dimyristoylphosphatidylglycerol (DMPG) exhibits a broad phase transition region characterized by several superimposed calorimetric peaks. Peculiar properties, such as sample transparency, are observed only in the transition region. In this work we use differential scanning calorimetry (DSC), turbidity, and optical microscopy to study the narrowing of the transition region with the increase of ionic strength (0-500 mM NaCl). Upon addition of salt, the temperature extension of the transition region is reduced, and the number of calorimetric peaks decreases until a single cooperative event at T(m) = 23 degrees C is observed in the presence of 500 mM NaCl. The transition region is always coupled with a decrease in turbidity, but a transparent region is detected within the melting process only in the presence of up to 20 mM NaCl. The vanishing of the transparent region is associated with one of the calorimetric peaks. Optical microscopy of giant vesicles shows that bilayers first rupture when the transition region is reached and subsequently lose optical contrast. Fluorescence microscopy reveals a blurry and undefined image in the transparent region, suggesting a different lipid self-assembly. Overall sample turbidity can be directly related to the bilayer optical contrast. Our observations are discussed in terms of the bilayer being perforated along the transition region. In the narrower temperature interval of the transparent region, dependent on the ionic strength, the perforation is extensive and the bilayer completely loses the optical contrast.

Influence of salt on the structure of DMPG studied by SAXS and optical microscopy

Aqueous dispersions of 50 mM dimyristoylphosphatidylglycerol (DMPG) in the presence of increasing salt concentrations (2-500 mM NaCl) were studied by small angle X-ray scattering (SAXS) and optical microscopy between 15 and 35 degrees C. SAXS data show the presence of a broad peak around q approximately 0.12 A(-1) at all temperatures and conditions, arising from the electron density contrasts within the bilayer. Up to 100 mM NaCl, this broad peak is the main feature observed in the gel and fluid phases. At higher ionic strength (250-500 mM NaCl), an incipient lamellar repeat distance around d=90-100 A is detected superimposed to the bilayer form factor. The data with high salt were fit and showed that the emergent Bragg peak is due to loose multilamellar structures, with the local order vanishing after approximately 4d. Optical microscopy revealed that up to 20 mM NaCl, DMPG is arranged in submicroscopic vesicles. Giant (loose) multilamellar vesicles (MLVs) start to appear with 50 mM NaCl, although most lipids are arranged in small vesicles. As the ionic strength increases, more and denser MLVs are seen, up to 500 mM NaCl, when MLVs are the prevailing structure. The DLVO theory could account for the experimentally found interbilayer distances.

Melting regime of the anionic phospholipid DMPG: new lamellar phase and porous bilayer model.

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) at pH above the apparent pK of DMPG and concentrations in the interval 70-300 mM have been investigated by small (SAXS) and wide-angle X-ray scattering, differential scanning calorimetry, and polarized optical microscopy. The order-disorder transition of the hydrocarbon chains occurs along an interval of about 10 degrees C (between T(m)(on) approximately 20 degrees C and T(m)(off) approximately 30 degrees C). Such melting regime was previously characterized at lower concentrations, up to 70 mM DMPG, when sample transparency was correlated with the presence of pores across the bilayer. At higher concentrations considered here, the melting regime persists but is not transparent. Defined SAXS peaks appear and a new lamellar phase L(p) with pores is proposed to exist above 70 mM DMPG, starting at approximately 23 degrees C (approximately 3 degrees C above T(m)(on)) and losing correlation after T(m)(off). A new model for describing the X-ray scattering of bilayers with pores, presented here, is able to explain the broad band attributed to in-plane correlation between pores. The majority of cell membranes have a net negative charge, and the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance.

Structural study of the aggregates formed by the dinucleoside phosphate G2 in aqueous solution

Abstract X-ray diffraction results in magnetically oriented samples of aqueous solutions of the sodium salt of the dinucleoside phosphate G2 in the cholesteric N∗ phase are presented. The symmetry of the diffraction confirms a model of stacking of tetrameric disks forming cylindrical aggregates in the N∗ phase. A weak band at 7 A not previously observed gives further information on the inner structure of the cylindrical aggregates. Analysis of the diffraction peak position in low angle evidences a change in functional behaviour of the distance α between cylinder axes as a function of the volume concentration cv of G2. In the H phase, a α cv −1/2 as it should for infinite cylinders; in the N∗ phase, a α cv −1/3, typical of finite cylinders. The cross-over of functional behaviour occurs at the N∗[sbnd]H phase transition. There is therefore cylinder growth along the N∗ phase.

First-order transition between nematic phases in lyotropic liquid crystals

Abstract Experimental evidence for the existence of a first order transition between Nc–Nd uniaxial lyonematic phases as a function of the variable M d (number of decanol molecules per amphiphilic molecule) is reported. The relevance of this evidence to molecular models for micellar aggregates is discussed. The evidence is for a change in micellar symmetry at this transition.

Changes in aggregate form, size and flexibility along phase sequences in lyotropic liquid crystals

A comparison is made between existing theories for self-assembling systems and experimental phase diagrams of complex systems made up of amphiphile/water/additives, with emphasis on phase transitions by changes in concentration. Evidence for different types of hexagonal (H) phases, in systems with detergent and lipid amphiphiles, are reviewed. It is shown that characteristics of the H phase and of the phase sequence (cubic phases with lipids and nematic phases with detergents) are revealed through the exponent defining the variation of the hexagonal parameter with amphiphile concentration. Emphasis is also given to results obtained in the ternary system sodium dodecyl (lauryl) sulfate / water / decanol, which exhibits the phase sequence isotropic (I) - H - nematic cylindrical (Nc) { [nematic biaxial (Nb)] - nematic discotic (Nd) - lamellar (L), with co-surfactant addition. Existing theories for self-assembling systems of rigid and exible rods predict the phase sequence I - (Nc) -H asa function of increased particle volume fraction up, with a triple point separating I - H and I - Nc - H phase sequences. Possible reasons for the non-trivial experimental I -H - Nc inversion in phase sequence are discussed. A complex path through the phase diagram is able to explain the experimental results in terms of changes in micellar growth and exibility induced by the co-surfactant. The role of the surfactant parameter, that expresses the curvature of the polar-apolar interfaces, explains much of the observed behavior, including the Nc - Nd transition with increase in decanol / amphiphile molar ratio.

SAS from inhomogeneous particles with more than one domain of scattering density and arbitrary shape

The Small Angle Scattering from inhomogeneous particles is very important in the investigation of biological systems, where the scattering objects might be formed by many domains. For these systems, the scattering intensity, as well as its Fourier transform, are written as a sum of different contributions. In this work, for the simple case of two domains, the form factors of different scattering particle shapes, such as the oblate, prolate and biaxial ellipsoids, spherocylinder and disks were calculated by using a Monte Carlo method. Finally, SAS data of sodium lauryl sulfate micelles in water were analysed by using a model that accounts for polydispersity and micelles of different shape.