Hexatic order and surface ripples in spherical geometries.

The charge-density distribution in multielectron bubbles is calculated with use of a generalized Thomas-Fermi method. The exchange and correlation effects are found crucial; these result in a density profile much narrower than that obtained by simple estimates. The observed stability of the bubbles is explained in terms of the nonlinear coupling between the radial and angular modes of oscillation. Above a threshold oscillation amplitude a period-doubling instability sets in.

An estimate is made of the frequencies of oscillation of compressional and surface modes of an electron bubble in liquid helium. The liquid is treated as an incompressible fluid with a surface tension determined by the equilibrium bubble radius. There are deviations from Rayleigh's classical formula arising from the quantum potential energy of deformation of an electron in a deformed cavity. These contributions are included in the calculation and a modified formula is obtained.

We present calculations of a number of properties of electron bubbles in liquid helium. The size and shape of bubbles containing electrons in different quantum states is determined based on a simplified model. We then find how the geometry of these bubbles changes with the applied pressure. The radiative lifetime of bubbles with electrons in excited states is calculated. Finally, we use a quantum Monte Carlo method to determine the properties of a bubble containing two electrons. We show that this object is unstable against fission.

We study the formulation of bond-orientational order in an arbitrary two dimensional geometry. We find that bond-orientational order is properly formulated within the framework of differential geometry with torsion. The torsion reflects the intrinsic frustration for two-dimensional crystals with arbitrary geometry. Within a Debye-Huckel approximation, torsion may be identified as the density of dislocations. Changes in the geometry of the system cause a reorganization of the torsion density that preserves bond-orientational order. As a byproduct, we are able to derive several identities involving the topology, defect density and geometric invariants such as Gaussian curvature. The formalism is used to derive the general free energy for a 2D sample of arbitrary geometry, both in the crystalline and hexatic phases. Applications to conical and spherical geometries are briefly addressed.

This paper reviews the progress in the study of the structureof the surface of pure liquids made in the last 10-20 years.This area of research has benefited enormously in recent yearsfrom developments in experimental techniques (especially x-rayscattering methods based on third-generation synchrotronsources, and advanced optical techniques) and theory (particularly computer simulation and density functionaltheory). The review is predominantly experimentally based, andfocuses very much on some recent exciting new results anddevelopments. Specific systems or phenomena that are discussedare molecular ordering and orientation at the surface, atomic-scale layering at liquid metal surfaces, surface freezing inliquid alkanes and alcohols, surface melting and pre-melting atthe water surface, thermal roughness (capillary waves) insimple liquids, and visco-elastic surface waves. Both theliquid-vapour and liquid-solid interfaces are considered. Somedetails of the principal experimental techniques exploited,x-ray reflectivity, diffuse x-ray scattering, grazing incidencex-ray diffraction, ellipsometry, light scattering, will begiven, The experimental developments have gone very much inconjunction with substantial theoretical advances, and theprincipal theoretical concepts will be briefly discussed.

The lipid headgroup plays an important role in the association of polymers with lipid bilayer membranes. Herein, we report how a glycerol headgroup versus a choline headgroup affects the interaction of poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) block copolymers with lipid bilayer vesicles. Unilamellar vesicles composed of phosphatidylcholine and phosphatidylglycerol at various molar ratios were used as model membranes. The interactions between the block copolymers and lipid bilayers were quantified by pulsed-field gradient nuclear magnetic resonance (PFG-NMR) based on the distinctly different mobilities of free and bound polymers. All the investigated polymer species showed significantly higher binding with 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) sodium salt (POPG) liposomes than with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, indicating stronger association with the glycerol headgroup compared to the choline headgroup. This effect did not become significant until the composition of mixed POPC/POPG liposomes contained more than 20 mol % POPG. A plausible explanation for the enhanced polymer binding with POPG invokes the role of hydrogen bonding between the glycerol headgroup and the ether moieties of the polymers.

Incorporation of quantum dots into the lipid bilayer of giant unilamellar vesicles and its stability

The frequency-dependent complex viscosity of dispersions of various lipid vesicles was measured. Two clearly distinguishable features in these curves could be observed, suggesting two relaxation processes. The first relaxation time is shown to be dependent on the third power of the vesicle radius. This implies an elasticity of entropic nature, which is the result of the translational ordering of the vesicles that are subjected to shear flow and forced out of their equilibrium distribution. A comparison with linear viscoelastic measurements of dispersions of ‘‘real’’ hard-sphere (silica) particles shows remarkable agreement, and trends are extended. Furthermore, the measurements support theoretical work. For the second relaxation process, a linear dependency of the relaxation time on the radius of the vesicle is observed. In addition, the relaxation time and the effective viscosity were found to be linearly proportional. A combination of various similar theories that treat the dynamics of spherical capsules in terms of capsule wall viscoelasticity proved capable of explaining the dependency of the second relaxation time on the product of radius and effective viscosity. The second relaxation process was found to be pertaining to the surface shear viscoelasticity of the vesicle wall. Thus, from the analysis of the data with theory, the surface shear modulus and the surface shear viscosity of the lipid bilayer could be deduced. Furthermore, a small contribution of the curvature modulus was detected in our measurements, for which an estimate could be given. With the aid of literature data for the curvature modulus, the dilatational modulus, and the present value of the surface shear modulus found in this work, Young’s modulus and the Poisson ratio of the investigated bilayer could be established.

We describe experiments we have performed in which we are able to image the motion of individual electrons moving in liquid helium 4. Electrons in helium form bubbles of radius ~19 Å.We use the negative pressure produced by a sound wave to expand these bubbles to a radius of about 10 m.The bubbles are then illuminated with light from a flash lamp and their position recorded.We report on several interesting phenomena that have been observed in these experiments.It appears that the majority of the electrons that we detect result from cosmic rays passing through the experimental cell.We discuss this mechanism for electron production.

Stabilization of liposome bilayers to freezing and thawing: Effects of cryoprotective agents and membrane proteins

Many hydrolytic enzymes are attached to the extracellular face of the plasma membrane of eukaryotic cells by a glycosylphosphatidylinositol (GPI) anchor. Little is currently known about the consequences for enzyme function of anchor cleavage by phosphatidylinositol-specific phospholipase C. We have examined this question for the GPI-anchored protein 5'-nucleotidase (5'-ribonucleotide phosphohydrolase; EC 3.1.3.5), both in the native lymphocyte plasma membrane, and following purification and reconstitution into defined lipid bilayer vesicles, using Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC). Membrane-bound, detergent-solubilized and cleaved 5'-nucleotidase all obeyed Michaelis-Menten kinetics, with a Km for 5'-AMP in the range 11-16 microM. The GPI anchor was removed from essentially all 5'-nucleotidase molecules, indicating that there is no phospholipase-resistant pool of enzyme. However, the phospholipase was much less efficient at cleaving the GPI anchor when 5'-nucleotidase was present in detergent solution, dimyristoyl phosphatidylcholine, egg phosphatidylethanolamine and sphingomyelin, compared with the native plasma membrane, egg phosphatidylcholine and a sphingolipid/cholesterol-rich mixture. Lipid molecular properties and bilayer packing may affect the ability of PI-PLC to gain access to the GPI anchor. Catalytic activation, characterized by an increase in Vmax, was observed following PI-PLC cleavage of reconstituted 5'-nucleotidase from vesicles of several different lipids. The highest degree of activation was noted for 5'-nucleotidase in egg phosphatidylethanolamine. An increase in Vmax was also noted for a sphingolipid/cholesterol-rich mixture, the native plasma membrane and egg phosphatidylcholine, whereas vesicles of sphingomyelin and dimyristoyl phosphatidylcholine showed little activation. Km generally remained unchanged following cleavage, except in the case of the sphingolipid/cholesterol-rich mixture. Insertion of the GPI anchor into a lipid bilayer appears to reduce the catalytic efficiency of 5'-nucleotidase, possibly via a conformational change in the enzyme, and activity is restored on release from the membrane.

Bone is mineralized when hydroxyapatite crystals derived from calcium ions and inorganic phosphate (Pi) grow along collagen fibrils in the extracellular matrix. Mineralization is initiated by nucleation of those crystals. Mature osteoblasts secrete matrix vesicles into osteoid, which contain growing hydroxyapatite crystal seeds. After rupture of the lipid bilayer of those vesicles, crystals continue to grow as a mineralized nodule and adhere to collagen fibrils. It remains controversial whether nucleation occurs mainly in matrix vesicles or also extra-vesicularly around collagen fibrils. Mineralization is inhibited by pyrophosphate (PPi) and by SIBLING family proteins, which carry an acidic serine- and aspartate-rich motif (ASARM) and include osteopontin, dentin matrix protein 1 and MEPE. Intracellular and extracellular activity of these factors is regulated by the PPi-generating ectonucleotide pyrophosphatase/phosphodiesterase (ENPP1) , the PPi-transporter progressive ankylosis (ANK) protein, the PPi-degrading/Pi-generating ectoenzyme alkaline phosphatase (ALPL, TNAP) , and PHEX endopeptidase. Gain- or loss-of-function mutations in genes encoding these proteins are associated with mineralization disorders such as ectopic calcification and other pathologies.

X-ray studies of atomic layering at liquid metal surfaces

We define and study the three-dimensional windings along Brownian paths in the quaternionic Euclidean, projective and hyperbolic spaces. In particular, the asymptotic laws of these windings are shown to be Gaussian for the flat and spherical geometries while the hyperbolic winding exhibits a different long time-behavior. The corresponding asymptotic law seems to be new and is related to the Cauchy relativistic distribution.

The twin-arginine translocase (Tat) transports folded proteins across the cytoplasmic membrane of prokaryotes and the thylakoid membrane of plant chloroplasts. In Gram-negative bacteria and chloroplasts, the translocon consists of three subunits, TatA, TatB, and TatC, of which TatA is responsible for the actual membrane translocation of the substrate. Herein we report on the structure, dynamics, and lipid interactions of a fully functional C-terminally truncated 'core TatA' from Arabidopsisthaliana using solution-state NMR. Our results show that TatA consists of a short N-terminal transmembrane helix (TMH), a short connecting linker (hinge) and a long region with propensity to form an amphiphilic helix (APH). The dynamics of TatA were characterized using 15 N relaxation NMR in combination with model-free analysis. The TMH has order parameters characteristic of a well-structured helix, the hinge is somewhat less rigid, while the APH has lower order parameters indicating structural flexibility. The TMH is short with a surprisingly low protection from solvent, and only the first part of the APH is protected to some extent. In order to uncover possible differences in TatA's structure and dynamics in detergent compared to in a lipid bilayer, fast-tumbling bicelles and large unilamellar vesicles were used. Results indicate that the helicity of TatA increases in both the TMH and APH in the presence of lipids, and that the N-terminal part of the TMH is significantly more rigid. The results indicate that plant TatA has a significant structural plasticity and a capability to adapt to local environments.