Spectrin, human erythrocyte shapes, and mechanochemical properties.

Physical studies of human erythrocyte spectrin indicate that isolated spectrin dimers and tetramers in solution are worm-like coils with a persistence length of approximately 20 nm. This finding, the known polyelectrolytic nature of spectrin, and other structural information about spectrin and the membrane skeleton molecular organization have lead us to the hypothesis that the human erythrocyte membrane skeleton constitutes a two-dimensional ionic gel (swollen ionic elastomer). This concept is incorporated in what we refer to as the protein gel-lipid bilayer membrane model. The model accounts quantitatively for red elastic shear modulus and the maximum elastic extension ratio reported for the human erythrocytes membrane. Gel theory further predicts that depending on the environmental conditions, the membrane skeleton modulus of area compression may be small or large relative to the membrane elastic shear modulus. Our analyses show that the ratio between these two parameters affects both the geometry and the stability of the favored cell shapes and that the higher the membrane skeleton compressibility the smaller the values of the gel tension needed to induce cell shape transformations. The main virtue of the protein gel-lipid bilayer membrane model is that it offers a novel theoretical and molecular basis for the various mechanical properties of the membrane skeleton such as the membrane skeleton modulus of area compression and osmotic tension, and the effects of these properties on local membrane skeleton density, cell shape, and shape transformations.     

Ab initio predictions of structural and elastic properties of struvite: contribution to urinary stone research

In the present work, we carried out density functional calculations of struvite – the main component of the so-called infectious urinary stones – to study its structural and elastic properties. Using a local density approximation and a generalised gradient approximation, we calculated the equilibrium structural parameters and elastic constants C _ijkl . At present, there is no experimental data for these elastic constants C _ijkl for comparison. Besides the elastic constants, we also present the calculated macroscopic mechanical parameters, namely the bulk modulus (K), the shear modulus (G) and Young's modulus (E). The values of these moduli are found to be in good agreement with available experimental data. Our results imply that the mechanical stability of struvite is limited by the shear modulus, G. The study also explores the energy-band structure to understand the obtained values of the elastic constants.     

First-principles investigations on structural stability,elastic and electronic properties of Co7M6 (M= W,Mo, Nb) µ phases

The structural, elastic and electronic properties of Co₇M₆ (M?=?W, Mo, Nb) μ phases were investigated by first-principles calculations based on the density functional theory (DFT). The calculated cohesive energy indicates that Co₇M₆ (M?=?W, Mo, Nb) μ phases are thermodynamically stable. Besides, Co₇W₆ owns a higher structural stability than that of Co₇Mo₆ and Co₇Nb₆. The obtained elastic constant demonstrates that Co₇M₆ (M?=?W, Mo, Nb) are mechanically stable. With Voigt-Reuss-Hill (VRH) approximation, the elastic bulk modulus (B), shear modulus (G), Young's modulus (E) and Poisson's ratio (ν) were derived. The ductility and plasticity as well as the elastic anisotropy of the three phases were discussed in details. Finally, the density of states and charge density difference were also analysed to reveal the underlying mechanism of structural stability and the elastic properties.     

The human erythrocyte membrane skeleton may be an ionic gel

In the first paper in this series (Stokke et al. Eur Biophys J 1986, 13:203-218) we developed the general theory of the mechanochemical properties and the elastic free energy of the protein gel--lipid bilayer membrane model. Here we report on an extensive numerical analysis of the human erythrocyte shapes and shape transformations predicted by this new cell membrane model. We have calculated the total elastic free energy of deformation of four different cell shape classes: disc-shaped cells, cup-shaped cells, crenated cells, and cells with membrane invaginations. We find that which of these shape classes is favoured depends strongly on the spectrin gel osmotic tension, IIGu, and the surface tensions, IIEu and IIPu, of the extracellular and protoplasmic halves of the membrane lipid bilayer, respectively. For constant ratio IIEu/IIPu greater than O large negative or positive values of IIGu favour respectively the crenated and invaginated cell shape classes. For small absolute values of IIGu, IIEu, and IIPu, biconcave or cup-shaped cells are the stable ones. Our numerical analysis shows that the higher the membrane skeleton compressibility is, the smaller are the values of IIGu needed to induce cell shape transformation. We find that the stable and metastable shapes of discocytes and stomatocytes generally depend both on the shape of the stressfree membrane skeleton and the membrane skeleton compressibility.     

First-principles study of phase stability and elastic properties in metastable Ti-Mo alloys with cluster structure

This paper provided a novel approach for evaluating phase stability and elastic properties in metastable Ti–Mo alloys with low Mo content by first-principles combined with cluster structure. In 54-atom body-centered-cubic supercell by substituting Ti atoms with 2–7 Mo atoms (7.1–23.0?wt% Mo), individual cluster structure of β-phase was constructed by ‘-Mo-Ti-Mo-’ cluster unit having the lowest cohesive energy. The distorted supercell was more stable than undistorted one at a low Mo content. With increasing Mo content, the density of state at Fermi level decreased, and bonding electron number increased, indicating β-phase stability was gradually promoted. Tetragonal shear elastic constant (C′?=?(C₁₁?–?C₁₂)/2), shear modulus (G₁₁₁) and anisotropy factor (A?=?C₄₄/C′) exhibited a fluctuation with Mo addition, while the change trend of A was opposite to C′ and G₁₁₁. Calculated Young’s modulus exhibited similar changing trend to the C′, implying that the softening of C′ resulted in low Young’s modulus of β-phase. Measured Young’s modulus exhibited significant difference from calculated one, which was mainly caused by formation of α″-martensite and ω-phase. The values of C′, G₁₁₁ and A were considered to associate with not only elastic properties of β-phase itself but also transition from β-phase to α″-martensite and/or ω-phase.     

Effect of hydroperoxides on red blood cell membrane mechanical properties

We investigate the effect of oxidative stress on red blood cell membrane mechanical properties in vitro using detailed analysis of the membrane thermal fluctuation spectrum. Two different oxidants, the cytosol-soluble hydrogen peroxide and the membrane-soluble cumene hydroperoxide, are used, and their effects on the membrane bending elastic modulus, surface tension, strength of confinement due to the membrane skeleton, and 2D shear elastic modulus are measured. We find that both oxidants alter significantly the membrane elastic properties, but their effects differ qualitatively and quantitatively. While hydrogen peroxide mainly affects the elasticity of the membrane protein skeleton (increasing the membrane shear modulus), cumene hydroperoxide has an impact on both membrane skeleton and lipid bilayer mechanical properties, as can be seen from the increased values of the shear and bending elastic moduli. The biologically important implication of these results is that the effects of oxidative stress on the biophysical properties, and hence the physiological functions, of the cell membrane depend on the nature of the oxidative agent. Thermal fluctuation spectroscopy provides a means of characterizing these different effects, potentially in a clinical milieu.     

The Presence of Novel Plant Growth Regulators in Leaves of Distylium racemosum Sieb et Zucc

The scaling law derived from the percolation theory was applied to the concentration dependence of mechanical properties of polyacrylamide measured near the sol–gel transition point. The critical concentration of the sol–gel transition, ?_g, was estimated from the plot of concentration (?) vs. the reciprocal of viscosity (η) by extrapolating 1/η to zero. The critical exponent for the sol viscosity, s, which was estimated from the slope of the log(?_g–?) vs. log η plot was about 0.7. The estimated value of s was similar to the value predicted by the percolation theory based on the superconductor–normal conductor mixture model. The critical exponent for the gel elasticity, t, as estimated from the slope of the log(?–?_g) vs. log G′ plot, where G′ was the dynamic shear modulus of the gel at a frequency of 2Hz. The value of t was about 2, which was also similar to the value predicted by the percolation theory. These results indicated the at the concentration dependences of η and G′ of polyacrylamide near the sol–gel transition point were described by the percolation theory.     

The human erythrocyte membrane skeleton may be an ionic gel. III. Micropipette aspiration of unswollen erythrocytes

We have carried out a theoretical analysis of micropipette aspiration of unswollen erythrocytes using the protein-gel-lipid-bilayer membrane model and taking into account that the modulus of area compression of the membrane skeleton may depend on the environmental conditions. Our analysis shows that the aspiration pressure needed to obtain a certain membrane projection length is strongly dependent on the ratio between the membrane skeleton modulus of area compression and the elastic shear modulus. Our analysis therefore predicts that micropipette aspiration of unswollen erythrocytes may be a sensitive method for detection of changes in this ratio. The analysis thus also shows that micropipette aspiration of unswollen erythrocytes can not be used to determine the membrane shear modulus unless something is known about the membrane skeleton modulus of area compression.     

Study of human erythrocyte membrane protein interactions by selective solubilization of Triton-skeletons

Summary— The membrane skeleton, responsible for shape and mechanical properties of the red cell, was purified by the Triton extraction procedure in presence of 5 mM, 150 mM or 600 mM NaCl. The proportion of spectrin, protein 4.1 and actin present in erythrocyte skeletons does not depend on the molarity of NaCl used. In contrast ankyrin, protein band 3 and protein 4.2 are removed from skeletons as the ionic strength increased. Solubilization assays of membrane skeletons were used to study protein interactions inside the skeleton. Solubilization was performed by Tris, a non-selective disruptive reagent, or by p-mercuribenzene sulfonic acid (PMBS), which principally release spectrin and actin. Tris action was assessed by calculation of the percentage of solubilized proteins, which increased proportionally with Tris molarity. PMBS action was kinetically determined as the decrease in skeleton turbidity. With these two reagents, we observed a lower dissociation of skeletons prepared with high ionic strength buffer. Erythrocyte pretreatment with okadaic acid, an inhibitor of serine-threonine phosphatases, revealed a phosphorylation-induced skeleton gelation and a better resistance to Tris-solubilization.     

Viscoelasticity of packed erythrocyte suspensions subjected to low amplitude oscillatory deformation.

Concentrated adult erythrocyte suspensions were subjected to low amplitude oscillatory shear in a Weissenberg rheogoniometer equipped with a cone-and-plate assembly. The dynamic viscoelastic properties of the suspension were measured over a broad range of frequency by a numerical solution that accounted for fluid inertia. Variation of shear amplitude and cell volume percent, and comparison of buffered saline, plasma, and dextran as suspending media showed that the cellular elements had undergone small bending and shearing deformations. Studies of normal adult erythrocytes, hypotonically swollen cells, temperature-altered cells, and erythrocyte ghosts suggested that the method was evaluating membrane material properties. The normal membrane was found to exhibit a shear rate dependent elastic modulus that increased by more than a factor of 20 over a frequency range from 0.0076 Hz to 60 Hz. The membrane viscosity showed a substantial drop with frequency indicative of a frequency thinning phenomenon. At high frequency of deformation the viscous response of normal erythrocytes was no longer indicative of a membrane property due to the dominant influence of the internal hemoglobin solution. The studies generally supported the ability of the method to quantify relative membrane material properties and detect changes in membrane structure.     

Model of red blood cell membrane skeleton: electrical and mechanical properties

A theoretical membrane skeleton model of erythrocyte has been developed and successfully applied to interpret electrical and mechanical properties of the red blood cell spectrin-actin network. The model is based on the structure of the membrane skeleton that is comprised of unit cells each containing an actin protofilament and shooting forth a few spectrin heterodimers. The loose ends of the heterodimers of adjacent cells can form bonds with each other giving rise to an integrated network. The number of bonds depends on the temperature. The bond length being excessive (2.6 times the distance between the centers of adjacent cells), the bonds are flexible, and can thus be regarded as entropy springs. The advanced model has been employed to calculate the shear modulus of the membrane skeleton as well as to establish its temperature dependence. In a wide range of temperatures mu(T) is a decreasing function well fitting the experimental data. The relationship between the membrane bilayer-free size of the skeleton and the ionic strength of the solution has been derived to appear in good agreement with the results obtained previously. Experimental data combined with the advanced theory yield the average number of heterodimers per unit cell, m0, as equal to ca. 5; the spectrin heterodimer charge has been estimated.     

Electrogenic bicarbonate secretion in the turtle bladder: Apical membrane conductance characteristics

Summary We have recently shown that stimulation of electrogenic HCO ₃ ^– secretion is accompanied by a simultaneous increase in short-circuit current (I _sc, equivalent to HCO ₃ ^– secretion rate under these conditions), apical membrane capacitance (C _a , proportional to membrane area), and apical membrane conductance (G _a , proportional to membrane ionic permeability). The current experiments were undertaken to explore the ionic basis for the increase inG _a and the possibility that the rate of electrogenic HCO ₃ ^– secretion is regulated by changes inG _a . Membrane electrical parameters were measured using impedance-analysis techniques before and after stimulation of electrogenic HCO ₃ ^– secretion with cAMP in three solutions which contained different chloride concentrations. In another series of experiments, the effects of an anion channel blocker, anthracene-9-carboxylic acid (9-AA), were measured after stimulation of electrogenic HCO ₃ ^– secretion with cAMP. The major conclusions are: (i) a measurable apical Cl^– conductance exists in control hemibladders; (ii) the transport-associated increase inG _a includes a Cl^–-conductive component; (iii)G _a also appears to reflect a HCO ₃ ^– conductance; (iv) the relative magnitudes of the apical membrane conductances to Cl^– and HCO ₃ ^– are similar; (v) 9-AA reducesG _a andI _sc appear cAMP-stimulated hemibladders; and (vi) alterations inI _sc appear to be mediated by changes inG _a .     

Determination of the number of cross-links in a protein gel from its mechanical and swelling properties

The relation between the chemical structure of a protein and the physical properties of a heat-set gel of that protein has been investigated. The physical properties of the gel are determined by means of mechanical experiments in which the viscoelastic properties of the gel are determined in terms of the storage shear modulus, the loss modulus and the stress-strain curve. The storage shear modulus defined the solid (elastic) character of the gel. The chemical structure of the protein and the nature of the solvent determine the nature and number of cross-links in the gel. The cross-links in gels formed by heating concentrated solutions of ovalbumin in 6M urea solutions were found to be disulfide bridges and the mechanical properties of these ovalbumin/urea gels approximated those of an ideal rubber. The latter finding enables one to calculate the number of cross-links per ovalbumin molecule from the value of the storage modulus, using the classical theory of rubber elasticity. This theory, together with the Flory-Huggins lattice model, can also be used to calculate the number of cros-links per ovalbumin molecule from the swelling behavior of ovalbumin/urea gels. The number of cross-links per ovalbumin molecule calculated from these two types of experiments are in mutual agreement and correspond with the number of thiol groups in ovalbumin. We conclude, thereforee, that theories of polymer physics can be used to relate the chemical structure of a protein to the physical properties of its gel.     

Electrical properties of the cellular transepithelial pathway inNecturus gallbladder: III. Ionic permeability of the basolateral cell membrane

Summary The ionic permeability of the basolateral membrane ofNecturus gallbladder epithelium was studied with intracellular microelectrode techniques. After removal of most of the subepithelial tissue (to reduce unstirred layer thickness), impalements were performed from the serosal side, and ionic substitutions were made in the serosal solution while a microelectrode was kept in a cell. Thus, it was possible to obtain continuous (and reversible) records of transepithelial and cell membrane potentials and to measure intermittently the transepithelial resistance and the ratio of cell membrane resistances. From these data and the mean value of the equivalent resistance of the cell membranes in parallel (obtained from cable analysis in a different group of tissues), absolute cell membrane and shunt resistances and equivalent electromotive forces (emf's) were calculated. From the changes of basolateral membrane emf (E _b ) produced by the substitutions, the conductance (G) and permeability (P) of the membrane for K, Cl and Na were estimated. Potassium-for-sodium substitutions produced large reductions of both cell membrane potentials, ofE _b , and of the resistance of the basolateral membrane (R _b ), indicating highG _K andP _K . Chloride substitution with isethionate or sulfate resulted in smaller changes of cell membrane potentials andE _b and in no significant change ofR _b , indicating small but measurable values ofG _Cl andP _Cl . Sodium substitutions with N-methyl-d-glucamine (NMDG) resulted in cell potential changes entirely attributable to the biionic potential produced in the shunt pathway (P _Na >P _NMDG ), and in no significant changes ofP _b orE _b , indicating thatG _Na andP _Na are undetectable. The question of the mechanism of Cl transport across the basolateral membrane was addressed by comparing the mean rate of transepithelial Cl transport (J _Cl ^net ) and the predicted passive Cl flux across the basolateral membrane (from the membrane Cl conductance, potential, and Cl equilibrium potential). The conclusion is that only a very small fraction of the Cl flux across the basolateral membrane can be electrodiffusional. Since the paracellular Cl conductance is also too low to account forJ _Cl ^net , these results suggest the presence of a neutral mechanism of Cl extrusion from the cells. This could be a NaCl pump, a downhill KCl transport mechanism, or a Cl–HCO₃ exchange mechanism.     

First principles investigation of pressure dependent stability,phonon, Debye temperature,physical, mechanical and thermodynamic properties of Rh3Al intermetallic compound

ABSTRACT

Pressure dependence of stability, phonon, Debye temperature, physical, mechanical and thermodynamic properties of Rh₃Al intermetallic compound were investigated by first-principles The calculated cohesive energy (E_c), formation enthalpy (ΔH) show that Rh₃Al is a thermodynamically stable compound. Properties related to the phonons of Rh₃Al were also obtained. In addition, the transverse sound velocity (ν_s), longitudinal sound velocity (ν_l), average sound velocity (ν_m) and Debye temperature (Θ_D) of Rh₃Al were calculated by using the VRH method along with pressure range from 0 to 60?GPa. The values of lattice parameters, bulk modulus and its first-order pressure derivative are consistent well with other works. The band structure indicates that Rh₃Al compound exhibits a metallic character. Moreover, the total density of states, partial density of states, Mulliken charges and electron density difference have been analysed to explain the physical properties. Based on the stress–strain approach and the Born stability criteria, the mechanical properties were evaluated by elastic constants (C_ij), other modulus (B, E, G), (B/G) ratio, Poisson’s ratio (ν), the anisotropic index (A), hardness (H) and compressibility (K) for this intermetallic compound. Finally, the thermodynamic properties, including enthalpy, free energy, entropy and heat capacity are discussed range from 0 to 1000?K.     

Two-component coarse-grained molecular-dynamics model for the human erythrocyte membrane

We present a two-component coarse-grained molecular-dynamics model for simulating the erythrocyte membrane. The proposed model possesses the key feature of combing the lipid bilayer and the erythrocyte cytoskeleton, thus showing both the fluidic behavior of the lipid bilayer and the elastic properties of the erythrocyte cytoskeleton. In this model, three types of coarse-grained particles are introduced to represent clusters of lipid molecules, actin junctions, and band-3 complexes, respectively. The proposed model facilitates simulations that span large length scales (approximately micrometers) and timescales (approximately milliseconds). By tuning the interaction potential parameters, we were able to control the diffusivity and bending rigidity of the membrane model. We studied the membrane under shearing and found that at a low shear strain rate, the developed shear stress was due mainly to the spectrin network, whereas the viscosity of the lipid bilayer contributed to the resulting shear stress at higher strain rates. In addition, we investigated the effects of a reduced spectrin network connectivity on the shear modulus of the membrane.     

Erythrocyte membrane proteins and membrane skeleton

Considerable advances in the research field of erythrocyte membrane were achieved in the recent two decades. New findings in the structure-function correlation and interactions of erythrocyte membrane proteins have attracted extensive attention. Interesting progress was also made in the molecular pathogenesis of erythrocyte membrane disorders. Advances in the composition, function and interaction of erythrocyte membrane proteins, erythrocyte membrane skeleton, and relevant diseases are briefly described and summarized here on the basis of domestic and world literatures. Translated from Life Science Research, 2005, 9(4): 283–291 [译自: 生命科学研究]     

Basolateral membrane potassium conductance is independent of sodium pump activity and membrane voltage in canine tracheal epithelium

Summary When secretagogues stimulate Cl secretion in canine tracheal epithelium, apical membrane Cl conductance (G _a ^Cl ) increases, and then basolateral membrane K conductance (G _b ^K ) increases. Conversely, inhibition ofG _a ^Cl results in a secondary decrease inG _b ^K . The coordination of the two membrane conductances and regulation ofG _b ^K is critical for maintaining constant intracellular ion concentrations and transepithelial Cl secretion. The purpose of this study was to test two hypotheses about the regulation ofG _b ^K . First, we asked whetherG _b ^K is directly linked to the activity of the Na,K-ATPase. We found that pump activity could be dissociated from K conductance. Inhibition of the Na pump with ouabain, in nonsecreting tissues led to an increase inG _b . Elevation of the bathing solution K concentration produced a similar effect. Addition of ouabain to secreting tissues did not appear to alterG _b . These results indicate thatG _b ^K does not directly parallel Na pump activity. Second, we asked whether changes inG _b ^K are voltage dependent. We prevented secretagogue-induced depolarization of the electrical potential difference across the basolateral membrane _b by clamping _b at its resting value during stimulation of Cl secretion with epinephrine. Despite maintaining _b constant, the typical changes in transepithelial resistance and the ratio of membrane resistances persisted. This observation indicates that depolarization is not required for the secretagogue-induced increase inG _b ^K . In addition we examined the effect of depolarizing and hyperpolarizing _b by passing transepithelial current in secreting and nonsecreting epithelia. Despite depolarizing and hyperpolarizing _b within the physiologic range, we observed no significant changes in transepithelial resistance or the ratio of membrane resistance that would suggest a change inG _b ^K . This observation indicates that changes in _b are not sufficient to alterG _b ^K . Thus,G _b ^K appears to be regulated by factors other than membrane voltage, or direct coupling to the Na pump.     

Dynamic viscoelastic properties of solutions of shear-degraded deoxyribonucleic acid

Calf thymus and salmon sperm deoxyribouncleie acid were degraded by high-shear stirring to molecular weights M in the range of 1.3–3.2 × 10⁶ and purified by chromatography on methylated bovine serum albumin. Dynamic viscoelastic properties of the fragmented products, in aqueous glycerol solutions in the concentration range of c = 0.003–0.01 g./ml., were investigated with the apparatus of Birnboim and Ferry. At values of the product cM higher than 4 × 10³, the frequency dependence of the components of the complex shear modulus, G′ and G″, displayed a plateau region in which G′ > G″ – ων₁η_S, similar to that observed in concentrated solutions of coiling polymers where it is attributed to an entanglement network (ω is radian frequency, ν₁ volume fraction of solvent, and η₈, solvent viscosity). The width of this plateau region on the logarithmic frequency scale is given by Δ = 3.8 (log cM – 3.56). At lower values of cM, the frequency dependence is intermediate between those predicted by the theory of Zimm for flexible coiled macromolecules and by the theory of Kirkwood and Auer for rods. Fitting to the Zimm theory gives highly discrepant values for molecular weights, while fitting the low-frequency end of the dispersion to the Kirkwood-Auer theory gives reasonable agreement for both molecular weight and rotary diffusion coefficient. It is concluded that the helical fragments appear as nearly rigid rods in their behavior at very low frequencies, but at higher frequencies reveal substantial bending flexibility.     

Phase stability,elastic, anisotropic and thermodynamic properties of HoT2Al20 (T = Ti,V, Cr) intermetallic cage compounds

The structural stability, elastic properties, anisotropy, dynamics stability and thermodynamics properties were explored for pure Al and HoT₂Al₂₀ intermetallics from the first-principles method. The formation enthalpy and phonon frequencies indicate that these HoT₂Al₂₀ intermetallics maintain structural stability. The elastic constants C_ijs and moduli B, G, E and H_v indicate these intermetallics possess higher hardness and the better resistance to deformation. The values of Poisson’s ratio and B/G demonstrate that HoT₂Al₂₀ intermetallics are brittle materials. The anisotropic constants and anisotropic acoustic velocities confirm that HoT₂Al₂₀ intermetallics exhibit anisotropic properties. Importantly, the calculated thermal quantities demonstrate that these new HoT₂Al₂₀ intermetallics possess the better thermodynamic properties at high temperature.