Low temperature Ga surface diffusion from focused ion beam milled grooves

Ga diffusion from focused ion beam (FIB) milled grooves has been studied using x-ray photoemission electron microscopy (XPEEM) and mirror electron microscopy (MEM). We analyze the surface chemistry of the FIB structures measuring the Ga presence in the top layers of the milled grooves and morphological defects inside the grooves. The Ga is initially strictly confined to the grooves. However, annealing at temperatures as low as 150 degrees C leads to rapid and significant Ga surface diffusion from the FIB structures. The out-diffused Ga forms a thin layer extending up to several microns laterally in a non-regular pattern. The diffusion is significantly enhanced at small crystallites at the edges of the grooves. We explain the general behavior with an atomic scale model in which interstitial Ga in the milled areas diffuses out and substitutes silanol groups on the surface.

Impact of oxygen coadsorption on intercalation of cobalt under the h-BN nanomesh

The process of penetration of cobalt atoms through the h-BN nanomesh on Rh(111) is investigated with both spectroscopic and microscopic techniques. It is discovered that oxygen coadsorption can drastically modify the physical properties and behavior of the deposited Co clusters upon postannealing. In the absence of oxygen, Co forms small nanoparticles in the pores (bonding parts) of the h-BN nanomesh, which start to agglomerate at elevated temperatures without any considerable intercalation. However, even a tiny amount of coadsorbed oxygen reduces cobalt agglomeration and greatly promotes its intercalation and trapping under h-BN. The oxygen exposure necessary for a complete intercalation of 1-2 monolayers of Co is very low, and the formation of oxidic species can be easily avoided. The nanomesh structure remains intact upon intercalating submonolayer amounts of Co, while further intercalation gradually distorts and finally destroys the periodic corrugation. Fortunately, this process is not accompanied by damaging the h-BN sheet itself, and the original structure can be restored by removing Co upon annealing at higher temperatures.

Direct observation of atomic scale surface relaxation in ortho twin structures in GaAs by XSTM

We have studied the (110) GaAs surface of a structure containing ortho twins by cross-sectional scanning tunnelling microscopy and we have compared the experimental results with ab initio density functional theory calculations and STM simulations. Both experimentally and theoretically we find that the surface of different twin crystallites are significantly displaced with respect to each other, parallel to the twin boundary. This result is explained by a surface relaxation of the atoms in the (110) GaAs surface and the difference between the atomic configuration of the ortho twins.

Step-orientation-dependent oxidation: from 1D to 2D oxides

Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.

Kinetics of the Reduction of the Rh(111) Surface Oxide: Linking Spectroscopy and Atomic-Scale Information

The reduction of the surface oxide on Rh(111) by H(2) was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H(2) does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H(2) pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use Johnson-Mehl-Avrami-Kolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements.

One-dimensional PtO2 at Pt steps: formation and reaction with CO

Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) oxide structure forms at the steps of the Pt(332) surface after exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.

Career stage and police cynicism

This study examined the relationship of police tenure and career stage and cynicism among 766 Norwegian constables and managers. Constables with less tenure and in an early career stage, but not managers, scored lower on cynicism.

A surface x-ray study of the structure and morphology of the oxidized Pd(001) surface

The oxidation of Pd(100) and the formation of PdO was studied in situ using surface x-ray diffraction. A bulklike, epitaxial PdO film is formed at oxygen partial pressures beyond 1 mbar and sample temperatures exceeding 650 K. The main orientation is PdO(001)/Pd(001), based upon bulk reflections from the PdO film. By comparing with measurements from the Pd crystal truncation rods, we estimate an rms surface roughness of 6 A, in good agreement with previous high pressure scanning tunneling microscopy measurements. Finally, we observed the transformation from the (radical5 x radical5) surface oxide to PdO bulk oxide at 675 K and 50 mbar O(2) pressure.

High-coverage oxygen structures on Rh111: adsorbate repulsion and site preference is not enough

A new O induced structure on Rh(111) displaying a (2 sqaureroot[3] x 2sqaureroot[3])R30 degrees periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the (2sqaureroot[3] x 2sqaureroot[3])R30 degrees and (2 x 2)-3O structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.

Self-limited growth of a thin oxide layer on Rh(111)

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.

Kinetic hindrance during the initial oxidation of Pd(100) at ambient pressures

The oxidation of the Pd(100) surface at oxygen pressures in the 10(-6) to 10(3) mbar range and temperatures up to 1000 K has been studied in situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions, we observe either a thin (sqrt[5]xsqrt[5])R27 degrees surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K.

Identification of step atoms by high resolution core level spectroscopy

Vicinal Rh(111) surfaces are studied with high resolution core level photoemission. We demonstrate the possibility to distinguish between the different kinds of surface atoms on these surfaces by virtue of their 3d core level binding energies. In particular, the low coordinated step atoms are found to exhibit a clear fingerprint in Rh 3d spectra. We demonstrate how this may be used to show that initial oxygen adsorption occurs on the steps and not on the terraces of the vicinal surfaces.

Improving subjective health at the worksite: a randomized controlled trial of stress management training, physical exercise and an integrated health programme

Our objective was to evaluate the effect of 12 weeks of stress management training (SMT), physical exercise (PE) and an integrated health programme (IHP) in a worksite setting on subjective health complaints. To do this, we randomly split 860 employees into the following groups: control (n = 344), PE (n = 189), IHP (comprising physical exercise and health information) (n = 165) and SMT (n = 162). There were no significant effects on subjective health complaints, sick leave or job stress. However, strong and specific positive effects were experienced for the particular goal areas defined for each intervention. The PE group showed improved general health, physical fitness and muscle pain, while the SMT group showed improved stress management. The IHP group showed the strongest effects, affecting most goals set for treatment.

Unusual Multilayer Surface Alloy: Al(100)-c(2 x 2)-2Li

An unusual multilayer surface alloy is formed by adsorption of one monolayer Li on Al(100). It is shown by low energy electron diffraction that the first three layers consist of a mixed Al-Li layer, a pure Al layer, and a second Al-Li layer. Thus the alloy has the same layer stoichiometry as the (100) surface of the metastable Al(3)Li bulk alloy. However, the relative orientation of the two mixed layers is the same as that in the Al(3)Ti-type structure. These findings are confirmed by total-energy calculations, which lead further to the prediction that the bulk Al(3)Li alloy has a faulted, Al(3)Ti-type surface.

Deconvolution can be used in electrooptic studies to correct for non-ideal electric excitation pulses only when the electric dipole moment of the studied molecules is predominantly induced

The electric field pulses used for most measurements of transient electrooptic properties such as birefringence and dichroism, are rectangular and assumed to be ideal, but in practice do all such pulses have non-zero rise and fall times. Claims have been made that this non-ideality may be taken into account by employing standard deconvolution techniques. We find that this approach yields exact results in the zero electric field limit when the electric dipole moment of the studied macromolecules is predominantly induced. However, for finite electric field strengths and/or macromolecules with partly or fully permanent electric dipole moments, we find that the deconvolution method yields erroneous estimates of the electrooptic relaxation times. When the decay time of the electric pulse and the electrooptic decay time are equal, and the system is operated in the Kerr domain, this systematic error for macromolecules with purely permanent electric dipole moment equals 37%. In a research field where the uncertainty of the reported relaxation times normally is assumed to be only a few percent this is an error that may seriously mislead unsuspecting users. We find that this systematic error can readily be avoided by employing standard numerical integration of a set of coupled first-order differential equations instead of the standard deconvolution techniques.

Development of human oocytes matured in vitro for 28 or 36 hours

OBJECTIVE

To compare the effects on human oocytes of in vitro maturation periods of 28 hours and 36 hours.

DESIGN

Retrospective analysis.

SETTING

University teaching hospital.

PATIENT(S)

A total of 48 infertile patients undergoing 55 cycles who volunteered for the experimental treatments.

INTERVENTION(S)

Immature oocytes were aspirated with use of transvaginal ultrasonography. Oocytes were matured, fertilized by intracytoplasmic sperm injection, and cultured for 2.5 or 3 days before being replaced into the uterus.

MAIN OUTCOME MEASURE(S)

Oocyte maturation, fertilization and cleavage rates, and clinical pregnancy rate.

RESULT(S)

Maturation of oocytes for either 28 hours or 36 hours before insemination resulted in similar rates of maturation, fertilization, and cleavage. The clinical pregnancy rate was similar in both groups (14%-15%).

CONCLUSION(S)

Shortening the in vitro maturation period does not compromise subsequent embryonic development.

Impact of a participatory organizational intervention on job characteristics and job stress

Increased employee control and participation are recommended to achieve both "flexible organization" and improvements in health, as outlined in occupational stress intervention models. This study evaluates the impact of a participatory organizational intervention on job stress and job characteristics. The intervention was carried out in two post offices in the Norwegian Postal Service. "Local theories" were seen as the key drivers for organizational improvement and increased control. The underlying dynamics of the intervention were to manipulate employees' learning opportunity and decision authority so as to improve work environment and health. Work groups, in dialogue with a steering committee, conducted diagnosis, action planning, and action taking. Work conditions deteriorated during the observation period in the control groups. In one of the intervention groups, this negative trend was reduced by the intervention. Lack of positive results in the other intervention group may have been due to organizational restructuring and turbulence.

Inhomogeneous alginate gel spheres: an assessment of the polymer gradients by synchrotron radiation-induced X-ray emission, magnetic resonance microimaging, and mathematical modeling

It has been previously demonstrated that calcium alginate gels prepared by dialysis often exhibit a concentration inhomogeneity being the polymer concentration considerably lower in the center of the gel than at the edges. Inhomogeneity may be a preferred structure in microcapsules due to low porosity and higher stability so that it is interesting to evaluate the polymer gradient in spherically symmetrical small alginate beads (1.0-0.7 mm diameter) obtained in different conditions. In this paper, two complementary techniques have been used to investigate this aspect. The concentration gradient of alginate has been analyzed by measuring both the spatial distribution of calcium ions in sections of alginate gel spheres, by means of x-ray fluorescence spectroscopy, and the T2 relaxation behavior on intact gel beads using magnetic resonance microimaging. The experimentally determined gradients from three-dimensional gels provide data to reevaluate the parameter estimates in the recently reported mathematical model for alginate gel formation (A. Mikkaelsen and A. Elgsaeter, Biopolymers, 1995, Vol. 36, pp. 17-41). The model may account for the gels being less inhomogeneous when nongelling sodium or magnesium ions are added during gelation.

Transient electric birefringence of human erythroid spectrin dimers and tetramers at ionic strengths of 4 mM and 53 mM

In conventional electrooptic studies the sample ionic strength must for technical reasons be kept below about 3 mM, which is only 2% of the ionic strength at physiological conditions. In particular for flexible polyelectrolytic macromolecules it can in general not be ruled out that both the conformational average and dynamics at ionic strength 3 mM and below may differ significantly from what it is at physiological conditions. Here we report on the first electrooptic study of human erythroid spectrin dimers and tetramers at ionic strengths higher than 3 mM. All measurements in this study were carried out at both ionic strength 4 mM (2.5 mM HEPES + 1 mM NaCl) and 53 mM (2.5 mM HEPES + 50 mM NaCl). Spectrin tetramers were studied only at 4 degrees C whereas the dimers were studied at both 4 degrees C and 37 degrees C. At 4 degrees C there is a striking quantitative similarity between the transient electric bire-fringence (TEB) of spectrin dimers and tetramers. Also, the TEB of spectrin dimers at 37 degrees C was very similar to the results at 4 degrees C. The contour length and the molecular weight of spectrin dimers and tetramers are known. The dominating TEB relaxation time is in all cases only a fraction of what is predicted theoretically if the spectrin dimers and tetramers are assumed to be stiff and extended molecules. In sum, the new TEB data constitute strong electrooptic evidence confirming that spectrin dimers and tetramers have a highly flexible structure, and demonstrate for the first time that a major part of the intrachain dynamics of the spectrin is quite insensitive to an increase of the ionic strength from 4 mM to 53 mM. Use of the reversing electric field pulse technique for all conditions studied yields TEB data suggesting that the orientation of both spectrin dimers and tetramers in an electric field is dominated by a permanent rather than an induced electric dipole moment.

Electric birefringence of recombinant spectrin segments 14, 14-15, 14-16, and 14-17 from Drosophila alpha-spectrin

Members of the spectrin protein family can be found in many different cells and organisms. In all cases studied, the major functional role of these proteins is believed to be structural rather than enzymatic. All spectrin proteins are highly elongated and consist mainly of homologous repeats that constitute rigid segments connected in tandem. It is commonly believed that the details of the spectrin function depend critically on the flexibility of the links between the segments. Here we report on a work addressing this question by studying the transient electric birefringence of recombinant spectrin fragments consisting of segments 14, 14-15, 14-16, and 14-17, respectively, from Drosophila alpha-spectrin. Transient electric birefringence depends sharply on both molecular length and flexibility. We found that the birefringence relaxation time of segment 14 measured at 4 degrees C, but scaled to what is expected at 20 degrees C, equals 16 ns (+/-15%) at pH 7.5 and ionic strength 6 mM. This is consistent with this single segment being rigid, 5 nm long and having an axial ratio equal to about two. Under the same conditions, segments 14-15, 14-16 and 14-17 show relaxation times of 45, 39 and 164 ns (all +/-20%), respectively, scaled to what is expected at 20 degrees C. When the temperature is increased to 37 degrees C the main relaxation time for each of these multisegment fragments, scaled to what is expected at 20 degrees C, increased to 46, 80, and 229 ns (all +/-20%), respectively. When the ionic strength and the Debye shielding is low, the dynamics of these short fragments even at physiological temperature is nearly the same as for fully extended weakly bending rods with the same lengths and axial ratios. When the ionic strength is increased to 85 mM, the main relaxation time for each of these multisegment fragments is reduced 20-50% which suggests that at physiological salt and temperature conditions the links in 2-4-segment-long fragments exhibit significant thermally induced flexing. Provided that the recombinant spectrin fragments can serve as a model for native spectrin, this implies that, at physiological conditions, the overall conformational dynamics of a native spectrin protein containing 20-40 segments equals that of a flexible polymer.

Presence and dehydration of ikaite, calcium carbonate hexahydrate, in frozen shrimp shell

Ikaite, calcium carbonate hexahydrate, has by means of X-ray diffraction analyses of frozen samples been identified as the mineral component of the white spots formed in the shell of frozen shrimp during storage. When the shrimp thaw and the shell material is dried and kept at room temperature, ikaite rapidly transforms into a mixture of anhydrous calcium carbonate forms. X-ray diffraction analyses and Raman spectra of synthetic ikaite as well as the dehydration product confirm the assignments, and the rate constant for dehydration is approximately 7 x 10(-)(4) s(-)(1) at ambient temperature. Differential scanning calorimetry showed that dehydration of synthetic ikaite is an entropy-driven, athermal process and confirms that a single first-order reaction is rate-determining. Ikaite is found to be stable in aqueous solution at temperatures below 5 degrees C and in the shell of frozen shrimps but decomposes on thawing to form anhydrous calcium carbonates.

Electrooptic analysis of macromolecule dipole moments using asymmetric reversing electric pulses

The use of symmetric reversing electric field pulses in electrooptic studies of rigid macromolecules in order to determine the ratio between the permanent and the induced dipole moments is well established. Application of this method to studies of small macromolecules requires a field reversal time of only a few nanoseconds. No high current pulse generator capable of producing symmetric kV pulses with such a short reversal time is available for studies of small macromolecules in physiological salt solutions, but it has long been known how to make such reversing pulses that are asymmetric. In order to take advantage of the opportunity offered by the latter fact, we here present a theoretical analysis in the thermal domain of the electrooptic properties of solutions containing rigid macromolecules with axial symmetry when exposed to asymmetric reversing electric field pulses. The analytical expressions needed for quantitative determination of the ratio between the permanent and the induced electric dipole moments of rigid macromolecules using electrooptic data obtained employing reversing electric pulses with given asymmetry are presented. The feasibility of this new approach is demonstrated by including experimental electric birefringence data for a 12 kDa protein (segment 14 of alpha-spectrin from Drosophila brains) in near physiological salt solutions obtained using a coaxial cable pulser producing 2 microseconds long pulses with a reversal time of about 15 ns.

Acid-catalysed reduction of ferrylmyoglobin: product distribution and kinetics of autoreduction and reduction by NADH

The pH dependence of iron(II)/iron(III) product distribution, following reduction of the hypervalent iron in equine ferrylmyoglobin by the protein moiety of the pigment (so-called autoreduction) and by NADH (nicotinamide adenine dinucleotide, reduced) and the rate of reduction was found to depend different on pH. Autoreduction is specific acid catalysed and has a more modest temperature dependence than autoxidation of oxymyoglobin, with the activation parameters delta H# = 58.5 +/- 0.4 kJ.mol-1 and delta S# = 2.7 +/- 0.1 J.mol-1.K-1 in 0.16 mol.l-1 NaCl. The product of autoreduction is the iron(III) pigment metmyoglobin, which is slightly modified in the protein moiety. The reaction has a positive kinetic salt effect from which it is deduced that the reactive centre of ferrylmyoglobin has a charge of +1 in agreement with the structure Fe(IV) = O. Reduction by NADH involves parallel reactions of two pigment forms in acid/base equilibrium with each other with a pKa equal to 4.9, both forms yielding metmyoglobin as well as the iron(II) pigment, oxymyoglobin, as products. The protonated form reacts faster than the deprotonated form, and two-electron transfer has greater importance for the protonated form with a limiting Fe(II)/Fe(III) product ratio of 0.6 in acidic solution compared to 0.12 in alkaline solution. A square root dependence of rate on NADH concentration suggests involvement of NAD.radicals with a disproportionation as the termination reaction.

The menopausal syndrome: a factor analytic replication

The authors contend that conclusions regarding the existence of a menopausal syndrome based on clinical trials with oestrogen therapy cannot be generalized to the general population. As an alternative strategy for investigating the nature of the menopausal syndrome the authors performed a factor analysis of the results from a symptom checklist. The list was included in a postal questionnaire sent to a representative sample of 2349 women aged between 45 and 55 years residing in the city of Oslo, Norway. A varimax, rotated five-factor solution yielded the best result both theoretically and empirically. The five factors were labelled as vague somatic complaints, nervousness, mood lability, vasomotor complaints and urogenital complaints. Five variables were constructed on the basis of the factor scores. One-way analysis of variance showed that vasomotor complaints associated with excessive sweating, hot flushes and vaginal dryness, constituted the only variable significantly related to menopausal development. The latter was measured according to time elapsed since the last menstrual period.

Psychosocial determinants of climacteric complaints

A total of 2349 Norwegian women aged 45-55 years, were investigated using postal questionnaires. Associations between menopausal development, psychosocial factors and climacteric complaints were analyzed. Factor analysis of 24 climacteric complaints identified five factors (vague somatic complaints, nervous complaints, mood lability, vasomotor and urogenital complaints) which were analyzed in relation to stage of menopausal development and a number of psychosocial variables. Five types of variable contributed to the variance in the five previously identified factors, viz. style of reacting to menstruation earlier in life, mother's climacteric complaints, negative expectations regarding the menopause, social network, sociodemographic factors, and chronological age. Menopausal development played a modest role in explaining the variance in all except vasomotor complaints, the latter being associated with current cigarette smoking. Traditional sex-role identification was associated with nervous complaints. The authors conclude that hot flushes, excessive sweating and vaginal dryness are the only complaints clearly attributable to menopausal development.

Frequency dependence of the shear moduli of spectrin studied using a multiple lumped resonator viscoelastometer

The frequency dependence (119-7860 Hz) of the storage and loss shear moduli, G' and G'', of human erythrocyte spectrin dimer crude solutions at 22.5 degrees C has been measured using a Birnboim-Schrag multiple lumped resonator viscoelastometer. The measurements were carried out on solutions of ionic strength 1 mM containing 1.1-3.7 mg ml-1 spectrin. This corresponds to the terminal zone for G' and G''. Analysis of the data using the standard theory of hybrid relaxation spectra yields a relaxation time of 22.5 +/- 1 microseconds. The pure spectrin dimer relaxation time is estimated to be 16 +/- 3 microseconds. This result suggests that at an ionic strength of 1 mM, the spectrin dimers are extended and that the main relaxation process is simple end-over-end rotation.

The molecular basis of erythrocyte shape

Recent discoveries about the molecular organization and physical properties of the mammalian erythrocyte membrane and its associated structural proteins can now be used to explain, and may eventually be used to predict, the shape of the erythrocyte. Such explanations are possible because the relatively few structural proteins of the erythrocyte are regularly distributed over the entire cytoplasmic surface of the cell membrane and because the well-understood topological associations of these proteins seem to be stable in comparison with the time required for the cell to change shape. These simplifications make the erythrocyte the first nonmuscle cell for which it will be possible to extend our knowledge of molecular interactions to the next hierarchical level of organization that deals with shape and shape transformations.

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.

The human erythrocyte membrane skeleton may be an ionic gel. I. Membrane mechanochemical properties

Biochemical and biophysical observations indicate that the erythrocyte membrane skeleton is composed of a swollen network of long, flexible and ionizable macromolecules located at the cytoplasmic surface of the fluid membrane lipid bilayer. We have analyzed the mechanochemical properties of the erythrocyte membrane assuming that the membrane skeleton constitutes an ionic gel (swollen ionic elastomer). Using recently established statistical thermodynamic theory for such gels, our analysis yields mathematical expressions for the mechanochemical properties of erythrocyte membranes that incorporate membrane molecular parameters to an extent not achieved previously. The erythrocyte membrane elastic shear modulus and maximum elastic extension ratio predicted by our membrane model are in quantitative agreement with reported values for these parameters. The gel theory predicts further that the membrane skeleton modulus of area compression, KG, may be small as well as large relative to the membrane elastic shear modulus, G, depending on the environmental conditions. Our analysis shows that the ratio between these two parameters affects both the geometry and the stability of the favoured cell shapes.

The human erythrocyte membrane skeleton may be an ionic gel. II. Numerical analyses of cell shapes and shape transformations

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.

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.

A computerized low-shear pendulum viscoelastometer, stress-relaxation, shear creep, and dynamic elastic moduli measurements of soft biogels

A computerized low-shear pendulum viscoelastometer that is well suited for measurements of the viscoelastic properties of soft biogels is described. The instrument has three modes of operation: (i) measurement of dynamic elastic moduli, (ii) stress relaxation, and (iii) shear creep-creep recovery. The main mechanical parts of the instrument include a specimen cuvette and a pendulum with a thin metal plate extending into the specimen cuvette. The cuvette and pendulum can undergo independent angular movements relative to a horizontal axis located about 12 cm above the specimen cuvette. The absolute angular position of the cuvette is under on-line computer control through a stepping motor driven micrometer. The angular position of the pendulum relative to the cuvette is measured using an inductive position detector. The necessary software for automatic operation, data acquisition and processing has been developed for all the three modes of operation. The instrument is well suited for measurements of dynamic elastic moduli in the range 0.02-150 dyn/cm2. Calibration data obtained using 100% ethanol are presented. The instrument suitability for measurements of stress relaxation and shear creep-creep recovery is illustrated by presenting the results of such measurements on a 5.5-mg/ml erythrocyte spectrin gel in isotonic solution.