Viscoelastic properties of aqueous solutions of an exocellular polysaccharide from cyanobacteria

The viscoelastic properties of aqueous solutions of the exocellular polysaccharide of Cyanospira capsulata have been studied, over a wide range of polymer concentrations, using small deformation oscillatory, steady and transient shear methods. The viscoelastic spectra generally resemble those of an entangled network, although notable deviations can be observed in the low frequency dependence of G′ and G″. At higher polymer concentrations, the viscoelastic spectrum shows solid-like behaviour over a wide range of frequencies. The superposition of η^*(ω) and η( ) curves occurs only at low frequencies, at higher frequencies the slope of η^*(ω) is lower than that of η( ). By studying the time evolution of shear stress after the inception of a steady shear rate (stress overshoot), the recovery of non-linear properties after steady shearing flow is seen to occur after times of c. 10³ s (in the case of 1·1% w/v solutions).

The overall viscoelastic properties appear original in comparison with those of the two structurally limiting types of polysaccharide, the ‘ordered’ chain xanthan and the ‘random coil’ guar. A rationale for this ‘anomalous’ viscoelastic behaviour can be tentatively proposed in terms of flickering intermolecular cross-interactions between semi-flexible segments, which occur in addition to the usual topological constraints.     

Viscoelastic properties of human tracheobronchial mucin in aqueous solution

Human tracheobronchial mucin isolated from cystic fibrosis patients (CF HTBM) was purified using a combination of gel filtration and density gradient centrifugation. The resulting mucin was fractionated to reduce polydispersity and to facilitate studies of the molecular weight dependence of mucin viscoelasticity in concentrated solution. The viscoelastic properties of CF HTBM were examined in distilled water, 0.1M salt solutions and chaotropic solvents. In controlled strain experiments (strain ≥ 5%) with increasing mucin concentration, a crossover from sol to gel behavior is observed. The gel strength, as measured by the magnitude of the storage modulus at comparable mucin concentrations, is greatest for distilled water, intermediate for 0.1M NaCl, and lowest far 6M GdnHCl. In distilled water, high molecular weight mucin undergoes a sol-gel transition at ～ 12 mg/mL, and shows evidence of a plateau modulus at higher concentrations. The storage and loss moduli of concentrated high molecular weight fractions in 6M GdnHCl exhibit a power law dependence on frequency typical of weak gels near the sol–gel transition at 20 mg/mL. Similar rheology is observed in 0.1M NaCl and 0.091M NaCl/3 mM CaCl₂, but with evidence for additional weak associations at low frequency. The power law exponent in these systems is 0.70 ± 0.02, in good agreement with prediction for networks formed by a percolation mechanism. Low molecular weight fractions in these solvents exhibit a fluid-like viscoelastic response. However, low molecular weight mucin in distilled water shows a strain-dependent increase in elasticity at low frequency indicative of weak intermolecular associations. Comparison of the rheological behavior of CF HTBM with our earlier studies of ovine submaxillary mucin lends support to the idea that carbohydrate side-chain interactions are important in the gelation mechanism of mucins. © 1995 John Wiley & Sons, Inc.     

Viscoelastic properties of alginate aqueous solutions in the presence of salts

The influence of added salts on the dynamic viscoelastic properties are investigated for aqueous solutions of alginates that have various molecular weights and mannuronate/guluronate (M/G) ratios. The dynamic moduli of the systems increase with increasing concentration of the added salt in the low-frequency region. The effect is notable in the order of KCl < NaCl < MgCl₂ ? CaCl². The values of the dynamic moduli in the rubbery plateau are independent of the addition of the salts, irrespective of the M/G ratio of the alginate. These facts strongly suggest that the structure that is formed by the interaction between the alginates and the metal ions does not work as cross-linking points but as heterogeneous relaxation units having a relatively long relaxation time from a rheological viewpoint.     

Viscoelastic properties of passive skeletal muscle in compression: stress-relaxation behaviour and constitutive modelling

The compressive properties of skeletal muscle are important in impact biomechanics, rehabilitation engineering and surgical simulation. However, the mechanical behaviour of muscle tissue in compression remains poorly characterised. In this paper, the time-dependent properties of passive skeletal muscle were investigated using a combined experimental and theoretical approach. Uniaxial ramp and hold compression tests were performed in vitro on fresh porcine skeletal muscle at various rates and orientations of the tissue fibres. Results show that above a very small compression rate, the viscoelastic component plays a significant role in muscle mechanical properties; it represents approximately 50% of the total stress reached at a compression rate of 0.5% s⁻¹. A stiffening effect with compression rate is observed especially in directions closer to the muscle fibres. Skeletal muscle viscoelastic behaviour is thus dependent on compression rate and fibre orientation.

A model is proposed to represent the observed experimental behaviour, which is based on the quasi-linear viscoelasticity framework. A previously developed strain-dependent Young's Moduli formulation was extended with Prony series to account for the tissue viscoelastic properties. Parameters of the model were obtained by fitting to stress-relaxation data obtained in the muscle fibre, cross-fibre and 45° directions. The model then successfully predicted stress-relaxation behaviour at 60° from the fibre direction (errors <25%). Simultaneous fitting to data obtained at compression rates of 0.5% s⁻¹, 1% s⁻¹ and 10% s⁻¹ was performed and the model provided a good fit to the data as well as good predictions of muscle behaviour at rates of 0.05% s⁻¹ and 5% s⁻¹ (errors <25%).     

Predicting the behaviour of pesticides in soil from their physical and chemical properties

Increasingly stringent environmental requirements for pesticides mean that both biological activity and favourable environmental behaviour must be assessed at an early stage in pesticide discovery. Soil behaviour is governed by the physical properties of the molecule: partition coefficient, dissociation constant, vapour pressure and melting point, which control potential movement under particular soil and environmental conditions and the soil persistence. Established chemical structure-physical property correlations generally allow physical properties to be estimated for the large number of compounds in a synthesis programme with adequate precision. Stability to chemical or biological transformations in soil is more difficult to estimate but a combination of measurement for a few compounds and analogy with known chemical and biological transformation rates for various functional groups can give useful structure-stability correlations.     

Viscoelastic properties of a skin-and-muscle compartment in the right and the left hands

The aim of this research was to discriminate and categorize isolateral stress-strain characteristics of the musculocutaneous compartment (MCC) using the model of the first dorsal interosseous muscle (FDI) in vivo. In our approach the MCC was progressively isokinetically compressed with a solid vertical bar in 0.25 mm steps, the force and deflection being measured and recorded electronically. The subject maintained a constantly relaxed position. Twenty-two strongly right-handed young males each yielded three data acquisition sequences from each hand. The elastic modulus and the specific energy of deformation were determined both for the total MCC and separately for the skin and muscle moieties. No unilateral pattern of dominance could be interpreted from the analysis of the range of indices. There was a highly variable individual pattern of bilateral dominance with no specific indication or predictability; only one subject yielded a data profile confirming the classical approach to right-hand dominance. As the muscle was relaxed and the integument layers were under minimal cortical influence, we think that the classical theory of primary cortical influence in motor lateralization does not adequately explain our recorded patterns of mechanical response of the MCC under study.     

Viscoelastic properties of skin in Mov-13 and Tsk mice

Viscoelastic properties of skin samples were measured in three types of mice (tight skin, Tsk, control and Mov-13), that are known to differ with regard to content of type I collagen. The experimental design used uniaxial stretching and measured the creep response and the complex compliance. The creep response was measured directly. The complex compliance was determined using a Wiener-Volterra constitutive model for each sample. The models were calculated from data obtained by applying a stress input having a pseudo-Gaussian waveform and measuring the strain response. The storage compliance of Mov-13 and control skin were similar and were greater than Tsk (p<0.001). The loss compliance of each group was significantly different (p<0.001) from each other group; Tsk had the lowest and control had the highest loss compliance. The phase angle of the Mov-13 and Tsk were similar and were less than the controls (p<0.001). The creep response was fit with a linear viscoelastic model. None of the parameters in the creep model differed between groups. The results indicate that gene-targeted and mutant animals have soft tissue mechanical phenotypes that differ in complex ways. Caution should be exercised when using such animals as models to explore the role of specific constituents on tissue properties.     

Viscoelastic modelling of impacted morsellised bone accurately describes unloading behaviour: an experimental study of stiffness moduli and recoil properties

Morsellised bone impaction grafting is commonly used for revision arthroplasty surgery. Several reports have described the mechanical behaviour of this bone material during impaction and loading. In this study we observed the unloading progress. The loose morsellised bone was modified by particle size, particle size distribution, water and fat content. Bone pellets were constructed using different impaction energies. After impaction, the pellets were loaded statically, after which their swelling was recorded at three unloading levels. We deduced two time-dependent recoil properties, the time resistant number (TRN) and the half total swelling time (HTST), and also one stiffness property, the unloading confined modulus of elasticity (UCME). In impacted morsellised bone, the progress of swelling is visco-elastic. Bone pellets with an even distribution of particle sizes have the most rapid recoil. Those with a high liquid content recoil more slowly, and to a significantly greater extent, than pellets with low liquid content. The recoil of pellets with low liquid content is instantaneous, i.e. unrecordable, and the displacement is significantly less than in other pellet samples.     

Viscoelastic properties of the tongue and soft palate using MR elastography

Biomechanical properties of the human tongue are needed for finite element models of the upper airway and may be important to elucidate the pathophysiology of obstructive sleep apneoa. Tongue viscoelastic properties have not been characterized previously. Magnetic resonance elastography (MRE) is an emerging imaging technique that can measure the viscoelastic properties of soft tissues in-vivo. In this study, MRE was used to measure the viscoelastic properties of the tongue and soft palate in 7 healthy volunteers during quiet breathing. Results show that the storage shear modulus of the tongue and soft palate is 2.67±0.29 and 2.53±0.31 kPa (mean ± SD), respectively. This is the first study to investigate the mechanical properties of the tongue using MRE, and it provides necessary data for future studies of patient groups with altered upper airway function.     

Viscoelastic material properties of the myocardium and cardiac jelly in the looping chick heart

Accurate material properties of developing embryonic tissues are a crucial factor in studies of the mechanics of morphogenesis. In the present work, we characterize the viscoelastic material properties of the looping heart tube in the chick embryo through nonlinear finite element modeling and microindentation experiments. Both hysteresis and ramp-hold experiments were performed on the intact heart and isolated cardiac jelly (extracellular matrix). An inverse computational method was used to determine the constitutive relations for the myocardium and cardiac jelly. With both layers assumed to be quasilinear viscoelastic, material coefficients for an Ogden type strain-energy density function combined with Prony series of two terms or less were determined by fitting numerical results from a simplified model of a heart segment to experimental data. The experimental and modeling techniques can be applied generally for determining viscoelastic material properties of embryonic tissues.     

Viscoelastic properties of proteoglycan subunits and aggregates in varying solution concentrations

Using a cone-on-plate mechanical spectrometer, we have measured the linear and non-linear rheological properties of cartilage proteoglycan solutions at concentrations similar to those found in situ. Solutions of bovine nasal cartilage proteoglycan subunits (22S) and aggregates (79S) were studied at concentrations ranging from 10 to 50 mg ml-1. We determined: (1) the complex viscoelastic shear modulus G (omega) under small amplitude (0.02 radians) oscillatory excitation at frequencies (omega) ranging from 1.0 to 20.0 Hz, (2) the non-linear shear rate (gamma) dependent apparent viscosity napp (gamma) in continuous shear, and (3) the non-linear shear rate dependent primary normal stress difference sigma 1 (gamma) in continuous shear. Both the apparent viscosity and normal stress difference were measured over four decades of shear rates ranging from 0.25 to 250 s-1. Analysis of the experimental results were performed using a variety of materially objective non-linear viscoelastic constitutive laws. We found that the non-linear, four-coefficient Oldroyd rate-type model was most effective for describing the measured flow characteristics of proteoglycan subunit and aggregate solutions. Values of the relaxation time lambda 1, retardation time lambda 2, zero shear viscosity no, and nonlinear viscosity parameter muo were computed for the aggregate and subunit solutions at all of the solute concentrations used. The four independent material coefficients showed marked dependence on the two different molecular conformations, i.e. aggregate or subunit, of proteoglycans in solution.     

Application of CP-MAS and liquid-like solid-state NMR experiments for the study of the ripening-associated cell wall changes in tomato

¹³C and ¹H NMR spectra of an ethanol insoluble material (EIM) prepared from the pericarp of mature-green (MG) and red-ripe (RR) tomato fruits were acquired in ‘liquid-like’ and cross-polarisation with dipolar decoupling and magic angle spinning (CPMAS) conditions using the same triple resonance probe. Such a strategy allowed acquisitions of various NMR experiments aimed at detecting compositional differences as well as distinguishing differences in molecular mobility for various constituent polysaccharides related with the two ripening stages. Increase of the proton dipolar decoupling power levels from 3 to 50–55 kHz during single pulse ¹³C acquisition, led to more intense signals for pectic and hemicellulosic polysaccharides. This behaviour was interpreted as reflecting motional restrictions of these polysaccharides inside the porous cell wall network. Measurements of the proton rotating frame relaxation times T_1ρ in the ‘liquid-like’ conditions and of the proton transverse relaxation times T₂ from CPMAS spectra, revealed changes in mobilities for some pectic polysaccharides in relation with ripening, particularly for the H1 and H5 protons of -1,5 arabinan (Ara) side chains of rhamnogalacturonans. These data are discussed in relation with known pectic modifications occurring during ripening and associated with the tomato fruit softening.     

Heteroxylans from maize bran in aqueous solution. Part II: Studies of polyelectrolyte behaviour

Potentiometric and viscosimetric behaviours of four samples of heteroxylans, extracted from corn bran at different temperatures with varying type and concentration of alkali were studied in dilute solutions. All four samples behave as typical polyelectrolytes with a low charge density parameter (λ = 0.15). They could be described by the Lifson and Katchalsky model pK₀≈3.15), indicating a rather homogeneous repartition of the charges along the macromolecules. Their intrinsic viscosities varied linearly with the reciprocal of the square root of the ionic strength. Their flexibility parameters, B (≈0.024), were similar to those of semi-flexible macromolecules, and their intrinsic viscosities extrapolated to infinite ionic strength were between 144 and 177ml/g. Their conformational dimensions depended on the type of counter-ion in solution (dimensions minimal with divalent cations and decreasing with size of monovalent alkali metal ions), of the temperature and the pH of the solution. The radius of gyration for unchanged heteroxylans was estimated from intrinsic viscosities extrapolated to infinite ionic strength (R²_g∞^1/2 = 21−25 nm).     

Surface properties of membrane systems. Transport of staphylococcal delta-toxin from aqueous to membrane phase

Hemolytic delta-toxin from Staphylococcus aureus was soluble in either water, methanol or chloroform/methanol (2 : 1, v/v). The toxin spread readily from distilled water into films with pressures (pi) of 10 dynes/cm on water and 30 dynes/cm on 6 M urea; from chloroform/methanol it produced 40 dynes/cm pressure on distilled water. The toxin adsorbed barely from water (pi = 1 dyne/ cm) but it did rapidly from 6 M urea (pi = 35 dynes/cm). The protein films had unusually high surface potentials, which increased with the film pressure and decreased with increasing both pH and urea concentration in the aqueous phase. The fluorescence of 1-aniline 8-naphthalene sulfonate with delta-toxin was much greater than that with RNAase and dipalmitoyl phosphatidylcholine itself, indicating probably a marked lipid-binding character of the toxin. By circular dichroism the alpha-helix content of delta-toxin was 42% in water, 45% in methanol, 24% in 6 M urea. Infrared spectroscopy showed predominant alpha-helix in both 2H2O and deuterated chloroform/methanol as well as in films spread from either solvent on 2H2O. In spreading from 6 M [2H]urea, in which the major infrared absorption was that of [2H]urea with peaks at 1600 and 1480 cm(-1), the delta-toxin film showed prevalently non-alpha-helix structures with major peak intensities at 1633 cm(-1) > 1680 cm(-1), indicating the appearance of new beta-aggregated and beta-antiparallel pleated sheet structures in the film. The data prove that (1) high pressure protein films can consist of alpha-helix as well as non-alpha-helix structures and, differently from another cytolytic protein, melittin, delta-toxin does not resume the alpha-helix conformation in going into the film phase from the extended chain in 6 M urea; (2) conformational changes are important in the transport of proteins from aqueous to lipid or membrane phase; (3) delta-toxin is by far more versatile in structural dynamics and more surface active than alpha-toxin.     

Isolation, properties and behaviour of tyramine-producing lactic acid bacteria from wine

Wines containing high levels of biogenic amines were investigated for the presence of tyramine-producing strains. Two different Lactobacillus brevis (IOEB 9809 and IOEB 9901) able to produce the amine were isolated. None of the isolated strains identified as Oenococcus oeni formed tyramine. In addition, other Lact. brevis and Lact. hilgardii strains from our collection (IOEB) and the American Type Culture Collection (ATCC) were strong tyramine producers. Lactobacillus brevis IOEB 9809 and Lact. hilgardii IOEB 9649 were found to produce tyramine and phenylethylamine simultaneously. The conditions that can influence tyramine formation in wine were evaluated for three strains of Lact. brevis (IOEB 9809 and IOEB 9901) and Lact. hilgardii (IOEB 9649). Tyrosine was the major factor affecting tyramine formation and was enhanced by the presence of sugars, mainly glucose. Tyrosine decarboxylase (TDC) activity greatly depended on the presence of the precursor, which suggested that tyrosine induced the TDC system. These results indicate that Lactobacillus could be the lactic acid bacteria responsible for tyramine production in wine.     

Learning in honeybees: from molecules to behaviour

Studies in a variety of organisms as diverse as molluscs, insects, birds and mammals have shown that memories can exist in a variety of temporal domains ranging from short-term memories in the range of minutes to long-term memories lasting a lifetime. While transient covalent modifications of proteins underlie short-term memory, the formation of long-term memory requires gene expression and protein synthesis. Different intracellular signalling cascades have been implicated in distinct aspects of learning and memory formation. Little is known however, about how learning in intact animals is related to the modulation of these signalling cascades and how this contributes to distinct neuronal and behavioural changes in vivo. Associative learning in the honeybee provides the opportunity to study processes of memory formation by analysing its progression through different phases, across levels of behaviour, neural circuits, and cellular signalling pathways. The findings reveal evidence that various cellular signalling pathways in the neuronal circuit of distinct brain areas play a role in different processes during learning and memory formation.     

Viscosity behaviour and persistence length of sodium xanthan in aqueous sodium chloride

Zero-shear-rate intrinsic viscosities [eta] of sodium xanthan in aqueous NACl at 25 degrees C were determined for five samples ranging in weight- average molecular weight from 2 x 10(5) to 4 x 10(6) at salt concentrations Cs between 0.005 and 1 M, at which the polysaccharide maintains its double-helical structure. The measured [eta] for every sample was almost independent of Cs, in contrast to usual observations on flexible polyelectrolytes. The persistence length q of sodium xanthan was determined as a function of Cs by use of the theory of Yamakawa et al. for [eta] of an unperturbed worm-like cylinder, and from its Cs dependence the intrinsic persistence length q(o) ( = q at infinite ionic strength) was estimated to be 106 nm. This q(o) value was roughly twice as large as that of double-stranded DNA, indicating a high intrinsic rigidity of the xanthan double helix. The electrostatic contribution ( = q - q(o)) to q was only about 10% even at the lowest Cs of 0.005 M. Thus, it was concluded that above Cs = 0.005 M, the double- helical structure of sodium xanthan is hardly stiffened by electrostatic interactions between charged groups.