Micro‐mechanical measurement of the torsional modulus of DNA

The torsional modulus C of DNA is determined from the difference between the work of stretching a single overwound molecule and the work done in stretching one underwound by the same number of turns. The value obtained C/k_BT=86±10 nm is within the range (75±25 nm) estimated by more indirect methods.This revised version was published online in October 2005 with corrections to the Cover Date.     

Cofilin increases the torsional flexibility and dynamics of actin filaments

We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from approximately 4 degrees (C = 2.30 x 10(-27) Nm2 radian(-1)) to approximately 17 degrees (C = 0.13 x 10(-27) Nm2 radian(-1)) at 25 degrees C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate P(i) release from filaments and modulate interactions with other regulatory actin filament binding proteins.     

The Growth Form of Croton pullei (Euphorbiaceae) - Functional Morphology and Biomechanics of a Neotropical Liana

Abstract: Croton pullei (Euphorbiaceae) is a woody climber of the lowland rainforest in French Guyana and Surinam. During ontogeny, a shift from a juvenile free-standing growth phase to an older supported growth phase is observed. The following biomechanical parameters were studied: structural Young's modulus, structural torsional modulus, flexural stiffness and bend to twist ratios. Changes in anatomical development were also analysed for different stages of development of C. pullei which differ significantly in their mechanical properties. Free-standing plants show a nearly constant structural Young's modulus and structural torsional modulus during ontogeny, with flexural stiffness increasing proportionally with the axial second moment of area. These patterns are typical for “semi-self-supporting plants". In contrast, supported plants show a significant decrease in structural Young's modulus in older stem parts, as well as a decrease in structural torsional modulus. Due to the decrease in structural Young's modulus, flexural stiffness does not increase proportionally with the axial second moment of area. These patterns are typical for non-self-supporting lianas. In all supported plants, a sudden transition occurs from early dense wood to a wood type with a much higher proportion of large diameter vessels. In contrast, only the dense wood type is present in all tested free-standing plants. The data are compared with results from other climbing species of the same study area and discussed with reference to observed features characterizing the growth form and life history of C. pullei.     

How to become a tree without wood – biomechanical analysis of the stem of Carica papaya L

Carica papaya L. does not contain wood, according to the botanical definition of wood as lignified secondary xylem. Despite its parenchymatous secondary xylem, these plants are able to grow up to 10‐m high. This is surprising, as wooden structural elements are the ubiquitous strategy for supporting height growth in plants. Proposed possible alternative principles to explain the compensation for lack of wood in C. papaya are turgor pressure of the parenchyma, lignified phloem fibres in the bark, or a combination of the two. Interestingly, lignified tissue comprises only 5–8% of the entire stem mass. Furthermore, the phloem fibres do not form a compact tube enclosing the xylem, but instead form a mesh tubular structure. To investigate the mechanism of papaya's unusually high mechanical strength, a set of mechanical measurements were undertaken on whole stems and tissue sections of secondary phloem and xylem. The structural Young's modulus of mature stems reached 2.5 GPa. Since this is low compared to woody plants, the flexural rigidity of papaya stem construction may mainly be based on a higher second moment of inertia. Additionally, stem turgor pressure was determined indirectly by immersing specimens in sucrose solutions of different osmolalities, followed by mechanical tests; turgor pressure was between 0.82 and 1.25 MPa, indicating that turgor is essential for flexural rigidity of the entire stem.     

Molecular investigation of the mechanical properties of single actin filaments based on vibration analyses

The mechanical vibration properties of single actin filaments from 50 to 288 nm are investigated by the molecular dynamics simulation in this study. The natural frequencies obtained from the molecular simulations agree with those obtained from the analytical solution of the equivalent Euler–Bernoulli beam model. Through the convergence study of the mechanical properties with respect to the filament length, it was found that the Euler–Bernoulli beam model can only be reliably used when the single actin filament is of the order of hundreds of nanometre scale. This molecular investigation not only provides the evidence for the use of the continuum beam model in characterising the mechanical properties of single actin filaments, but also clarifies the criteria for the effective use of the Euler–Bernoulli beam model.     

Development and Growth Form of the Neotropical Liana <Emphasis Type="Italic">Croton nuntians</Emphasis>: The Effect of Light and Mode of Attachment on the Biomechanics of the Stem

The neotropical liana Croton nuntians (Euphorbiaceae) can occur in a variety of different growth habits. Juvenile freestanding plants are mechanically stable without support and resemble morphologically young trees or shrubs, whereas adult plants are climbers. Ontogenetic variation of bending and torsion properties of different growth phases are analyzed by measurements of flexural stiffness, structural bending modulus, torsional stiffness and structural torsional modulus. Mechanical and anatomical data show two fundamentally different patterns for juvenile freestanding and adult climbing plants. In freestanding plants, mechanical properties and the contribution of cortex, wood, and pith to the stem cross-section vary only little during ontogeny as is typical for semi-self-supporting plants. In contrast, climbing plants become significantly more flexible during ontogeny, as is characteristic for lianas. This is accompanied by a transition to the formation of a less dense wood type with large diameter vessels and an increasing contribution of flexible tissues (less dense wood and cortex) to the cross-sectional area and the axial second moment of area of the stems. Depending on the environmental conditions, freestanding plants can differ considerably in their appearance due to differences in branching system or stem taper. Therefore the influence of light quantity, measured as percentage of canopy opening, on the mechanical properties and the stem anatomy was tested. Freestanding plants grown with strong shade are significantly more stiff in bending compared with plants grown with a moderate light environment.     

Lipid tail chain asymmetry and the strength of membrane-induced interactions between membrane proteins

Many lipids are composed of asymmetric tail chains that differ by their molecular weight (MW) and/or degree of saturation. Previous studies found that membrane moduli vary with the degree of lipid tail asymmetry. However, to date little is known regarding the effect (if any) of tail asymmetry on the membrane-induced interactions between embedded proteins. In this paper we use a self-consistent field model to examine the effect of lipid tail asymmetry on membrane proteins. We first examine the case where the overall tail length (sum of both chains) is held constant, which implies that the membrane thickness remains constant as well, independent of tail asymmetry. We find that, in these systems, the membrane area stretch and bending moduli decrease with increasing chain asymmetry, thereby reducing the magnitude of the membrane-induced barrier to protein aggregation. Since in symmetric lipid bilayers the energy barrier is typically of order ∼ 1-2 times the thermal energy kT, the asymmetry-induced reduction in barrier height may increase the probability of protein aggregation significantly. In systems where one tail chain is held constant, increasing asymmetry involves changes in the bilayer thickness which are found to dominate any effect arising from the asymmetry.     

The Motor Disorder of Classic Lesch‐Nyhan Disease

Reports describing the neurological features of Lesch‐Nyhan disease (LND) vary widely, thereby implying the involvement of different neurological substrates. The movement abnormalities in 20 patients with LND were investigated. Dystonia was the most frequent and severe movement disorder. At rest, hypotonia was more frequent than hypertonia. These findings are compatible with basal ganglia dysfunction in LND.     

Anharmonic compression of the glitter lattice

A previous report has described the crystal structure of glitter, which is a dense 3-,4-connected net composed of ethylenic columns that run parallel to the c-axis of the unit cell. Such a structure invites speculation as to its relative stiffness along that axis. A semiempirical expression due to Cohen was used in a previous communication to estimate its zero-pressure bulk modulus. This estimate exceeds that of any known material at 440 GPa. Further, by treating the ethylenic units as harmonic springs, a correction was computed for the elastic deformation of the carbon-carbon double bonds along the c-axis. This correction is on the order of 300 GPa for deformations of the double bonds of approximately 0.1 angstroms. The present communication treats the ethylenic units along the c-axis of glitter as anharmonic springs obeying a Morse potential and a Morse's law force. Within the anharmonic approximation, at modest bond length deformations, x', the bulk modulus at pressure of the glitter lattice exceeds 1 TPa.     

Hydrodynamic characteristics and equilibrium rigidity of pullulan molecules

The hydrodynamic characteristics of the polysaccharide pullulan (polymaltotriose) in water have been investigated and its molecular characteristics have been determined. Experimental values varied over the following ranges: velocity sedimentation coefficient (S): 0.9 < S < 11.2, translational diffusion coefficient (10⁷ cm² s⁻¹): 1.1 < D < 14.7 and intrinsic viscosity (cm³ g⁻¹): 6.7 < [η] < 164, which corresponds to a change in molecular weight (× 10³) in the range 3.9 < M_SD < 644. On the basis of analysis of the literature and our experimental data, excluded volume effects have been shown to have a prevailing influence on the chain length of these polysaccharides. The equilibrium rigidity and hydrodynamic chain diameter of pullulan were evaluated on the basis of the theory of hydrodynamic properties of a wormlike necklace, taking into account excluded volume effects. At low M (< 30 × 10³) the translation friction data (in contrast to viscometric data) cannot be described in the framework of the theory of linear molecules.     

An ultrasonic technique for the measurement of the elastic moduli of human cornea

In the hopes of reducing the unpredictability associated with refractive surgical procedures and ultimately improving surgical techniques, many investigators have attempted to determine the elastic moduli of the cornea. Techniques such as stress-strain tests of corneal strips and the measurement of mercury drop displacement in a whole eye under increasing pressure have resulted in a range of values for Young's modulus from 10⁵ to 10⁷ N m⁻². Both of these methods are limited because these mechanical tests cannot be performed in the physiological state and because of the large amount of force applied during testing. We used an ultrasonic technique to determine the elastic moduli of the human cornea. Two groups of six corneas prepared under different conditions (in saline and in dextran) were examined separately and the shear waves were generated and detected in these 12 human eye bank eyes. All the waveforms were digitized and saved in files of binary format. Fast Fourier transformation (FFT) was applied to calculate the speed and attenuation of the shear wave. Using the resulting wave speeds and attenuation coefficients, the Young's moduli of the corneal samples were calculated as (5.3±1.1)×10⁶ N m⁻² and (2.0±1.0)×10⁷ N m⁻² for cornea samples prepared in saline and in dextran at 2.25 MHz, respectively.     

The role of striatal glutamate receptors in models of Parkinson's disease

Summary The aim of the study was to examine the effect of antagonists of the NMDA receptor on the parkinsonian-like muscle rigidity in rats. Reserpine and haloperidol increased the muscle resistance of the hind foot to passive movements, as well as the reflex electromyographic (EMG) activity in the gastrocnemius and tibialis anterior muscles. MK-801 (0.32-1.28 mg/kg sc), an uncompetitive antagonist of the NMDA receptor, and L-701,324 (5-40 mg/ kg ip), an antagonist of the glycine site, reduced the muscle tone and the reflex EMG activity enhanced by reserpine or haloperidol. AP-5 (2 and 5 ,g/ 0.5 pl), a competitive antagonist of the NMDA receptor, and 5,7-dichlorokynurenic acid (1.0-4.5g/0.5 pl), the glycine site antagonist injected bilaterally into the rostral striatum, inhibited the muscle rigidity induced by haloperidol. In contrast, AP-5, injected alone bilaterally into the intermediate-caudal striatum induced muscle rigidity. The present results suggest that: (1) the inhibitory effect of the NMDA receptor antagonists on the parkinsonian-like muscle rigidity depends, at least partly, on their action on the rostral striatum; (2) the blockade of NMDA receptors in the intermediate-caudal striatum may reduce the beneficial impact of these compounds.     

The twist-to-bend compliance of the Rheum rhabarbarum petiole: integrated computations and experiments

Plant petioles can be considered as hierarchical cellular structures, displaying geometric features defined at multiple length scales. Their macroscopic mechanical properties are the cumulative outcome of structural properties attained at each level of the structural hierarchy. This work appraises the compliance of a rhubarb stalk by determining the stalk’s bending and torsional stiffness both computationally and experimentally. In our model, the irregular cross-sectional shape of the petiole and the layers of the constituent tissues are considered to evaluate the stiffness properties at the structural level. The arbitrary shape contour of the petiole is generated with reasonable accuracy by the Gielis superformula. The stiffness and architecture of the constituent layered tissues are modeled by using the concept of shape transformers so as to obtain the computational twist-to-bend ratio for the petiole. The rhubarb stalk exhibits a ratio of flexural to torsional stiffness 4.04 (computational) and 3.83 (experimental) in comparison with 1.5 for isotropic, incompressible, circular cylinders, values that demonstrate the relative structural compliance to flexure and torsion.     

Decrease of elastic moduli of DOPC bilayers induced by a macrolide antibiotic, azithromycin

The elastic properties of membrane bilayers are key parameters that control its deformation and can be affected by pharmacological agents. Our previous atomic force microscopy studies revealed that the macrolide antibiotic, azithromycin, leads to erosion of DPPC domains in a fluid DOPC matrix [A. Berquand, M. P. Mingeot-Leclercq, Y. F. Dufrene, Real-time imaging of drug-membrane interactions by atomic force microscopy, Biochim. Biophys. Acta 1664 (2004) 198-205.]. Since this observation could be due to an effect on DOPC cohesion, we investigated the effect of azithromycin on elastic properties of DOPC giant unilamellar vesicles (GUVs). Microcinematographic and morphometric analyses revealed that azithromycin addition enhanced lipid membranes fluctuations, leading to eventual disruption of the largest GUVs. These effects were related to change of elastic moduli of DOPC, quantified by the micropipette aspiration technique. Azithromycin decreased both the bending modulus (k_c, from 23.1 ± 3.5 to 10.6 ± 4.5 k_BT) and the apparent area compressibility modulus (K_app, from 176 ± 35 to 113 ± 25 mN/m). These data suggested that insertion of azithromycin into the DOPC bilayer reduced the requirement level of both the energy for thermal fluctuations and the stress to stretch the bilayer. Computer modeling of azithromycin interaction with DOPC bilayer, based on minimal energy, independently predicted that azithromycin (i) inserts at the interface of phospholipid bilayers, (ii) decreases the energy of interaction between DOPC molecules, and (iii) increases the mean surface occupied by each phospholipid molecule. We conclude that azithromycin inserts into the DOPC lipid bilayer, so as to decrease its cohesion and to facilitate the merging of DPPC into the DOPC fluid matrix, as observed by atomic force microscopy. These investigations, based on three complementary approaches, provide the first biophysical evidence for the ability of an amphiphilic antibiotic to alter lipid elastic moduli. This may be an important determinant for drug: lipid interactions and cellular pharmacology.     

两种梭梭根部输导组织坚韧度及其抗旱性

以梭梭和白梭梭一年生盆栽幼苗为试材,测定60％（对照）、40％和20％的土壤相对含水量（sRwc）处理20d后两种梭梭同化枝的电导率和含水量,地上和地下部水势,根部木质素、纤维素、半纤维素含量,根肉质化程度和根长度。结果表明：两种梭梭同化枝含水量随着SRWC的下降均保持较高的水平;SRWC为40％和20％时,两种同化枝电导率的变化不显著,且均保持较低的值;两种梭梭地下部与地上部水势差值随土壤含水量的降低而增大;SRWC为40％的土壤条件促进两种梭梭的根系生长,20％的SRWC条件下仍保持与对照一样的水平;不同SRWC条件下,梭梭和白梭梭根部的木质素、纤维素、半纤维素含量的变化幅度均较小,且保持很高的水平,总含量分别为46．9％-53．3％和50．6％-57．6％。由此推断,在干旱胁迫下两种梭梭的根系依赖于较强的根部榆导组织坚韧度,往土壤深层扎根找水,适应干旱环境。     

Relationship between vibrotactile detection threshold in the Pacinian channel and complex mechanical modulus of the human glabrous skin

The goal of this study was to investigate the relationship between the psychophysical vibrotactile thresholds of the Pacinian (P) channel and the mechanical properties of the skin at the fingertip. Seven healthy adult subjects (age: 23–30) participated in the study. The mechanical stimuli were 250-Hz sinusoidal bursts and applied with cylindrical contactor probes of radii 1, 2, and 3.5?mm on three locations at the fingertip. The duration of each burst was 0.5?s (rise and fall time: 50?ms). The subjects performed a two-interval forced-choice task while the stimulus levels changed for tracking the threshold at 75% probability of detection. There were significant main effects of contactor radius and location (two-way ANOVA, values of p?<?0.001). The thresholds decreased as the contactor radius increased (i.e., spatial summation effect) at all locations. The thresholds were lowest near the whorl at the fingertip. Additionally, we measured the mechanical impedance (specifically, the storage and loss moduli) at the contact locations. The storage moduli did not change with the contactor location, but the loss moduli were lowest near the whorl. While the loss moduli decreased, the storage moduli increased (e.g., more springiness) as the contactor radius increased. There was moderate and barely significant correlation between the absolute thresholds and the storage moduli (r?=?0.650, p?=?0.058). However, the correlation between the absolute thresholds and the loss moduli was high and very significant (r?=?0.951, p?<?0.001). The results suggest that skin mechanics may be important for locally shaping psychophysical detection thresholds, which would otherwise be expected to be constant due to uniform Pacinian innervention density at the fingertip.     

New target for rice lodging resistance and its effect in a typhoon

We demonstrated the new target for lodging resistance in rice (Oryza sativa L.) by the analysis of physiological function of a locus for lodging resistance in a typhoon (lrt5) with the near isogenic line under rice “Koshihikari” genetic background (tentatively named S1). The higher lodging resistance of S1 was observed during a typhoon in September 2004 (28 days after heading), when most other plants in “Koshihikari” became lodged. Visual observations showed that bending of the upper stems triggered lodging during the typhoon; the upper stem of “Koshihikari” buckled completely, whereas that of S1 remained straight. In addition to the strong rain and winds during the typhoon, the weight of the buckled upper plant parts increased the pressure on adjacent plants and caused a domino effect in “Koshihikari”. Young’s modulus, an indicator of the rigidity of the culm, was significantly higher in S1 than in “Koshihikari”. In the upper culm, the starch content in S1 was 4.8 times the value in “Koshihikari”, and senescence was delayed in the upper leaves of S1. These results suggest that the rigidity of the upper culm by the higher starch content (as a result of delayed senescence in the upper leaves) may be responsible for the higher lodging resistance during a typhoon in rice. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Constant surface tension molecular dynamics simulations of lipid bilayers with trehalose

Surface areas and fluctuations evaluated from 50 ns molecular dynamics simulations of fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayers in a 1:2 trehalose:lipid ratio carried out at surface tensions 10, 17 and 25 dyn/cm/leaflet are compared with those of pure bilayers under the same conditions. Trehalose increases the surface area, as consistent with the surface tension lowering observed in simulations at constant area. The system bulk elastic modulus K _b  = 1.5 ± 0.3 × 10¹⁰ dyn/cm². It is independent of bilayer surface area and trehalose content within statistical error. In contrast, the area elastic modulus K _a shows a strong area dependence. At 64 Å²/lipid (the experimental surface area), K _a  = 138 ± 26 dyn/cm for a pure DPPC bilayer and 82 ± 10 dyn/cm for one with trehalose; i.e. trehalose increases fluidity of the bilayer surface at this area per lipid.