Mechanical components of motor enzyme function.

Motor enzymes use energy from ATP dephosphorylation to generate movement by a mechanical cycle, moving and pushing in one direction while attached to their cytoskeletal substrate, and recovering by moving relative to their substrate to a new attachment site. Mainstream models assert that movement while attached to the substrate results from preexisting strain in the attached motor. The additional underlying ideas can be described in terms of three components for strain amplification: a rotating lever arm, multiple attached states, and elastic compliance. These components determine how energy is recovered during the mechanical cycle and stored in a strained motor. They may coexist in a real motor; the challenge is to determine the contributions of each component. Because these components can generate similar relationships between strain energy and strain, standard measurements of motor function do not discriminate easily between these components. However, important information could be is provided by observations that suggest weak coupling between chemical and mechanical cycles, observations of negative force and movement events in single motor experiments, and the discovery that two motors that move in opposite directions have very similar structures. In models incorporating changes in conformation between attached states, these observations are only explained easily if the conformational changes are tightly coupled to changes in the strength of motor-substrate binding.     

Mechanical properties of actin

We used a cone and plate rheometer to evaluate the mechanical properties of actin over a wide range of oscillation frequencies and shear rates. Remarkably, both filamentous and nonfilamentous actin behaved as viscoelastic solids in both oscillatory and shear type experiments, providing that they were given ample time to equilibrate. Actin was purified by gel filtration from rabbit skeletal muscle and Acanthamoeba. Nonfilamentous actin in 2 different buffers had similar properties. In a low ionic strength buffer the absence of filaments was confirmed by electron microscopy, ultracentrifugation, and the fluorescence of pyrene-labeled actin. In 0.6 M KI, actin was monomeric by gel filtration. Filamentous actin had similar properties in 2 mM MgCl2 with either 50 mM KC1 or 500 mM KC1. Under all 4 of these conditions, actin required about 1000 min at 25 degrees C for the rheological properties to equilibrate. Under conditions where the oscillation of the rheometer did not affect the mechanical properties, all of the actin preparations had dynamic viscosities that were inverse functions of the frequency and dynamic elasticites that leveled off at low frequencies as expected for viscoelastic solids. For filamentous actin, the values of these parameters were about 2 times higher than for nonfilamentous actin. In shear experiments, both filamentous and nonfilamentous actin exhibited shear rate-dependent yield stresses. When filamentous and nonfilamentous actin structures were disrupted by transient shearing, the dynamic elasticity recovered to 90% in 30 min. Ovalbumin in the low ionic strength buffer also behaved as a viscoelastic material with elasticity and viscosity about 10 times lower than nonfilamentous actin, while cytochrome c behaved as a Newtonian fluid with a viscosity of 0.02 poise.     

Antibiotic properties of lignin components

Inhibitory effects of compounds with guaiacyl and syringyl structure, representing the structure of native lignin, were studied on model cultures of bacteria, yeasts, yeast-like microorganisms and moulds. Isoeugenol exhibited the most pronounced inhibitory effect on growth of the studied microorganisms.     

Mechanical properties of pleural membrane

The pleural membrane is modeled as a planar collection of interconnected randomly oriented line elements. By assuming that the line elements follow the strain field of a continuum, a strain-energy function is formulated. From the strain-energy function, an explicit stress-strain equation for large deformations is derived. In the linear approximation of the stress-strain equation the shear modulus and the area modulus of the membrane are respectively found to be 2.4 and 2.8 times the tension at the reference state. The stress-strain equation for large deformations is used to predict the displacement field around a circular hole in pleura. Good agreement is found between these predictions and measurements made on ablated pleura from dog lungs. From these theoretical and experimental results the conclusion is drawn that the pleura has a significant role in carrying shear forces and maintaining the lung's shape.     

Mechanical properties of cytoskeletal polymers

The mechanical properties of cytoplasm are dominated by microfilaments, microtubules, and intermediate filaments, collectively termed the cytoskeleton. This review discusses how the physical properties of these biopolymer systems are related to their molecular structures and interactions, and how remodelling of these biopolymers in vivo affects cell shape and motility.     

Mechanical properties of DNA films

The Young's dynamical modulus (E) and the DNA film logarithmic decrement (theta) at frequencies from 50 Hz to 20 kHz are measured. These values are investigated as functions of the degree of hydration and temperature. Isotherms of DNA film hydration at 25 degrees C are measured. The process of film hydration changing with temperature is studied. It is shown that the Young's modulus for wet DNA films (E = 0.02-0.025 GN m-2) strongly increases with decreasing hydration and makes E = 0.5-0.7 GN m-2. Dependence of E on hydration is of a complex character. Young's modulus of denatured DNA films is larger than that of native ones. All peculiarities of changing of E and theta of native DNA films (observed at variation of hydration) vanish in the case of denatured ones. The native and denatured DNA films isotherms are different and depend on the technique of denaturation. The Young's modulus of DNA films containing greater than 1 g H2O/g dry DNA is found to decrease with increasing temperature, undergoing a number of step-like changes accompanied by changes in the film hydration. At low water content (less than 0.3 g H2O/g dry DNA), changing of E with increasing temperature takes place smoothly. The denaturation temperature is a function of the water content.     

Mechanical properties of DNA-like polymers

The molecular structure of the DNA double helix has been known for 60 years, but we remain surprisingly ignorant of the balance of forces that determine its mechanical properties. The DNA double helix is among the stiffest of all biopolymers, but neither theory nor experiment has provided a coherent understanding of the relative roles of attractive base stacking forces and repulsive electrostatic forces creating this stiffness. To gain insight, we have created a family of double-helical DNA-like polymers where one of the four normal bases is replaced with various cationic, anionic or neutral analogs. We apply DNA ligase-catalyzed cyclization kinetics experiments to measure the bending and twisting flexibilities of these polymers under low salt conditions. Interestingly, we show that these modifications alter DNA bending stiffness by only 20%, but have much stronger (5-fold) effects on twist flexibility. We suggest that rather than modifying DNA stiffness through a mechanism easily interpretable as electrostatic, the more dominant effect of neutral and charged base modifications is their ability to drive transitions to helical conformations different from canonical B-form DNA.     

Mechanical properties of cranial sutures

Many bones in mammalian skulls are linked together by cranial sutures, connective tissue joints that are morphologically variable and show different levels of interdigitation among and within species. The goal of this investigation was to determine whether sections of skull with cranial sutures have different mechanical properties than adjacent sections without sutures, and if these properties are enhanced with increased interdigitation. To test these hypotheses, bending strength and impact energy absorption were measured for samples of goat (Capra hircus) cranial bone without sutures and with sutures of different degrees of interdigitation. Bending strength was measured under both dynamic (9.7 mm displacement s-1) and relatively static (0.8 mm s-1) conditions, and at either speed, increased sutural interdigitation provided increased strength during three-point bending. However, except for very highly interdigitated sutures loaded slowly, sutures were not as strong in bending as bone. In contrast, sutures absorbed from 16% to 100% more energy per unit volume during impact loading than did bone. This five-fold increase in energy absorption by the sutures was significantly correlated with increased sutural interdigitation.     

Mechanical properties of collagen fibrils

The formation of collagen fibers from staggered subfibrils still lacks a universally accepted model. Determining the mechanical properties of single collagen fibrils (diameter 50-200 nm) provides new insights into collagen structure. In this work, the reduced modulus of collagen was measured by nanoindentation using atomic force microscopy. For individual type 1 collagen fibrils from rat tail, the modulus was found to be in the range from 5 GPa to 11.5 GPa (in air and at room temperature). The hypothesis that collagen anisotropy is due to the subfibrils being aligned along the fibril axis is supported by nonuniform surface imprints performed by high load nanoindentation.     

Mechanical properties of zona pellucida hardening

We have investigated the changes in the mechanical properties of the zona pellucida (ZP), a multilayer glycoprotein coat that surrounds mammalian eggs, that occur after the maturation and fertilization process of the bovine oocyte by using atomic force spectroscopy. The response of the ZP to mechanical stress has been recovered according to a modified Hertz model. ZP of immature oocytes shows a pure elastic behavior. However, for ZPs of matured and fertilized oocyte, a transition from a purely elastic behavior, which occurs when low stress forces are applied, towards a plastic behavior has been observed. The high critical force necessary to induce deformations, which supports the noncovalent long interaction lifetimes of polymers, increases after the cortical reaction. Atomic force microscopy (AFM) images show that oocyte ZP surface appears to be composed mainly of a dense, random meshwork of nonuniformly arranged fibril bundles. More wrinkled surface characterizes matured oocytes compared with immature and fertilized oocytes. From a mechanical point of view, the transition of the matured ZP membrane toward fertilized ZP, through the hardening process, consists of the recovery of the elasticity of the immature ZP while maintaining a plastic transition that, however, occurs with a much higher force compared with that required in matured ZP.     

Mechanical properties of the upper airway

The series and shunt components of the impedance of the upper airway (Zuaw) were evaluated from measurements obtained during a Valsalva maneuver by means of a modified forced oscillation technique. When the cheeks are supported, the upper airway can be represented by a single distributed transmission line. The homogeneity of this line was confirmed by measuring separately Zuaw and the impedance of the mouth. Correction of the impedance of the respiratory system, determined by means of the forced oscillations technique, for the shunt properties of Zuaw results in some modifications of the frequency dependence of resistance (Rrs) in healthy adults and in marked changes of the absolute values of Rrs in children and in patients with obstructive lung disease.