Structural and mechanical properties of biopolymer gels

A torsion pendulum for measurement of the viscoelasticity of biopolymers and its application to actin networks

This report describes the design, construction, and method of operation of a torsion pendulum which is specifically designed for the measurement of soft and fragile biopolymer gels. The pendulum can be assembled and employed in a standard biological laboratory and provides data that currently require access to specialized equipment usually limited to physics or material science laboratories. This instrument measures the shear moduli of viscoelastic materials by applying either steady or oscillating shear forces to a disc-shaped sample and measuring the resulting angular displacement of a pendulum attached to one face of the sample. The device is easily constructed using commercially available materials and no specialized machinery. Shear stresses as low as 0.03 Pa and shear rates as low as 0.00003 s-1 can be measured in steady shear experiments, and dynamic shear moduli from 1 to 2500 Pa measured by oscillatory measurements with sample volumes as low as 0.5 ml. The use of the torsion pendulum is illustrated by measuring the effects of two different actin binding proteins on the viscoelasticity of actin filament networks.

Creep measurements on gelatin gels

The present work describes creep measurements on a series of concentrations of gelatin gels well above the critical gel concentration C0, using a high precision constant stress rheometer. Results for the concentration dependence of compliance are close to those expected both from theory and from dynamic oscillatory measurements of gel modulus. The concentration dependence of viscosity follows an approximate power law behaviour, with eta proportional C1.1. This exponent is consistent with relaxation in the sol fraction, and in regions of dangling chain attached to the gel. At concentrations closer to C0 we predict that a higher power law regime will prevail.

Calcium-induced gelation of low methoxy pectin solutions--thermodynamic and rheological considerations

Rheological changes occurring during the gelation of low methoxy pectins in the presence of calcium ions were investigated. The onset of gelation was found to correspond to the same macroscopic rheology (i.e. value of G') whatever the gelling conditions. Sol-gel diagrams for calcium pectin systems were mapped out in terms of calcium level, temperature and pectin concentration. The thermodynamic parameters for the cross-linking process were derived.

Probe diffusion in cross-linked actin gels

The diffusion of monodisperse polystyrene latex spheres (PLS) in column-purified 0.65 mg/mL actin solutions, polymerized with 100 mM KCl in the absence and presence of a cross-linker, actin binding protein (ABP), has been studied using dynamic light scattering. Measurements over a wide range of scattering angles from 90 degrees to 8 degrees, corresponding to inverse scattering vector probing distances of about 40-400 nm, respectively, give a measure of both the fraction of PLS mobile over the probing distance (from the normalized time autocorrelation function amplitude) and the average diffusion coefficient of the mobile PLS. Both 100- and 500-nm diameter PLS are fully mobile in polymerized actin solutions over distances of less than 100 nm, as reported previously (Newman, J., Schick, K. S. & Gukelberger, G. Biophys. J. 53, 573a, and Newman, J., Mroczka, N. & Schick, K. L. Biopolymers, 28, 655-666). At increasing probing distances, or when ABP is added at molar ratios of 1:750 or 1:150, greater fractions of the PLS are immobilized, up to almost 99% at the conditions of a 400-nm probing distance with 500-nm probes and at a ratio of 1:150 added ABP to actin. The degree of immobilization correlated well with the amount of added ABP, the size of the PLS, and the probing distance. At increasing probe distances, as the degree of immobilization increases, the remaining mobile fraction of PLS has an increasing average diffusion coefficient. These results suggest a range of pore sizes in the actin gels with a mean size of a few hundred nanometers.

Interpretation of the renaturation kinetics of gelatin solutions

Renaturation kinetics of dilute gelatin solutions are studied by optical rotation and u.v. absorption. Comparing initial slopes of renaturation curves for concentrations below 10 mg.ml-1, the rate limiting process appears as the sum of a constant rate intramolecular nucleation and a bimolecular nucleation. To fit the whole renaturation curves, a kinetic model using Monte Carlo calculations was tested, consisting of random nucleation, rapid total propagation and slow reversion depending on the size of helical segment. Mismatch on the final renaturation extent indicates that propagation is more limited than proposed.