Microrheological studies reveal semiflexible networks in gels of a ubiquitous cell wall polysaccharide.

Microrheological measurements have been carried out on ionotropic gels made from an important cell wall polysaccharide, using diffusing wave spectroscopy and multiple particle tracking. These gels were formed by the interaction of calcium ions with negatively charged groups on the polymer backbone, which is a copolymer of charged and uncharged sugars, galacturonic acid, and its methylesterified analog, respectively. The results suggest that semiflexible networks are formed in these systems, with a low frequency, frequency independent storage modulus (G'>G''), and a high frequency scaling of both G' and G'' with omega3/4. The differences observed between gels obtained using polysaccharide samples with different amounts and patterns of the charged ion-binding groups could comfortably be accommodated within this theoretical framework, assuming that the elementary semiflexible elements of the network are filaments consisting of two polymer chains bridged with calcium. In particular, a sample that was engineered to possess a blockwise intramolecular distribution of calcium chelating moieties clearly exhibited the high frequency scaling of both moduli with omega3/4 across some three orders of magnitude, and the concentration dependences of the elastic modulus, at both high and low frequency, were found to follow power laws with predicted exponents. Furthermore, quantitative agreement of the moduli with theory was found for realistic estimates of the molecular parameters, suggesting that the physics of semiflexible networks is not only exploited by protein components of the cytoskeleton but also by polysaccharides in plant cell walls.

DWS microrheology of a linear polysaccharide.

Diffusing wave spectroscopy has been used to measure the rheological behavior of pullulan (M(w) = 1 x 10(5)) aqueous solutions up to concentration of 40 g/dL. It was found that these solutions were mainly viscous, with the loss modulus G'' higher than the elastic modulus G'. The plot of the specific viscosity eta(sp) as a function of pullulan concentration showed two critical concentrations c = 4 g/dL and c = 15 g/dL. For c < c, eta( sp) approximately c(1.25+/-0.05); for c < c < c, eta( sp) approximately c(2+/-0.05); and for c > c, eta( sp) approximately c(4.5+/-0.5). These results are in very good agreement with those reported in the literature.

Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy.

Diffusing-wave spectroscopy (DWS) has been used to study the stability of sodium-caseinate-stabilized emulsions. The emulsions underwent creaming as a result of depletion flocculation when excess sodium caseinate was added. The creaming process was monitored over a 3-h period and each autocorrelation function was collected for 2 min to ensure adequate signal-to-noise ratio. The temporal variation of average particle size times the coefficient of viscosity of the continuous phase was derived from the backscattering measurements, and the variation of the scattering mean free path length with time was found from the backscattering and transmission measurements. It was confirmed that the creaming process was delayed at high oil concentrations, presumably due to the formation of oil droplet networks.

Dynamic light scattering investigation of sodium caseinate and xanthan mixtures.

Aqueous solutions of sodium caseinate and xanthan at pH 7 and containing 0.1 M NaCl, and their mixtures were investigated using dynamic light scattering. Sodium caseinate solutions showed a bimodal distribution of relaxation rates; with the aggregate peak distribution predominating. Xanthan solutions showed a single distribution at low concentrations (< or =0.06 wt.%) and a bimodal distribution at higher concentrations. The sodium caseinate-xanthan mixture modes were independent of the total biopolymer concentration, and behaved as a superposition of sodium caseinate solution alone and xanthan solution alone. This indicates that there is no interaction between xanthan and sodium caseinate in the range of concentrations considered in this study.