group^.^-^ We could not obtain evidence for such secondary aggregation in CHAPS.Thus, GFC can provide very accurate C, values for CHAPS, which make possible the determination of aggregation numbers and stepwise aggregation constants. These results will be helpful J . Phys. Chem. 1991Chem. , 95, 1850Chem. -1858 for applications of CHAPS in membrane biochemistry. Furthermore, the present approach &n be used for other compounds, such as bile salts, drugs, nucleosides, and dye^.^-'^ The odd-even alternation observed in the aggregation number dependence of stepwise aggregation constants is a novel phenomenon, which has been revealed by the determination of very accurate V, values.The properties of aqueous solutions of low molecular weight triblock copolymers of PEO/PPO/PEO have been investigated, mainly by dynamic and static light scattering. At low concentrations (C < 10%) and temperature (<25 O C ) the relaxation time distributions from dynamic light scattering show the coexistence of the monomer (RH = 18 A), micelles (RH = 80 A), and micellar aggregates in relative proportions which depend critically on temperature and concentration. Micelles are formed at about C = 5% at 25 O C . At 40 "C and above micelles are present at all concentrations used (C > 0.3%). At infinite dilution the hydrodynamic radius of the micelles is approximately constant over the temperature range 15-50 OC. At finite concentrations the apparent micellar radius increases with increasing temperature. The growth into asymmetric particles with increasing concentration is stronger as demonstrated by ultracentrifugation and combining static and dynamic light scattering data. At higher concentrations, a solidlike gel is formed at a well-defined temperature as shown by oscillatory shear measurements. It is characterized by a dynamic correlation length which decreases monotonically with increasing concentration to about 20 A.

No abstract

No abstract

Actin is the major protein of eukaryote peripheral cytoplasm where its mechanical effects could determine cell shape and motility. The mechanical properties of purified F-actin, whether it is a viscoelastic fluid or an elastic solid, have been a subject of controversy. Mainstream polymer theory predicts that filaments as long as those found in purified F-actin are so interpenetrated as to appear immobile in measurements over a reasonable time with available instrumentation and that the fluidity of F-actin could only be manifest if the filaments were shortened. We show that the static and dynamic elastic moduli below a critical degree of shear strain are much higher than previously reported, consistent with extreme interpenetration, but that higher strain or treatment with very low concentrations of the F-actin severing protein gelsolin greatly diminish the moduli and cause F-actin to exhibit rheologic behavior expected for independent semidilute rods, and defined by the dimensions of the filaments, including shear rate independent viscosity below a critical shear rate. The findings show that shortening of actin filaments sufficiently to permit reasonable measurements brings out their viscoelastic fluid properties. Since gelsolin shortens F-actin, it is likely that the effect of high strain is also to fragment a population of long actin filaments. We confirmed recent findings that the viscosity of F-actin is inversely proportional to the shear rate, consistent with an indeterminate fluid, but found that gelsolin abolishes this unusual shear rate dependence, indicating that it results from filament disruption during the viscosity measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this work was to examine possible similarities between the relaxation time distributions for polystyrene in semidilute solutions in the solvent bis(2-ethylhexyl) phthalate (dioctyl phthalate, DOP) obtained from dynamic light scattering (DLS) and dynamic mechanical (DM) measurements. The paper deals mainly with the influence of concentration and molecular weight but also with the role of solvent quality through change in temperature. The DLS and DM relaxation functions have been analyzed by inverse Laplace transformation (ILT) and the DLS data also by fitting the slow, q-independent, part of the spectrum to a general exponential function. It was found that with both techniques the dynamical processes probed at long times are characterized by a similar wide range of relaxation times and that the latter increases strongly with concentration and molecular weight. However, the distribution of relaxation times from DLS is much more sensitive in the slow part of the spectrum to changes in solvent quality.