Robert Ivkov

The polymerization of actin: Structural changes from small-angle neutron scattering

We present a new analysis of small-angle neutron-scattering data from rabbit muscle actin in the course of the polymerization from G-actin to F-actin as a function of temperature. The data, from Ivkov et al. [J. Chem. Phys. 108, 5599 (1998)], were taken in D2O buffer with Ca2+ as the divalent cation on the G-actin in the presence of ATP and with KCl as the initiating salt. The new analysis of the data using modeling and the method of generalized indirect fourier transform (O. Glatter, GIFT, University of Graz, Austria, http://physchem.kfunigraz.ac.at/sm/) provide shapes and dimensions of the G-actin monomer and of the growing actin oligomer in solution as a function of temperature and salt concentration. This analysis indicates that the G-actin monomer, under the conditions given above, is a sphere 50-54 A in diameter as opposed to the oblate ellipsoid seen by x-ray crystallography. The F-actin dimensions are consistent with x-ray crystal structure determinations.

The polymerization of actin: Study by small angle neutron scattering

We report measurements of small angle neutron scattering from solutions of rabbit muscle G-actin at 3.00 mg/mL in D2O buffer solution, with [Ca2+]=0.52 mM and with [KCl]=15, 8.9, and 5.4 mM. We observe the onset of the polymerization of G-actin to F-actin as the temperature is increased. The polymerization takes place on a time scale of 30–45 min for each temperature jump of 2 °C–3 °C. As the temperature is increased further, the average size of the polymers increases, and the characteristic length scale (or correlation length), ξ, of the F-actin in the dilute solution grows: ξ is about 10 A below Tp, and about 70 A a few degrees above Tp. The transition is sharper for lower concentrations of KCl. For the sample with [KCl]=8.9 mM, we observe a peak in ξ at about 2 °C above Tp, which indicates a crossover into the semidilute regime. The transition is essentially reversible, but shows evidence of incomplete depolymerization on cycling. We are unable to apply the available theoretical model for reversible po...

Inelastic neutron scattering from filled elastomers

Inelastic neutron scattering experiments are powerful techniques for evaluating local molecular dynamics. These methods are especially sensitive to hydrogen atoms containing motions. An overview of these experimental techniques is presented. Neutron filter analyzer and time-of-flight spectroscopy methods are used to characterize the local dynamics of polymers in the presence and absence of fillers. Of particular interest is the comparison between bound rubber attached to the filler surface and the pure, unbound rubber. A commercial synthetic polyisoprene containing approximately 100% cis-1,4 isomers was compounded with three different carbon blacks: N299, G299 (graphitized N299), and N762. Soxhlet extraction on each of the samples was performed so that corresponding samples containing purely bound rubber with filler were obtained. The filter analyzer and time-of flight spectra show distinct differences between the bound and pure rubber as well as differences based on carbon black type. Correlation of the spectral differences to the type of carbon black and initial concentration of carbon black are discussed.

Neutron Reflectivity from the Monolayer of SAN Random Copolymer

Abstract The detailed structural aspects of the monolayer of SAN random copolymer with 22.6 wt% AN content at the air/water interface were investigated using neutron reflectivity(NR) technique. NR data at various surface pressures were analyzed to obtain the thickness, the density, and the concentration distribution. Well defined double-layered structure could fit the data.

The Significance of Percolation on the Dynamics of Polymer Chains Bound to Carbon Black

A critical need in the fundamental understanding of reinforcement in filled polymers is the characterization of the polymer-filler interface and the dynamics of the polymer in this interfacial regime. In carbon black filled polymers, one of the central themes in the mechanism of reinforcement is that of “bound” polymer. Understanding the dynamics of this bound polymer may be key to arriving at an understanding of reinforcement mechanisms in filled polymers. The interactions between polymers and filler surfaces are also key in the development of more advanced nanocomposite materials. We have previously utilized inelastic neutron scattering methods to examine the variation of bound polymer dynamics as a function of carbon black type for a single, initial carbon black concentration. An apparent change in the distribution in backbone motions was observed in the bound polymer compared with the pure polymer. In this study, we extend our prior work to examine the bound polymer dynamics as a function of the type of carbon black and the initial concentration of carbon black. The results suggest that two types of dynamic behavior are observed as a function of the initial carbon black concentration. This critical cutoff concentration may be related to the percolation threshold of the carbon black and suggests that quantifying the amount of bound polymer is insufficient for understanding the relationship between mechanical behavior and bound polymer content.