Characterization of natural rubber using size-exclusion chromatography with online multi-angle light scattering. Study of the phenomenon behind the abnormal elution profile

Revisiting the Molar Mass and Conformation of Derivatized Fractionated Softwood Kraft Lignin

The limited utilization of reliable tools and standards for determination of the softwood kraft lignin molar mass and the corresponding molecular conformation hampers elucidation of the structure-property relationships of lignin. At issue, conventional size exclusion chromatography (SEC) is unable to robustly measure the molar mass because of a lack of calibration standards with a similar structure to lignin. In the present work, the determination of the absolute molar mass of acetylated technical lignin was revisited utilizing SEC combined with multi-angle light scattering with a band pass filter to suppress the fluorescence. Fractionated lignin isolated using sequential techniques of solvent and membrane methods was used to enhance the clarity of light-scattering profiles by narrowing the molar mass distribution of lignin fractions. Further information on the molecular conformation of derivatized samples was studied utilizing a differential viscometer, and chemical structures were identified by NMR spectroscopy analysis. Through the help of fractionation, intrinsic viscosity values were determined for the different fractions as a function of molecular weight cut-off membranes. The derivatized acetone-soluble lignin was found to possess a lower molecular weight and an extremely compact structure relative to the derivatized acetone-insoluble fraction based on a significantly lower "α" value in the Mark-Houwink-Sakurada plot (0.15 acetone-soluble vs 0.33 acetone-insoluble). The differences in geometry were supported by the linkage analysis from NMR showing the acetone-soluble part containing fewer native linkages. In both of these examples, kraft lignin behaved like a solid sphere, limiting the ability to provide entanglements between molecular chains. From this standpoint, macroscopic properties of lignin are justified with this knowledge of a dense and extremely compact structure.

EFFECT OF MASTICATION ON THE STRUCTURE OF MICROGEL PRESENT IN NATURAL RUBBER

Mechanical or thermal mastication experiments were performed on three commercial natural rubber (NR) samples of TSR10 grade made from latex of three different clones (GT1, PB235, and RRIM600). The mesostructure (different gel or aggregate fractions, structure of random coils of cis-1,4-polyisoprene) of all the NR samples was fully characterized by size exclusion chromatography coupled with multiangle light scattering (SEC-MALS), using pretreated SEC columns. This method was used to quantify and investigate the structure of the little-studied smaller microaggregates, constituting the microgel fraction smaller than 1 μm (Microgel<1μ) of NR. The three unmasticated NR samples showed no difference in terms of microaggregate structure. Conversely, microaggregates appeared denser after mastication. This phenomenon was found to depend on the mastication conditions, as mechanically masticated NR samples had smaller (lower radius of gyration) and more compact microaggregates than thermally masticated samples. Macrogel also behaved differently depending on the mastication conditions. Mechanical mastication conditions allowed a higher degradation of the macrogel compared with thermal mastication conditions.

Modulation of the oligomerization state of p53 by differential binding of proteins of the S100 family to p53 monomers and tetramers

We investigated the ways S100B, S100A1, S100A2, S100A4, and S100A6 bind to the different oligomeric forms of the tumor suppressor p53 in vitro, using analytical ultracentrifugation and multiangle light scattering. It is established that members of the S100 protein family bind to the tetramerization domain (residues 325-355) of p53 when it is uncovered in the monomer, and so binding can disrupt the tetramer. We found a stoichiometry of one dimer of S100 bound to a monomer of p53. We discovered that some S100 proteins could also bind to the tetramer. S100B bound the tetramer and also disrupted the dimer by binding monomeric p53. S100A2 bound monomeric p53 as well as tetrameric, whereas S100A1 only bound monomeric p53. S100A6 bound more tightly to tetrameric than to monomeric p53. We also identified an additional binding site for S100 proteins in the transactivation domain (1-57) of p53. Based on our results and published observations in vivo, we propose a model for the binding of S100 proteins to p53 that can explain both activation and inhibition of p53-mediated transcription. Depending on the concentration of p53 and the member of the S100 family, binding can alter the balance between monomer and tetramer in either direction.