The use of various binitroxides, with both radical sites of similar reactivity, as mediator in the controlled radical polymerization of styrene was examined and the rate constants of the reactions involved were determined at 130 °C. Typical features of a controlled radical polymerization were observed in the early stage of the polymerization, leading to the formation of two-arm macromolecules containing the binitroxide at the core. At higher conversion, however, continuous decomposition reaction resulted in a break of the two-arm macromolecules into a dead chain having an unsaturation end group and a living chain capped by a modified binitroxide. The latter had the free nitroxide site inactivated by conversion into hydroxylamine. The extent of this side reaction was much larger than in classical nitroxide-mediated controlled radical polymerization of styrene. This feature was assigned to the structure of the growing chains. As they contain the binitroxide at the core, the activation reaction produces a propagating chain and a nitroxide to which another polymeric chain remains attached by the second alkoxyamine bond. Thus, deactivation of the propagating radical by nitroxide is a bimolecular process between two macromolecular species. This unique situation had no significant effect on the rate constant of the alkoxyamine homolytic dissociation, but more importantly decreased the rate constant of recombination. The latter was 30 times lower than that determined for the recombination of TEMPO with polystyryl radical at 130 °C. The slow recombination was assigned to steric hindrance and resulted in an unusually high concentration of nitroxide in the polymerization medium, together with a large concentration of propagating radicals. A consequence of the high concentration of both radicals was the enhanced rate of hydrogen transfer from the active species to the persistent radical, leading to alkoxyamine bond breaking and, hence, to arm separation.
Nanofiber scaffolds formed by self-assembling peptide RADA16-I have been used for the study of cell proliferation to mimic an extracellular matrix. In this study, we investigated the effect of RADA16-I on the growth of human leukemia cells in vitro and in nude mice. Self-assembly assessment showed that RADA16-I molecules have excellent self-assembling ability to form stable nanofibers. MTT assay displayed that RADA16-I has no cytotoxicity for leukemia cells and human umbilical vein endothelial cells (HUVECs) in vitro. However, RADA16-I inhibited the growth of K562 tumors in nude mice. Furthermore, we found RADA16-I inhibited vascular tube-formation by HUVECs in vitro. Our data suggested that nanofiber scaffolds formed by RADA16-I could change tumor microenvironments, and inhibit the growth of tumors. The study helps to encourage further design of self-assembling systems for cancer therapy.
Water‐soluble nitroxides with different structures were tested as mediators in the controlled free‐radical polymerization of sodium 4‐styrenesulfonate carried out in water at 130 °C. Nitroxides based on 1,1,3,3‐tetramethylisoindolin‐2‐oxyl and 1,1,3,3‐tetraethylisoindolin‐2‐oxyl with an ionic group on the aromatic ring (either a quaternary ammonium or a sulfonate substituent) exhibited better efficiency than water‐soluble derivatives of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy radical (TEMPO). The steric hindrance at the nitroxide site (ethyl or methyl substituent) had a much larger influence on the activation–deactivation equilibrium than the type of ionic group. With nitroxides bearing four ethyl substituents, the equilibrium constant was one order of magnitude larger than that obtained for nitroxides with four methyl substituents. Final molar masses matched the predicted values, with narrow distribution. “Livingness” of the poly(sodium 4‐styrenesulfonate)s was confirmed by successful reinitiation of sodium acrylate in water at 130 °C.
Coptis chinensis Franch. is one of the most important medicinal plants globally. However, this species contains relatively high concentrations of chromium (Cr) which potentially detrimental to human health. It is important to understand Cr localization and speciation in order to evaluate its accumulation and transportation mechanisms and minimize Cr transfer to humans. As little previous work in this area has been carried out, we utilized synchrotron radiation microscopic X-ray fluorescence (SR-μXRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to spatially locate Cr, X-ray absorption near-edge spectroscopy (XANES) to analyze Cr speciation, and inductively coupled plasma mass spectrometry (ICP-MS) to detect Cr subcellular concentration. Micromapping results showed that Cr was distributed predominantly within the vascular cylinder, the periderm and some outer cortex, and the cortex and some vascular bundles in root, rhizome, and petiole, respectively. XANES data showed that Cr(VI) can be reduced to Cr(III) when grown with Cr(VI), and yielded a novel conclusion that this plant contain elemental chromium. ICP-MS data showed that Cr was primarily compartmentalized in cell walls in all tissues. The new insights on Cr accumulation in C. chinensis Franch. provide a theoretical basis for the evaluation of Cr in other medicinal plants.
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