Probing the Hydrophobic Interactions of a Series of Pyrene End-Labeled Poly(ethylene oxide)s in Aqueous Solution Using Time-Resolved Fluorescence

The hydrophobic association of a series of poly­(ethylene oxide)­s covalently labeled at both ends with pyrene (PEO­(<i>X</i>)-Py₂ where <i>X</i> represents the number average molecular weight (<i>M</i>_n) of the PEO chains equal to 2, 5, 10, and 16.5 kDa) in aqueous solutions was investigated at different polymer concentrations (<i>C</i>_P) using steady-state and time-resolved fluorescence measurements. Phase separation was observed with PEO­(2 kDa)-Py₂ and PEO­(5 kDa)-Py₂ samples at high <i>C</i>_P. The steady-state fluorescence spectra showed that the ratios of excimer-to-monomer fluorescence intensities (<i>I</i>_E/<i>I</i>_M) of all PEO samples remained constant when <i>C</i>_P was below 4 × 10^–5 M and decreased dramatically with increasing PEO chain length due to a decrease in intramolecular pyrene excimer formation. The <i>I</i>_E/<i>I</i>_M ratio in this regime was found to scale as <i>M</i>_n^–2.3±0.2. For <i>C</i>_P > 4 × 10^–5 M, pyrene excimer is formed by both intra- and intermolecular interactions and the <i>I</i>_E/<i>I</i>_M ratio increases linearly with increasing <i>C</i>_P except for PEO­(2 kDa)-Py₂ which undergoes phase separation. The decays obtained at various polymer concentrations were fitted according to a “sequential model” (SM) which assumes that the pyrene excimer is formed in a sequential manner. The molar fractions of all excited pyrene species and the rate constants for pyrene excimer formation were determined from the global analysis of the monomer and excimer fluorescence decays. The fraction of pyrenes that formed excimer from ground-state pyrene aggregates (<i>f</i>_E0) was found to increase with <i>C</i>_P in the regime where the pyrene excimer is formed both intra- and intermolecularly and decrease with <i>M</i>_n in the regime where the pyrene excimer is formed only intramolecularly. The fraction of pyrene pendants subject to hydrophobic interactions were used to determine the hydrophobic capture radius (<i>R</i>_c) of pyrene in water from the distribution of PEO end-to-end distances. <i>R</i>_c was found to equal 2.2 ± 0.2 nm using <i>f</i>_E0

Interactions between Hydrophobically Modified Alkali-Swellable Emulsion Polymers and Sodium Dodecyl Sulfate Probed by Fluorescence and Rheology

The interactions between a pyrene-labeled hydrophobically modified alkali-swellable emulsion (Py-HASE) polymer and the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution were investigated with a fluorometer, a rheometer, and a combination of both instruments to probe the fluorescence of the polymer while the solution was being sheared. Different amounts of SDS were added to two solutions with Py-HASE concentrations of 8 and 57 g/L. The pyrene monomer and excimer decays of the Py-HASE solutions were acquired and globally fitted according to the fluorescence blob model (FBM) and the model free (MF) analysis. Both models yielded the same molar fractions of pyrenes that were isolated, aggregated, or forming excimer by diffusion. The average number of pyrenes per micelle, ⟨<i>n</i>⟩, was determined according to the FBM and found to equal 2.0 at the SDS concentration corresponding to a maximum in solution viscosity. For a Py-HASE concentration of 57 g/L, the solution viscosities at different SDS concentrations were measured from the Newtonian plateau regions and were found to peak at an SDS concentration of 11 mM. The steady-state fluorescence spectra were acquired at SDS concentrations of 0.1, 6.0, 11.1, and 17 mM while the 57 g/L Py-HASE solution was sheared. Although the solutions of Py-HASE and SDS were found to shear-thin substantially with the solution viscosity decreasing by up to 4 orders of magnitude, no change was observed in the fluorescence spectra acquired at shear rates ranging from 0.005 to 500 s^–1. The overlap of the fluorescence spectra under conditions where the solution viscosity decreased by 4 orders of magnitude suggested that the rearrangement of the hydrophobes from inter- to intramolecular associations that leads to shear-thinning occurs on a time scale that is much faster than that over which the rheology experiments are being conducted

Chirality-Controlled Carbon Nanotubes Fabricated by Self-Assembly of Graphene Nanoribbons

We demonstrate by molecular dynamics simulations that carbon nanotubes can activate and guide on their surfaces the fabrication of single-walled carbon nanotubes by self-assembly of edge-unpassivated twisted graphene nanoribbons. Temperature is a governing factor, which mainly controls the self-assembly process. Three types of stable configurations exist due to the self-assembly of twisted graphene nanoribbons at constant temperatures, i.e., a helical structure, a self-assembled carbon nanotube, and a nearly straight graphene strip, on a basal carbon nanotube. Raising the temperature gradually, the helical structure can spontaneously switch to a single-walled carbon nanotube or a nearly straight graphene strip. The straight graphene strip can further turn into a self-assembled carbon nanotube through annealing technique. Furthermore, the chirality of the self-assembled carbon nanotube can be predicted by the width of the twisted graphene nanoribbon and the radius of the basal carbon nanotube. Our finding should be useful for the design of nanodevices with chirality-controlled nanotubes

Fucoidan Induces Cancer Cell Apoptosis by Modulating the Endoplasmic Reticulum Stress Cascades

<div><p>Background</p><p>Cancer metastasis is the main cause leading to disease recurrence and high mortality in cancer patients. Therefore, inhibiting metastasis process or killing metastatic cancer cells by inducing apoptosis is of clinical importance in improving cancer patient survival. Previous studies revealed that fucoidan, a fucose-rich polysaccharide isolated from marine brown alga, is a promising natural product with significant anti-cancer activity. However, little is known about the role of endoplasmic reticulum (ER) stress in fucoidan-induced cell apoptosis.</p><p>Principal Findings</p><p>We reported that fucoidan treatment inhibits cell growth and induces apoptosis in cancer cells. Fucoidan treatments resulted in down-regulation of the glucose regulated protein 78 (GRP78) in the metastatic MDA-MB-231 breast cancer cells, and of the ER protein 29 (ERp29) in the metastatic HCT116 colon cancer cells. However, fucoidan treatment promoted ER Ca²⁺-dependent calmodulin-dependent kinase II (CaMKII) phosphorylation, Bcl-associated X protein (Bax) and caspase 12 expression in MDA-MB-231 cells, but not in HCT116 cells. In both types of cancer cells, fucoidan activated the phosphorylation of eukaryotic initiation factor 2 alpha (p-eIF2α)\CCAAT/enhancer binding protein homologous protein (CHOP) pro-apoptotic cascade and inhibited the phosphorylation of inositol-requiring kinase 1 (p-IRE-1)\X-box binding proteins 1 splicing (XBP-1s) pro-survival cascade. Furthermore, CHOP knockdown prevented DNA damage and cell death induced by fucoidan.</p><p>Conclusion/Significance</p><p>Fucoidan exerts its anti-tumor function by modulating ER stress cascades. Contribution of ER stress to the fucoidan-induced cell apoptosis augments our understanding of the molecular mechanisms underlying its anti-tumour activity and provides evidence for the therapeutic application of fucoidan in cancer.</p></div

Combinatory treatment of fucoidan and salubrinal enhances eIF2α phosphorylation and CHOP expression in MDA-MB-231 cells and HCT116 cells.

<p>Cells were treated with salubrinal (50 µM) alone or in combination with fucoidan (100 µg/ml) for 48 h. Cell lysates (30 µg) were applied for the analysis of the expression of p-eIF2α and CHOP by immunoblot. **p<0.01; ***p<0.001.</p

Effect of fucoidan on the relative phosphorylation of CaMKII (p-CaMKII/CaMKII) and Bax expression in MDA-MB-231 cells (A) and HCT116 cells (B).

<p>(A) Relative phosphorylation of CaMKII and Bax expression were progressively increased by fucoidan in MDA-MB-231 cells; (B) Relative phosphorylation of CaMKII was moderately stimulated (∼1.5-fold) at 10 µg/ml, followed by inhibition (∼1.4-fold) of fucoidan at 100 µg/ml. Bax expression was not affected by fucoidan in HCT116 cells. (C) Caspase 12 expression and cleavage. Fucoidan efficiently induces caspase 12 expression in MDA-MB-231 cells, rather than in HCT116 cells. No cleavage of caspase 12 was found in both types of cells. Data represent mean ± SD from triplicate experiments. *p<0.05; **p<0.01.</p

Fucoidan inhibits cell growth and promotes cell apoptosis.

<p>(A) Cell growth. Cells were treated with fucoidan at the indicated concentration and the viable cells were assessed using the CellTiter96 AQueous One Solution Cell Proliferation Assay. (B) Caspase 3 activation. Cells were treated with fucoidan for 3 days and the expression of cleaved caspase 3 was examined by Western blot. (C) TUNEL. For TUNEL analysis, cells were treated with fucoidan at 100 µg/ml for 3 days and the cell apoptosis was analysed using TUNEL assay. Apoptotic cells (green) were counted in five randomly selected fields (300 cells per field) and presented as a mean ± SD of percentage of apoptotic cells over the total cells (DAPI, blue). Data are expressed as a mean ± SD in triplicate experiments. *p<0.05; **p<0.01.</p

Fucoidan activates p-eIF2α\CHOP in MDA-MB-231 cells (A) and HCT116 cells (B).

<p>Cells were treated with fucoidan as described in the “Materials and Methods” and in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108157#pone-0108157-g002" target="_blank">Figure 2</a>. The relative phosphorylation of eIF2α (p-eIF2α/eIF2α) and CHOP expression were remarkably increased by fucoidan as assessed by immunoblot. TG treatment (2.0 µM, 24 h) induced activation of p-eIF2α and CHOP expression. Data are expressed as mean ± SD in triplicate experiments. *p<0.05; **p<0.01.</p

Effect of fucoidan on the expression GRP78 and ERp29.

<p>Cells at 70–80% confluence were treated with fucoidan at the indicated concentrations for 3 days and both the attached and suspended cells were harvested for protein expression analysis. (A) The expression of GRP78, but not ERp29, was inhibited by fucoidan in MDA-MB-231 cells; (B) The expression of ERp29 was significantly attenuated by fucoidan in HCT116 cells, while GRP78 was only slightly decreased (∼1.5-fold) in cells treated with fucoidan at >50 µg/ml. As a positive control, cells were treated with ER stress inducer TG (2.0 µM) for 24 h. Data represent mean ± SD in triplicate experiments. *p<0.05; **p<0.01.</p

Fucoidan suppresses p-IRE-1\XBP-1 splicing in MDA-MB-231 cells (A) and HCT116 cells (B).

<p>Cells were treated with fucoidan as described in the “Materials and Methods” and in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108157#pone-0108157-g002" target="_blank">Figure 2</a>. The phosphorylation of IRE-1 (p-IRE-1) and XBP-1 splicing were remarkably reduced by fucoidan as assessed by immunoblot. Its downstream target, p58^IPK, was not subsequently decreased. Data are expressed as mean ± SD in triplicate experiments. Note that TG-treated cells (2.0 µM, 24 h) showed an increased p-IRE-1 and XBP-1s in both cells. *p<0.05; **p<0.01.</p