We present a method in which we stabilize mechanically an optical tweezer setup over a period ranging up to 30 min. A feedback loop is used to correct the mechanical and thermal drifts. The position of a fixed object on the sample surface is measured with a CCD device and its fluctuations analyzed and used to maintain its position fixed with three piezoelectric devices. With this setup, we obtain a spatial stability of 1.5 nm in the radial direction and 5 nm in the axial direction. This method opens the route for real-time measurements of kinetics of macromolecules association, at a single molecule level, on very long time scales.
In this paper, the pitting of potato starch granules in aqueous suspensions (1%) by high-frequency high-power ultrasound (850kHz at a power of 0.2W, 2W or 3.7W; and also 500kHz and 1MHz at a power of 2W) is reported. The number of pits per starch granules was found to be independent of the amylose content of starches, and the surface properties of starch granules as modified through SDS and ethanol washing. At 850kHz, the maximum number of pits per starch granule, for both normal and waxy starches, did not exceed 11. However, a close inspection of fractionated starch granules based on their sizes showed that there is an optimum granule size for which a maximum pit number is obtained. For example, starch granules with diameter size range of ∼15 to ∼30μm had a maximum pit number (between 10 and 20 pits per granule) when sonicated (2W, 850kHz and 30min); while sonication of small (<10μm) and very large (>45μm) granules resulted in a smaller number of pits per granule (∼5). Further, the maximum number of pits per granules is also found to be proportional to the ultrasound frequency, with values of approximately 7, 10 and 11 at 0.50, 0.85, and 1MHz, respectively. FTIR measurements did not show any breakup of starch molecules.
α-Cyclodextrins (α-CDs) have the ability to form inclusion complexes with poly(ethylene oxide) (PEO) polymer chains. These pseudo-polyrotaxanes (PPRs) can be obtained by quenching an α-CD/PEO mixture in water from 70 °C down to a lower temperature (typically in the range from 5 to 30 °C) thanks to favorable interactions between α-CD cavities and PEO chains. Moreover, starting from a liquid α-CD/PEO mixture at a total mass fraction of 15% w/w at 70 °C, the formation of PPRs with time at a lower temperature induces a white physical gel with time, and phase separation is observed. We established that PPR molecules are exclusively found in the precipitated phase although unthreaded α-CD molecules and unthreaded PEO chains are in the liquid phase. At 30 °C, the physical gel formation is much slower than at 5 °C. At 30 °C, we established that, in a first step, α-CDs thread onto PEO chains, forming PPR molecules which are not in good solvent conditions in water. At a higher length scale, rapid aggregation of the PPR molecules occurs, and threaded α-CD-based nanocylinders form (cylinder length L = 5.7 nm and cylinder radius R = 4.7 nm). At a higher length scale, α-CD-based nanocylinders associate in a Gaussian way, engendering the formation of precipitated domains which are responsible for the high turbidity of the studied system. At the end of this first step (i.e., after 20 min), the system still remains liquid and the PPRs are totally formed. Then, in a second step (i.e., after 150 min), the system undergoes its reorganization characterized by a compacity increase of the precipitated domains and forms a physical gel. We found that PPRs are totally formed after 20 min at 30 °C and that the system stays in a nongel state up to 150 min. This opens new perspectives regarding the PPR chemical modification: between these two characteristic times, we can easily envisage an efficient chemical modification of the PPR molecules in water, as for instance an end-capping reaction leading to the synthesis of polyrotaxanes.
The structure of polyrotaxanes (PRs) based on α-cyclodextrins (α-CDs) threaded onto 22 kg mol−1 poly(ethylene oxide) (PEO) chains in concentrated solution (≈20% w/w) in dimethyl sulfoxide (DMSO) was studied by small-angle neutron scattering (SANS) measurements as a function of the temperature and the complexation degree N (i.e., the number of threaded α-CDs per PR which ranged from 7 up to 157). A multiblock copolymers behavior was revealed for PRs in DMSO. This multiblock behavior of PRs at 43 °C is due to the presence of two kinds of blocks which alternate along the PR. One block type is rigid and corresponds to α-CD rodlike tubes with a length Lrod ≈ 7 nm. The other block type corresponds to flexible naked PEO segments. When the PR mixtures are cooled down to 21 °C, they gelify slowly with time and form transparent physical gels. The gel structure is due to the multiblock copolymer behavior of PRs leading to the formation of regular bundles for which the characteristic sizes (L = 14 nm and R = 5.7 nm) are constant during the gelation process and are independent with N. These regular bundles contain naked PEO segment crystallites surrounded by α-CD rodlike tube aggregates at their extremities. Indeed, α-CD rodlike tubes, which are present in the initial state at 43 °C, act like a compatibilizer and thus lead to the nanoscale bundle sizes and thus to the transparency of the physical PR-based gels. Furthermore, we showed that the kinetics of the bundle formation is N-dependent. Indeed, at constant PR weight fraction in DMSO, the N value is a crucial parameter controlling the intrinsic flexibility of PRs (flexibility favored at low N values) and their prealignment (prealignment favored at high N values) and thus controlling the self-organization.
Ru(bpy)2dppz2+ has been widely used as a probe for exploring the structure of double-stranded DNA (dsDNA). The flexibility change of DNA helix is important in many of its biological functions but not well understood. Here, flexibility change of dsDNA helix caused by intercalation with Ru(bpy)2dppz2+ was investigated using the atomic force microscopy. At first, the interactions between ruthenium complex and dsDNA helix were characterized and the binding site size (p = 2.87 bp) and binding constant (Ka = 5.9 * 107 M-1) were determined by the relative extension of DNA helix using the equation of McGhee and von Hippel. By measuring intercalator-induced DNA elongation and the mean square of end-to-end distance at different molar ratios of Ru(bpy)2dppz2+ to dsDNA, the changes of persistence length under different ruthenium concentrations were determined by the worm-like chain model. We found that the persistence length of dsDNA decreased with increasing Ru(bpy)2dppz2+ concentration, demonstrating that the flexibility of dsDNA obviously enhanced due to the intercalation. Especially, the persistence length changed greatly from 54 to 34 nm on changing the molar ratio of ruthenium to dsDNA from 0 to 0.2. We speculated that the intercalation of dsDNA with Ru(bpy)2dppz2+ resulted in local deformation or bending of the DNA duplex. In addition, the thermal dynamic stability of DNA helix was measured with high resolution melting method which revealed the increase in thermal dynamic stability of DNA helix due to the ruthenium intercalation.
We attribute similarities in the rheology of many soft materials (foams, emulsions, slurries, etc.) to the shared features of structural disorder and metastability. A generic model for the mesoscopic dynamics of ``soft glassy matter'' is introduced, with interactions represented by a mean-field noise temperature x. We find power law fluid behavior either with (x<1) or without (1
<div>Abstract<p>Cisplatin-derived anticancer therapy has been used for three decades despite its side effects. Other types of organometallic complexes, namely, some ruthenium-derived compounds (RDC), which would display cytotoxicity through different modes of action, might represent alternative therapeutic agents. We have studied both <i>in vitro</i> and <i>in vivo</i> the biological properties of RDC11, one of the most active compounds of a new class of RDCs that contain a covalent bond between the ruthenium atom and a carbon. We showed that RDC11 inhibited the growth of various tumors implanted in mice more efficiently than cisplatin. Importantly, in striking contrast with cisplatin, RDC11 did not cause severe side effects on the liver, kidneys, or the neuronal sensory system. We analyzed the mode of action of RDC11 and showed that RDC11 interacted poorly with DNA and induced only limited DNA damages compared with cisplatin, suggesting alternative transduction pathways. Indeed, we found that target genes of the endoplasmic reticulum stress pathway, such as <i>Bip, XBP1, PDI</i>, and <i>CHOP</i>, were activated in RDC11-treated cells. Induction of the transcription factor CHOP, a crucial mediator of endoplasmic reticulum stress apoptosis, was also confirmed in tumors treated with RDC11. Activation of CHOP led to the expression of several of its target genes, including proapoptotic genes. In addition, the silencing of <i>CHOP</i> by RNA interference significantly reduced the cytotoxicity of RDC11. Altogether, our results led us to conclude that RDC11 acts by an atypical pathway involving CHOP and endoplasmic reticulum stress, and thus might provide an interesting alternative for anticancer therapy. [Cancer Res 2009;69(13):5458–66]</p></div>
