Optimization of a silica supported bis(butylcyclopentadienyl)-zirconium dichloride catalyst for ethylene polymerization

A series of heterogeneous α-olefin polymerization catalysts were prepared by the immobilization of bis(butylcyclopentadienyl)zirconium dichloride, (nBuCp)₂ZrCl₂, on a commercial silica support (Grace 948) using different procedures. The preparation parameters, namely, silica activation temperature, grafting temperature, grafting time, and solvent, were evaluated in terms of metal content on silica and ethylene homopolymerization activity. Metal contents were determined by Rutherford back-scattering spectrometry (RBS). In the temperature activation range between 373 and 723 K, silica surface saturation in Zr was found to be around 0.34 wt.-% Zr/SiO₂. However, polymer polydispersity is shown to decrease with increasing support activation temperature. A better control in the generation of the active surface species was achieved with thermal pretreatment temperatures close to 723 K. The grafting reaction was seen to be immediate. Longer grafting times or higher temperatures bore deactivated species. Practically all the systems were active in ethylene polymerization in the presence of MAO, but the highest yield was obtained after grafting at 353 K for 1 h in toluene solution, employing silica pretreated at 723 K.     

Stem–loop formation drives RNA folding in mechanical unzipping experiments

Accurate knowledge of RNA hybridization is essential for understanding RNA structure and function. Here we mechanically unzip and rezip a 2-kbp RNA hairpin and derive the 10 nearest-neighbor base pair (NNBP) RNA free energies in sodium and magnesium with 0.1 kcal/mol precision using optical tweezers. Notably, force–distance curves (FDCs) exhibit strong irreversible effects with hysteresis and several intermediates, precluding the extraction of the NNBP energies with currently available methods. The combination of a suitable RNA synthesis with a tailored pulling protocol allowed us to obtain the fully reversible FDCs necessary to derive the NNBP energies. We demonstrate the equivalence of sodium and magnesium free-energy salt corrections at the level of individual NNBP. To characterize the irreversibility of the unzipping–rezipping process, we introduce a barrier energy landscape of the stem–loop structures forming along the complementary strands, which compete against the formation of the native hairpin. This landscape correlates with the hysteresis observed along the FDCs. RNA sequence analysis shows that base stacking and base pairing stabilize the stem–loops that kinetically trap the long-lived intermediates observed in the FDC. Stem–loops formation appears as a general mechanism to explain a wide range of behaviors observed in RNA folding.

Unzipping experiments permit one to investigate the physico- chemical properties of nucleic acids, from the thermodynamics of duplex formation to the folding of secondary and tertiary structures. In particular, DNA hybridization finds diverse applications in the field of DNA nanotechnology, the construction of DNA origami, molecular robots, DNA walkers, switches, and nanomotors (1–5). In an unzipping experiment, the two strands of a duplex DNA or RNA molecule are mechanically pulled apart by exerting opposite forces on the two strands on one end. In this way, it is possible to measure a force–distance curve (FDC) that exhibits a sequence-dependent sawtooth pattern. DNA unzipping has been used to test the validity of the nearest-neighbor (NN) model (6–9) and to extract the 10 NN base pairs (NNBP) free-energy parameters at different salt conditions (10, 11). A precise knowledge of the NNBP energies might be also useful to unravel hidden energy codes in molecular evolution (12).Here we derive the 10 NNBP RNA energies from unzipping experiments carried out on a 2-kbp RNA hairpin in monovalent (sodium) and divalent (magnesium) salt conditions. The NN model has many parameters requiring a sufficiently long RNA hairpin to infer them from unzipping experiments reliably. Two are the main difficulties of these experiments: First, the molecular synthesis of a long (a few kilobases) RNA hairpin is challenging; second, the FDC along the RNA sequence alternates reversible unzipping regions with irreversible ones that exhibit hysteresis and multiple long-lived intermediates (13, 14). Compared to DNA, where unzipping is practically reversible, a similar derivation of the RNA energies from irreversible FDCs is not possible. Here we derive the full equilibrium FDC in RNA by the piecewise assembly of the reversible parts and the reconstructed equilibrium ones for the irreversible regions. These are obtained by repeatedly unzipping and rezipping the RNA hairpin in these irreversible regions and using statistical physics methods based on fluctuation theorems. This allows us to derive the NNBP energies for RNA in sodium and magnesium and compare them with the results reported by the literature (15–18). Moreover, we demonstrate the validity of an equivalence rule for the free-energy salt corrections between sodium and magnesium at the level of individual NNBP. We find that NNBP free-energy parameters for a given magnesium concentration are equal to those in 77(±49)-fold sodium. This result is compatible with the 100/1 rule of thumb by which the nonspecific RNA binding affinity of 10 mM Mg²⁺ approximately equals that of 1 M Na+ (19). We provide a solid verification of this phenomenological result by measuring the NNBP RNA energies in sodium and magnesium. We study the irreversibility and hysteresis in the FDCs and hypothesize that this is caused by the formation of stem–loop structures along the unpaired single strands. Remarkably, the hysteresis along the unzipping–rezipping pathway directly correlates with the barrier energy landscape defined by the stem–loops that are formed at the junction separating single strands and duplex. A sequence analysis of the irreversible regions of the 2-kbp RNA and experiments on specifically designed short-RNA sequences demonstrates that base stacking and base pairing within the single-stranded RNA (ssRNA) promote the formation of stem–loop RNA structures transiently stabilized at forces as high as 20 pN. The stem–loops mechanism explains the slow kinetics and multiple trapping conformations observed in RNA folding, with implications for the RNA folding problem (13, 20–23).     

Sustainable production of curable maltodextrin-based electrospun microfibers

Maltodextrins are inexpensive, water-soluble starch hydrolysis products composed of high molecular weight polysaccharide molecules. This feature allows their water solutions to be processed by electrospinning to produce bio-based microfibrous mats. Also, the presence of hydroxyl functions along the maltodextrin''s backbone enables cross-linking reactions to be performed, necessary to overcome the intrinsic solubility of the starting material, by exploiting suitable functional molecules. In this work, the electrospinning process to obtain fibre deposition from water solutions of five different commercial maltodextrins was firstly optimized. Well-defined fibres with diameters ranging between 1.1 μm and 1.5 μm were successfully obtained using water as the unique solvent. Subsequently, the same maltodextrin-containing water solutions with citric acid added were then processed again. The presence of citric acid did not hinder the spinnability of the studied systems, while the possibility to achieve a one-step thermal curing of the obtained fibres was proved via solubility tests, TGA, and FTIR-ATR analyses. Eventually, bio-based cross-linked mats with fibre diameters ranging from 0.7 μm to 1.4 μm were obtained from the electrospinning of commercial maltodextrins and citric acid, employing water as the unique solvent and environmentally friendly curing processes. This approach enables the reported mats to be further studied for environmental, pharmaceutical, and medical applications.

This paper reports a sustainable approach to obtain thermally curable bio-based microfibrous mats from the electrospinning of water solutions containing commercial maltodextrins and citric acid.     

Up-regulation of the proapoptotic mediators Bax and Bak after adenovirus-mediated p53 gene transfer in lung cancer cells.

Overexpression of wild-type p53 in cancer cells by adenovirus-mediated p53 gene transfer can result in the induction of apoptosis. To identify the potential mediators of this p53-induced apoptosis, we examined apoptotic protein levels in human lung cancer cells after Adp53 gene transfer. We observed up-regulation of Bax and Bak protein levels 18-36 h after transduction with Adp53 in H1299, H358, and H322 lung cancer cells. Contrary to expected observations, no changes in Bcl-2 and Bcl-X(L) protein levels were observed. Morphological cell changes and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling staining showed evidence of apoptosis in all cell lines 48 h after transduction with Adp53. These results indicate that the induction of apoptosis by adenovirus-mediated p53 transfer may be mediated by the induction of proapoptotic mechanisms rather than suppression of antiapoptotic mechanisms.     

Age at onset reveals different functional connectivity abnormalities in prodromal Alzheimer’s disease

Brain Imaging and Behavior - Age at symptom onset (AAO) underlies different Alzheimer’s disease (AD) clinical variants: late-onset AD (LOAD) is characterized by memory deficits, while...     

Key issues that need to be considered while revising the current annex of the European Council Recommendation (2003) on cancer screening

The 2003 European Council recommendation urging the Member States to introduce or scale up breast, cervical and colorectal cancer screening through an organized population-based approach has had a remarkable impact. We argue that the recommendation needs to be updated for at least two sets of reasons. First, some of the current clinical guidelines include new tests or protocols that were not available at the time of the Council document. Some have already been adopted by organized screening programs, such as newly defined age ranges for mammography screening, Human Papillomavirus (HPV)-based cervical cancer screening, fecal immunochemical test (FIT) and sigmoidoscopy for colorectal cancer screening. Second, the outcomes of randomized trials evaluating screening for lung and prostate cancer have been published recently and the balance between harms and benefits needs to be pragmatically assessed. In the European Union, research collaboration and networking to exchange and develop best practices should be regularly supported by the European Commission. Integration between primary and secondary preventive strategies through comprehensive approaches is necessary not only to maximize the reduction in cancer burden but also to control the rising trend of other noncommunicable diseases sharing the same risk factors.     

The effect of ICSI, maternal age, and embryonic stage on early clinical loss rate of twin versus singleton pregnancies

OBJECTIVES: To compare early loss rates between twin and singleton pregnancies following ART. STUDY DESIGN: First-trimester sonography counted the number of embryos with positive heartbeat in women undergoing IVF/ICSI and transfer of one to three embryos. The number of lost pregnancies was calculated from a second-trimester sonogram. Loss rates of the entire pregnancy were related to maternal age <38 or > or = 38 years, IVF or ICSI, and cleavage or blastocyst stage embryo transfers (in ICSI cases). RESULTS: Patients underwent IVF with (n = 672) and without (n = 189) ICSI. The overall odds of miscarrying the entire singleton pregnancy were 2.6 times that of a twin gestation (95% CI 1.5, 4.5). The disadvantage for singletons compared to twins seems more apparent in pregnancy after ICSI in the subgroup of patients <38 years (OR 2.9, 95% CI 1.5, 5.8). In this subgroup, the disadvantage conferred to singletons appeared only among days 2-3 embryo transfers (OR 3.0, 95% CI 1.3, 7.2). CONCLUSION: A significantly lower early spontaneous loss rate of twin pregnancies seems related to ICSI followed by cleavage stage embryo transfer in patients <38 years.