Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution

We report molecular dynamics simulations of a hydrophobic polymer-chain in aqueous solution between 260 K and 420 K at pressures of 1 bar, 3000 bar, and 4500 bar. The simulations reveal a hydrophobically collapsed structure at low pressures and high temperatures. At 3000 bar and about 260 K and at 4500 bar and about 260 K, however, an abrupt transition to a swelled state is observed. The transition is driven by a smaller volume and a remarkably strong lower enthalpy of the swelled state, indicating a steep positive slope of the corresponding transition line. The swelling is strongly stabilized by the energetically favorable state of water in the polymer's hydrophobic first hydration shell at low temperatures. This finding is consistent with the observation of a positive heat capacity of hydrophobic solvation. Moreover, the slope and location of the estimated swelling transition line for the collapsed hydrophobic chain coincides remarkably well with the cold denaturation transition of proteins.     

Formation of macroporous self-assembled hydrogels through cryogelation of Fmoc-Phe-Phe

In this study, it was found that macroporous hydrogels were formed when self-assembly of fluorenyl-9-methoxycarbonyl (Fmoc)-diphenylalanine (Phe-Phe) peptides was induced using glucono-δ-lactone (GdL) in apparently frozen samples. Formed cryogels exhibited a heterogeneous structure with pore walls of densely packed fibres of assembled dipeptides and pores in the range 10-100 μm. Hydrogels formed from the same composition above the freezing point exhibited a homogenous structure without any apparent porosity. The formed gels were characterised using microscopy techniques, CD-spectroscopy and stress sweeps. The cryogels exhibited less mechanical strength than the hydrogels that might be due to the heterogeneous structure of the former. It appeared that the self-assembled peptide both in the cryo- and hydrogel maintained the β-sheet structure commonly attributed to these.     

Impact of double cryogelation process on a macroporous dye-affinity hydrogel

Cryogels with interconnected channels allow high flow-through properties and mass transfer when dealing with complex mixtures such as non-clarified crude extracts. However, their mechanical strength can be challenged due to a large void volume inside the polymeric network. We have addressed this problem by forming a double-layer cryogel applied as a dye-affinity chromatography gel. In this study, poly(acrylamide-co-allyl glycidyl ether) cryogel was prepared at sub-zero temperature. The second layer was then prepared inside the primary cryogel under the same conditions to form a double-layer network. Cibacron Blue F3GA, a dye molecule, was immobilized on the surface of the cryogels. Bovine serum albumin was used as a model molecule to study the adsorption/elution procedure in batch and continuous modes. The maximum batch binding capacity and the dynamic binding capacity for the single-layer cryogel were 18 and 0.11, and for the double-layer cryogel were 7.5 and 0.9 mg/g of gel, respectively. However, the mechanical stability of the double-layer cryogel increased 7-fold (144 kPa). It was found that the kinetic and adsorption isotherms follow pseudo-second-order and Freundlich models, respectively. The regeneration of the columns after adsorption/elution cycles was evaluated, and no significant loss of capacity was observed after 10 cycles.     

Combined protein A imprinting and cryogelation for production of spherical affinity material

Cryogels have been demonstrated to be efficient when applied for protein isolation. Owing to their macroporous structure, cryogels can also be used for treating particle‐containing material, e.g. cell homogenates. Another challenging development in protein purification technology is the use of molecularly imprinted polymers (MIPs). These MIPs are robust and can be used repeatedly. The paper presents a new technology that combine the formation of cryogel beads concomitantly with making imprints of a protein. Protein A was chosen as the print molecule which was also be the target in the purification step. The present paper describes a new method to produce protein‐imprinted cryogel beads. The protein‐imprinted material was characterized and the separation properties were evaluated with regard to both the target protein and whole cells with target protein exposed on the cell surface. The maximum protein A adsorption was 18.1 mg/g of wet cryogel beads. The selectivity coefficient of protein A‐imprinted cryogel beads for protein A was 5.44 and 12.56 times greater than for the Fc fragment of IgG and protein G, respectively.     

Low-temperature and high-pressure structural behaviour of NaBi(MoO4)2—an X-ray diffraction study

NaBi(MoO₄)₂ has been characterized by single-crystal and powder X-ray diffraction in the temperature and pressure ranges 13-297 K and 0-25 GPa, respectively. The domain structure developing below proves that NaBi(MoO₄)₂ undergoes a ferroelastic phase transition associated with tetragonal I4₁/a to monoclinic I2/a symmetry change. The character of the unit cell evolution as a function of temperature indicates a continuous transition with the spontaneous strain as an order parameter. The structural distortion, due to small displacements of Bi³⁺ and Na⁺ ions, develops slowly. Therefore the overall changes, as measured in single-crystal diffraction at 110 and 13 K, appear to be subtle. High-pressure powder X-ray diffraction shows that the elastic behaviour is anisotropic, the linear compressibility along the a- and c-axes of the tetragonal unit cell being β_a=2.75(10)×10^-3 and , respectively. The cell contraction, stronger along the c-axis, causes the distances between the MoO₄ layers to be shortened. Consequently, the cation migration in the channels formed by MoO₄ tetrahedra becomes hindered, and any symmetry lowering phase transition is not observed up to 25 GPa. The zero-pressure bulk modulus is , and its pressure derivative .     

Low-temperature oxidation of polyethylene

To increase the surface energy of polyethylene (PE), the surface is subjected to oxidative treatment, such as chemical oxidation or Bunsen flame treatment. It is shown that oxidation of the polymer surface below 100° leads to increase in the contents of double bonds and of hydroxyl, carbonyl and carboxyl groups and also to branching of the molecules. Increase in the concentration of the polar groups causes increase in the surface free energy.     

Low-temperature cluster catalysis

Free and supported metal clusters reveal unique chemical and physical properties, which vary as a function of size as each cluster possesses a characteristic electron confinement. Several previous experimental results showed that the outcome of a given chemical reaction can be controlled by tuning the cluster size. However, none of the examples indicate that clusters prepared in the gas phase and then deposited on a support material are indeed catalytically active over several reaction cycles nor that their catalytic properties remain constant during such a catalytic process. In this work we report turn-over frequencies (TOF) for Pd(n) (n = 4, 8, 30) clusters using pulsed molecular beam experiments. The obtained results illustrate that the catalytic reactivity for the NO reduction by CO (CO + NO --> 1/2N(2) + CO(2)) is indeed a function of cluster size and that the measured TOF remain constant at a given temperature. More interestingly, the temperature of maximal reactivity is at least 100 K lower than observed for palladium nanoparticles or single crystals. One reason for this surprising observation is the character of the binding sites of these small clusters: N(2) forms already at relatively low temperatures (400 and 450 K) and therefore poisoning by adsorbed nitrogen adatoms is prevented. Thus, small clusters not only open the possibility of tuning a catalytic process by changing cluster size, but also of catalyzing chemical reactions at low temperatures.     

Low-temperature photolysis of azidostyrylquinolines

Low-temperature photolysis of 2-and 4-(4′-azidostyryl)quinolines and azidohemicyanine dye, 1-methyl-4-(4′-azidostyryl)quinolinium iodide, was studied in an ether-ethanol matrix at 77 K and a methyltetrahydrofuran matrix at 5 K by means of electronic absorption spectroscopy and ESR technique. The formation of corresponding triplet nitrenes with absorption bands at 380–440 nm and zero-field splitting parameters of |D/h _cl = 0.781–0.790 cm^?1 and E = 0 was detected. It was found that the introduction of the positive charge into the azidostyrylquinoline molecule resulted in a bathochromic shift of the nitrene absorption band by ～40 nm and a decrease in the D by 0.005 cm^?1 due to charge transfer from the nitrene center to the quinoline moiety.     

Low-temperature radiation chemistry

An overview of studies on the radiation-initiated chemical reactions of polymerization, copolymerization, graft polymerization, telomerization, oxidation, cryoozonolysis, hydrobromination of olefins, and chlorination of paraffins occurring at low and ultralow temperatures has been presented. Particular attention has been paid to self-oscillating reactions.     

Nanomaterials under high-pressure

The use of high-pressure for the study and elaboration of homogeneous nanostructures is critically reviewed. Size effects, the interaction between nanostructures and guest species or the interaction of the nanosystem with the pressure transmitting medium are emphasized. Phase diagrams and the possibilities opened by the combination of pressure and temperature for the elaboration of new nanomaterials is underlined through the examination of three different systems: nanocrystals, nano-cage materials which include fullerites and group-14 clathrates, and single wall nanotubes. This tutorial review is addressed to scientist seeking an introduction or a panoramic view of the study of nanomaterials under high-pressure.     

Low-temperature luminescence of aqueous systems

The emission and emission excitation spectra of various hydroxides, acids and halide salts, separately and in combination, have been measured in aqueous, methanolic and ethanolic solutions. The shifts of spectra as a function of solvent, the disappearance of emission in ethanol and the enhancement of the emissivity of the acids by the addition of halide salts suggest that the emitting entity is an exciplex of the hydrogen atom with water. The candidacy of(H₃O^.) or, better [(H₂O)_nH^.] as the emitter accords with all experimental information; however, GAUSSIAN 80 calculations for the lowest-energy quartet state suggest that this state is dissociative in the gas phase.     

Low-temperature regeneration of activated carbon

Activated granular carbon samples, routinely equilibrated with synthetic aqueous solutions of o-NO₂-phenol, were regenerated by heating up to 500 °C in a dynamic N₂ atmosphere. Kinetic evaluation of five subsequent thermodesorption cycles was accomplished by two different non-isothermal computation methods. Quantitative estimation of adsorbent-adsorbate interactions was carried out by means of the thermal coefficient of the massive adsorbate release.

---

Zusammenfassung Routinemäßig mit synthetischen wässrigen Lösungen von o-Nitrophenol äquilibrierte, granulierte Aktivkohle-Proben wurden durch Aufheizen auf 500 °C in einer dynamischen N₂-Atmosphäre regeneriert. Fünf aufeinanderfolgende Thermodesorptionszyklen wurden nach zwei verschiedenen nicht-isothermen Berechnungsmethoden kinetisch ausgewertet. Die quantitative Bestimmung der Adsorbent-Adsorbat-Wechselwirkungen wurde mittels der thermischen Koeffizienten der massiven Adsorbatdesorption ausgeführt.

---

, - , 500 ° . . — .

     

Low-temperature study of ultradisperse polytetrafluoroethylene

The temperature behavior of the IR and EPR spectra of the industrial polytetrafluoroethylene F-4 sample and ultradisperse polytetrafluoroethylene (UPTFE) sample obtained by the thermogas dynamic method has been studied. It is shown that the spiral-chain conformation is preserved for both structures and that ordering tends to increase at lower temperatures. The IR spectral parameters of UPTFE were found to change irregularly at 273 K and 233 K due to a rearrangement of the crystalline component of the polymer. The observed temperature changes in the EPR spectra of the middle radicals in the samples are caused by gradual freeze-out of the molecular motions in UPTFE to a temperature ～25° lower than in F-4, due to differences in the molecular and supramolecular structures of the samples.     

Low-temperature studies of encapsulated proteins

Water-soluble proteins encapsulated within reverse micelles may be studied under a variety of conditions, including low temperature and a wide range of buffer conditions. Direct high-resolution detection of information relating to protein folding intermediates and pathways can be monitored by low-temperature solution NMR. Ubiquitin encapsulated within AOT reverse micelles was studied using multidimensional multinuclear solution NMR to determine the relationship between protein structure, temperature, and ionic strength. Ubiquitin resonances were monitored by 15N HSQC NMR experiments at varying temperatures and salt concentrations. Our results indicate that the structure of the encapsulated protein at low temperature experiences perturbation arising from two major influences, which are reverse micelle-protein interactions and low-temperature effects (e.g., cold denaturation). These two effects are impossible to distinguish under conditions of low ionic strength. Elevated concentrations of nondenaturing salt solutions defeat the effects of reverse micelle-protein interactions and reveal low-temperature protein unfolding. High ionic strength shielding stabilizes the reverse micelle at low temperatures, which reduces the electrostatic interaction between the protein and reverse micelle surfaces, allowing the phenomenon of cold denaturation to be explored.     

Low-temperature deacylation of N-monosubstituted amides

[reaction: see text] The (PhO)(3)P.Cl(2) reagent, prepared in situ by titrating a solution of triphenyl phosphite with chlorine, is used to convert N-monosubstituted amides into their corresponding amines. The reaction, if compared to other traditional methods, shows the advantage of very mild conditions and low temperature (-30 degrees C-->rt).     

Low-temperature heat capacity of diglycylglycine

Heat capacity of tripeptide diglycylglycine was measured in a temperature range from 6.5 to 304 K. The results were compared with those for glycine and glycylglycine. Peptide bonding was found not to change C _P(T) virtually above 70 K, where heat capacity does not obey the Debye model. Comparison with literature data allows one to expect a significant difference in the heat capacity for enantiomorph and racemic species of valine and leucine, like it was found recently for D-and DL-serine.