Activity of TiO2 deposited by the CVD method on ammoxidized surface of a carbonaceous material in hydrogenation of styrene

Hydrogenation of styrene has been applied as a test reaction to study the catalytic activity of TiO₂ deposited by the CVD (chemical vapour deposition) method on the surface of a carbonaceous material enriched in nitrogen (C_N).     

Debye type dielectric relaxation and the glass transition of alcohols

Many hydrogen-bonded liquids, especially glass-forming cases, display a dielectric relaxation behavior that differs qualitatively from that of other simple liquids. The majority of models aimed at explaining this unusual dielectric behavior associate the prominent Debye process with structural relaxation, viscous flow, and the glass transition. We perform dielectric and calorimetric studies of glass-forming mixtures of 2-ethylhexylamine and 2-ethyl-1-hexanol across the entire composition range. The kinetic glass transition temperature derived from the large dielectric Debye peak decreases, whereas that of the much smaller and asymmetrically broadened peak increases upon addition of amine. Only the latter feature coincides with the calorimetric glass transition results, implying that molecular structure and dielectric polarization fluctuate on time scales that can differ by orders of magnitude in many hydrogen-bonding liquids.     

Layer by layer buildup of polysaccharide films: physical chemistry and cellular adhesion aspects

The formation ofpolysaccharide films based on the alternate deposition of chitosan (CHI) and hyaluronan (HA) was investigated by several techniques. The multilayer buildup takes place in two stages: during the first stage, the surface is covered by isolated islets that grow and coalesce as the construction goes on. After several deposition steps, a continuous film is formed and the second stage of the buildup process takes place. The whole process is characterized by an exponential increase of the mass and thickness of the film with the number of deposition steps. This exponential growth mechanism is related to the ability of the polycation to diffuse "in" and "out" of the whole film at each deposition step. Using confocal laser microscopy and fluorescently labeled CHI, we show that such a diffusion behavior, already observed with poly(L-lysine) as a polycation, is also found with CHI, a polycation presenting a large persistence length. We also analyze the effect of the molecular weight (MW) of the diffusing polyelectrolyte (CHI) on the buildup process and observe a faster growth for low MW chitosan. The influence of the salt concentration during buildup is also investigated. Whereas the CHI/HA films grow rapidly at high salt concentration (0.15 M NaCl) with the formation of a uniform film after only a few deposition steps, it is very difficult to build the film at 10(-4) M NaCl. In this latter case, the deposited mass increases linearly with the number of deposition steps and the first deposition stage, where the surface is covered by islets, lasts at least up to 50 bilayer deposition steps. However, even at these low salt concentrations and in the islet configuration, CHI chains seem to diffuse in and out of the CHI/HA complexes. The linear mass increase of the film with the number of deposition steps despite the CHI diffusion is explained by a partial redissolution of the CHI/HA complexes forming the film during different steps of the buildup process. Finally, the uniform films built at high salt concentrations were also found to be chondrocyte resistant and, more interestingly, bacterial resistant. Therefore, the (CHI/HA) films may be used as an antimicrobial coating.     

Field-induced structural ordering in electrorheological fluids

Structural relaxations of an electrorheological fluid (ERF) due to changes in the applied electrical field strength or shear rate are observed on time scales 1 s<t<40 000 s. Commercial ERFs consisting of mesoscopic polyurethane particles in a silicone oil matrix were studied by three different experimental techniques in order to obtain and compare the characteristic relaxation times. It is demonstrated that dielectric spectroscopy, viscosimetry and light transmission experiments represent the same results concerning the structural relaxation phenomena of ERFs when electrical fields are applied. The tendency of strong induced dipoles to align the particles in the direction of the field increases the effective dipole moment and therefore , the shear viscosity and the amount of light transmitted along the field direction in an ITO/glass sandwich cell. The optical experiment is capable of resolving fast processes within the first 1 ms if large electrical fields are applied. The effects of electrophoresis and shearing, which both counteract the field induced structures, are also addressed.     

Diluent effects on the Debye-type dielectric relaxation in viscous monohydroxy alcohols

With the recognition that the Debye-type dielectric relaxation of liquid monohydroxy alcohols does not reflect the structural relaxation dynamics associated with the viscous flow and the glass transition, its behavior upon dilution is expected to differ from that of real alpha-processes. We have investigated the Debye-type dielectric relaxation of binary alcohol/alkane mixtures across the entire concentration range in the supercooled regimes. The focus is on 2-ethyl-1-hexanol in two nonpolar liquids, 3-methylpentane and squalane, which are more fluid and more viscous than the alcohol, respectively. The Debye relaxation is found to occur only for alcohol mole fractions x > 0.2 and is always accompanied by a non-Debye relaxation originating from the alcohol component. Prior to its complete disappearance, the Debye relaxation is subject to broadening. We observe that the Debye dynamics of 2-ethyl-1-hexanol is accelerated in the more fluid 3-methylpentane, while the more viscous squalane leads to longer Debye relaxation times. The present experiments also provide evidence that the breakdown of the Debye relaxation amplitude does not imply the absence of hydrogen-bonded structures.     

Templates direct the sequence-specific anchoring of the C-terminus of peptido RNAs

When amino acids and ribonucleotides react in aqueous condensation buffer, they form peptido RNA with a phosphoramidate bond between the N-terminus of the peptide and the 5′-terminal phosphate of a ribonucleotide. If peptido RNA was the product of spontaneous reactions of amino acids and nucleotides, there must have been a transition to peptidyl tRNAs, where the C-terminus of the peptide is ester-linked to the 2′,3′-terminus of an oligonucleotide. Here we report how short peptido RNAs react with the 3′-terminus of oligodeoxynucleotides, templated by RNA strands. In our model system, the rate and yield of the anchoring of the C-terminus of the dipeptido dinucleotides to an amino group was found to depend on the sequence of the peptide, the 5′-terminal nucleotide of the dinucleotide and the RNA template. In all cases tested, highest yields were found for dinucleotides hybridizing next to the primer terminus. For the most reactive species, GlyPro-AA, anchoring yields ranged from 8–99%, depending on the template. When LeuLeu-AA, PhePhe-AA and GlyGly-AA were allowed to compete for anchoring on 3′-UUC-5′ as templating sequence, they gave a product ratio of 1 : 2 : 6, and this selectivity was almost independent of the terminal base of the primer. Our results show the control that a simple duplex context has over the covalent anchoring of peptido RNAs at a position known from peptidyl tRNAs. Processes of this type may have bridged the gap between untemplated condensation reactions and the highly specific processes of ribosomal protein synthesis.

The C-terminus of dipeptido dinucleotides reacts with the 3′-terminus of a primer strand in template directed fashion with a strong dependence on the structures of peptide and template.     

Heterogeneous thermal excitation and relaxation in supercooled liquids

We investigate a phenomenological model which rationalizes the effects of dielectric hole burning on the basis of heterogeneous dielectric and specific heat relaxation in supercooled liquids. The quantitative agreement between model predictions and dielectric hole-burning observations is lost if the assumption of correlated dielectric and thermal relaxation times is removed from the model. This suggests that dynamically distinct domains in real liquids are associated with a time constant which characterizes both the structural and thermal relaxation behaviors. The calculations demonstrate that the observed burn-induced modifications reflect the spectral selectivity and persistence time of the fictive temperatures within these domains, and that 100 or more cycles of the sinusoidal burn field can be required to saturate the heat accumulated in the slow degrees of freedom. It is also shown that the recovery of dielectric holes is entirely accounted for by the model, and that the persistence times do not provide direct insight into rate exchange processes. Additionally, the model predicts that the heating effects considered here are a significant source of nonlinear dielectric behavior, even in the absence of deliberate frequency selective hole burning.     

On the effect of covalently appended quinolones on termini of DNA duplexes

Quinolones are gyrase inhibitors that are widely used as antibiotics in the clinic. When covalently attached to oligonucleotides as 5'-acylamido substituents, quinolones were found to stabilize duplexes of oligonucleotides against thermal denaturation. For short duplexes, such as qu-T*GCGCA, where qu is a quinolone residue and T is a 5'-amino-5'-deoxythymidine residue, an increase in the UV melting point of up to 27.8 degrees C was measured. The stabilizing effect was demonstrated for all quinolones tested, namely nalidixic acid, oxolinic acid, pipemidic acid, cinoxacin, norfloxacin, and ofloxacin. The three-dimensional structure of (oa-T*GCGCA)2, where oa is an oxolinic acid residue, was solved by two-dimensional NMR spectroscopy and restrained molecular dynamics. In this complex, the oxolinic acid residues disrupt the terminal T1:A6 base pairs and stack on the G2:C5 base pairs. The displaced adenosine residues bind in the minor groove of the core duplex, while the thymidine residues pack against the oxolinic acid residues. The "molecular cap" thus formed fits tightly on the G:C base pairs, resulting in increased base-pairing fidelity, as demonstrated in UV melting experiments with the sequence oa-T*GGTTGAC and target strands containing a mismatched nucleobase. The structure of the "molecular cap" with its disrupted terminal base pair may also be helpful for modeling how quinolones block re-ligation of DNA strands in the active site of gyrases.     

Dynamics of glass-forming liquids. IX. Structural versus dielectric relaxation in monohydroxy alcohols

The prominent Debye-type but non-Arrhenius dielectric relaxation is a feature common to many monohydroxy alcohols in their liquid state. Although this exponential process is often considered to reflect the primary structural relaxation, only a faster, smaller, and nonexponential relaxation peak correlates with viscous flow and mechanical relaxation. We provide dielectric relaxation data for 2-methyl-1-butanol, 2-ethyl-1-hexanol, and 3,7-dimethyl-1-octanol across ten decades in time. Based on these and previous results, we show that there exists a variety of dielectric to mechanical relaxation time ratios in the viscous regime, but a universal value of 100 for that ratio appears to evolve in the high temperature limit. The temperature dependence for both the relaxation time and strength of the Debye peak differs from the typical behavior of structural dynamics in terms of the alpha process. The implications of these findings for rationalizing the Debye-type dielectric process of hydrogen-bonded liquids are discussed.