Thermal study of irradiated polyacetylene films

Polyacetylene films irradiated by-rays up to 100 MRad were studied by means of TMA, DTG and DSC methods.It is shown that as the irradiation dose increases the concentration of topological branching knots into the polymer chains and theT _g values decrease, the total mass loss and the enthalpy of the thermal isomerization reaction also decrease.     

A quantum chemical study of [1.1.1.1]pagodane and its related compounds

The PBE0/6-31G** quantum chemical method is used to determine the symmetry and equilibrium structural parameters of the molecules of [1.1.1.1]pagodane (C₂₀H₂₀, D _2h ), two dienes (C₂₀H₂₀, D _2h ), two diradicals (C₂₀H₂₀, C _2ν ), and two dications (C₂₀H₂₀²⁺, D _2h and C _2ν ). The energy of a highly symmetric dication with a rectangular cycle is lower by 36 kcal/mole than that of a low symmetric dication with a trapezoidal cycle. The polarization interaction with liquid methylene chloride causes its decrease by 147 kcal/mole.     

Changes of Parameters of Mixed Monolayers Based on Photosensitive Crown Ether in the Presence of Silver Ions

The use of the achievements attained in colloid chemistry and other fields of chemical sciences allows us to develop new nanoscale systems for chemosensorics. Mixed monolayers consisting of a new chemosensory amphiphilic dithia-18-crown-6, which contains a dye of 4-alkylpyridine series with polyvinyl stearate at the water–air interface, are obtained and studied. The influence of various concentrations of silver ions on these mixed monolayers is studied. A promising method for the detection of silver ions in water using the given nanoscale systems based on chemosensory amphiphilic dithia-18-crown-6 is proposed.     

Peculiarities of the SERS Spectra of 4,4′-Bipyridine Molecule in a Single Molecule Detection Regime

The paper describes briefly main statements of the theory of the SERS spectra with regards to the single molecule regime, when the enhancement achieves the values ~10¹⁴?10¹⁵. Analysis of the spectra of 4,4′-bypyridine, obtained on the dimer lattice of sharp nanoparticles points out that the observed enhancement is caused exclusively by a strong quadrupole light-molecule interaction, which manifests in the presence of lines, caused by vibrations with the unit irreducible representations of the D₂ and D_2h symmetry groups, which apparently describe the symmetry properties of the molecule. The study of the spectra, obtained by tip enhanced spectroscopy demonstrates that the strong quadrupole light-molecule interaction still has a governing role, however the strong dipole interaction still manifests in the existence of very weak forbidden lines. This result apparently is associated with another experimental geometry.     

Structure of Supramers Formed by the Amphiphile Biotin-CMG-DOPE

The synthetic function-spacer-lipid (FSL) amphiphile biotin-CMG-DOPE is widely used for delicate ligation of living cells with biotin residues under physiological conditions. Since this molecule has an “apolar-polar-hydrophobic” gemini structure, the supramolecular organization is expected to differ significantly from the classical micelle. Its organization is investigated with experimental methods and molecular dynamics simulations (MDS). Although the linear length of a single biotin-CMG-DOPE molecule is 9.5 nm, the size of the dominant supramer globule is only 14.6 nm. Investigations found that while the DOPE tails form a hydrophobic core, the polar CMG spacer folds back upon itself and predominantly places the biotin reside inside the globule or planar layer. MDS demonstrates that <10 % of biotin residues on the highly water dispersible globules and only 1 % of biotin residues in layer coatings are in an linear conformation and exposing biotin into the aqueous medium. This explains why in biotin-CMG-DOPE apolar biotin residues both in water dispersible globules and coatings on solid surfaces are still capable of interacting with streptavidin.     

Supramolecular nanostructures based on bacterial reaction center proteins and quantum dots

Design of the nanostructures based on membrane proteins (the key functional elements of biomembranes) and colloid nanoparticles is a fascinating field at the interface of biochemistry and colloids, nanotechnology and biomedicine. The review discusses the main achievements in the field of ultrathin films prepared from bacterial reaction center proteins and light-harvesting complexes, as well as these complexes tagged with quantum dots. The principles of preparation of these thin films and their structure and properties at different interfaces are described; as well as their characteristics estimated using a combination of the modern interfacial techniques (absorption and fluorescence spectroscopy, atomic force and Brewster angle microscopy, etc.) are discussed. Further approaches to develop the nanostructures based on the membrane proteins and quantum dots are suggested. These supramolecular nanostructures are promising prototypes of the materials for photovoltaic, optoelectronic and biosensing applications.     

Spin densities from subsystem density-functional theory: assessment and application to a photosynthetic reaction center complex model

Subsystem density-functional theory (DFT) is a powerful and efficient alternative to Kohn-Sham DFT for large systems composed of several weakly interacting subunits. Here, we provide a systematic investigation of the spin-density distributions obtained in subsystem DFT calculations for radicals in explicit environments. This includes a small radical in a solvent shell, a π-stacked guanine-thymine radical cation, and a benchmark application to a model for the special pair radical cation, which is a dimer of bacteriochlorophyll pigments, from the photosynthetic reaction center of purple bacteria. We investigate the differences in the spin densities resulting from subsystem DFT and Kohn-Sham DFT calculations. In these comparisons, we focus on the problem of overdelocalization of spin densities due to the self-interaction error in DFT. It is demonstrated that subsystem DFT can reduce this problem, while it still allows to describe spin-polarization effects crossing the boundaries of the subsystems. In practical calculations of spin densities for radicals in a given environment, it may thus be a pragmatic alternative to Kohn-Sham DFT calculations. In our calculation on the special pair radical cation, we show that the coordinating histidine residues reduce the spin-density asymmetry between the two halves of this system, while inclusion of a larger binding pocket model increases this asymmetry. The unidirectional energy transfer in photosynthetic reaction centers is related to the asymmetry introduced by the protein environment.     

Long-lived excited state of spiroanthroxazine after its matrix isolation in halogenated polyolefins by supercritical fluid impregnation

Supercritical fluid impregnation was used to perform matrix isolation of spiroanthroxazine (SAO) photochromic molecules in halogenated polymer films (polyvinylchloride and tetrafluoroethylene copolymer with vinylidene fluoride). An excited (colored) SAO form not relaxing to the ground (colorless) state was stabilized in polymers. Such a long-lived excited SAO form could exist as an isomer of the B form (B₅₃₅ with an absorption band at λ_max = 535 nm) in the fluorinated polymer matrix, for which the absorption maximum was shifted by more than 75 nm to the blue, and as the B form with a small admixture of the B₅₃₅-form (in polyvinylchloride).     

Paradox of the Variation of the Bulk Resistance of Potassium Ion-Selective Electrode Membranes within Nernstian Potentiometric Response Range

Russian Journal of Electrochemistry - Bulk resistance and other electrochemical properties of membranes of K+-selective electrodes (ISEs) containing valinomycin are measured by means of...     

High-resolution diffractometer for structural studies of polycrystalline materials

A high-resolution powder diffraction station with synchrotron radiation has been created. The diffractometer was developed on the basis of two Microcontrol precision goniometers. The latter provide the independent movement of a specimen and a detector with a minimal step of 2θ=0.001°. Using the Si(111) crystal as the analyzer, we obtained half widths of the (212), (203), and (301) reflections for α-SiO₂ (0.02–0.025° 2θ). The device was tested using α-Al₂O₃ as the standard. Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 85–91, March–April, 1994. Translated by T. Yudanova