Comprehensive conformational analysis of the nucleoside analogue 2'-beta-deoxy-6-azacytidine by DFT and MP2 calculations

A comprehensive conformational analysis of isolated 2'-beta-deoxy-6-azacytidine (d6AC), an analogue of therapeutically active 6-azacytidine (6AC), has been performed by means of ab initio calculations at the MP2/6-311++G(2df,pd)//DFT B3LYP/6-31G(d,p) level of theory. Among the 81 conformers located within a 7.83 kcal/mol Gibbs energy range at T = 298.15 K, 38 contain syn-oriented bases with respect to 2'-deoxyribose; the other conformers include anti-oriented bases. Energetic analysis of these conformers shows that conformational equilibrium of isolated d6AC at T = 298.15 K is shifted to syn conformation with a syn/anti ratio estimated as 61.4%:38.6%. As far as the sugar conformation is concerned, 40 conformers contain north (N) (with 0.3 degrees < or = P < or = 40.1 degrees), and the rest possess south (S) (with 157.1 degrees < or = P < or = 207.0 degrees) puckers, where P is the pseudorotational angle of the furanose ring. The S/N occupancy ratio is estimated as 80.2%:19.8% (T = 298.15 K). The two most stable conformers are energetically quasidegenerate and correspond to both C2'-endo/syn conformers differing only by orientation of the O3'H hydroxyl group. They are both stabilized by means of similar intramolecular H-bonds, i.e., O5'H...O2, C2'H2...O2, and C2'H2...O5'. As examined by AIM criteria, from 1 to 3 H-bonds per conformer were identified among 13 possible interactions: O5'H...O2, O5'H...N6, O3'H...O5', O5'H...O3', C1'H...O2, C2'H2...O2, C2'H2...O5', C3'H...O2, C3'H...N6, C5'H1...O2, C5'H2...O2, C5'H1...N6, and C5'H2...N6. The biological effect of d6AC is conceived as an inhibition of replicative DNA polymerase caused by an unusual orientation of the sugar residue against the base in the only A form DNA-like conformer.     

Spectral-luminescent and solvatochromic properties of 2-(3-coumarinyl)-5-(2′-(R-amino)-phenyl)-1,3,4-oxadiazoles

Spectral-luminescent properties of the newly synthesized 2-(3^′-coumarinyl)-5-(2′-(R-amino)-phenyl)-1,3,4-oxadiazoles has been investigated in solvents of various polarity and hydrogen-bonding ability. It has been found that for all the studied compounds no excited state intramolecular proton transfer occurs despite the presence of coumarinyl fragment - electron acceptor effect of the coumarinyl fragment is not sufficient to increase the excited state acidity of the amino group. It has been found that the absorption spectra of the studied compounds shift to higher energy with increase in solvent polarity, whereas corresponding fluorescence spectra shift to lower energy with solvent polarity increase. It has been suggested that long-wavelength shifts of the fluorescence spectra of the studied compounds with increase in solvent polarity is caused by the solvent relaxation. The observed solvent relaxation effect allow us to propose some of the studied compounds as potential probes to monitor changes in solvent relaxation in low-polar media and as potential probes for rigidochromic effect.     

Structure-electrical properties relationships of polymer composites filled with Fe-powder

Structure-properties relationships of composite materials, consisting of a polymer matrix and metal inclusions, is very important for designing new materials with desirable properties. In the present work the electrical and dielectric properties of several composites, consisting of a polymer matrix and iron (Fe) particles as filler, were investigated. Broadband dielectric relaxation spectroscopy measurements were carried out. The electrical behaviour of the composites is described in terms of the percolation theory. Percolation threshold values were calculated and the values of the dielectric permittivity critical exponent were found in good agreement with the theoretical ones. The influence of using different polymer matrices on the physical properties of the composites was also of particular interest. The results were related to the microstructure of the composites and a schematic model was proposed.     

Structure and dielectric properties of a thermoplastic blend containing dispersed metal

In the present work broadband dielectric relaxation spectroscopy measurements were employed to investigate the dielectric properties of polymer composites. A polyethylene/polyoxymethylene (PE/POM) thermoplastic blend was used as a matrix, while the inclusions were iron (Fe) particles. For comparison, the two pure polymers- PE and POM- were used as a matrix, too. In the PE/POM-Fe composites, the polymer matrix is two-phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. In PE-Fe and POM-Fe composites, the filler spatial distribution is random. The behaviour of all the composites studied is described in terms of the percolation theory. The PE/POM-Fe composites, based on the PE/POM blend, demonstrate different electrical behaviour compared to that of POM-Fe and PE-Fe systems. The percolation threshold value of the PE/POM-Fe composites was found much lower than that of the other two systems. The results were related to the microstructure of the composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behaviour of the PE/POM-Fe composites by taking into account the ordered distribution of the filler particles in a binary polymer matrix. Optical microscopy photographs confirm this model.     

Water sorption and electrical/dielectric properties of organic-inorganic polymer blends

Organic-inorganic polymer blends (OIPB) were obtained by reaction of organic and inorganic oligomers. The organic oligomer was synthesized with 2,4-toluene diisocyanate (TDI) and oligooxypropylene glycols (OPG) with various molecular weights (MW). The inorganic component was a water solution of sodium silicate. The OIPB obtained are hydrophilic and have great water sorption ability (the relative weight of sorbed water reaches 2000 %). The kinetics of water sorption and the changes of electrical conductivity during sorption were studied. Sorption ability, and mechanical, electrical and dielectric properties of OIPB depend on molecular weight of OPG: conductivity increases with increasing MW, whereas the sorption ability correlates with the mechanical properties. The influence of the inorganic phase content on the electrical and dielectric properties was studied as well.     

Structure‐dependent conductivity and microhardness of metal‐filled PVC composites

Metal‐filled composites of a commercial PVC (polyvinyl chloride) powder (mean particle size d_p ≈ 100 microns) and a metal powder (mean particle size d_f about 100 microns for copper, Cu, and about 10 microns for nickel, Ni) prepared by mechanical mixing in a ball mill, subsequent hot‐pressing at 443 K and rapid cooling to 300 K, were characterized by the room‐temperature measurements of electrical conductivity σ, density ρ and microhardness H. The sudden jumps of about 17 orders of magnitude followed by a much slower growth up to the limiting filler fraction ϕ* on the log σ vs. ϕ plots are the evidence for the onset of percolation transitions, at filler volume contents ϕ_c1 = 0.05 and 0.04 for PVC/Cu and PVC/Ni, respectively. For both systems, the values of H exhibited an initial steep increase up to ϕ_c2 = 0.07, followed by an apparent plateau extending up to ϕ = 0.18. However, drastic differences in the patterns of composition dependence of H were observed at higher metal loadings, i.e., a continuous increase of H up to the leveling‐off at ϕ* for PVC/Cu, in contrast to a sudden drop of H at ϕ = 0.20 and subsequent slow increase for PVC/Ni. For both composites the apparent density ρ′ of a polymer matrix remained the same as that of the neat PVC in the composition interval ϕ < 0.20, while at ϕ* > 0.20 a precipitous drop of ρ₁ was observed due to the formation of polymer‐free voids between filler particles (crowding effect) as ϕ approaches ϕ*. The observed effects were analyzed in terms of a tentative model envisaging cross‐overs from “dilute suspension regime” to “semi‐dilute suspension regime” in the concentration range ϕ_c1 to ϕ_c2, and from “semi‐dilute suspension regime” to “concentrated suspension regime” above ϕ = 0.20. Different behavior in this latter regime was explained by intrinsic differences in the structure of conductive infinite clusters between mixtures of particles of about the same size (PVC/Cu) and of widely different sizes (PVC/Ni).