Yoshiko Urabe

Solid-solid phase transformation of uniaxially oriented n-alkane crystals of high purity

Abstract The 13 C solid-state NMR spectrum and the spin-lattice relaxation time, T 1C , have been measured at different temperatures for a uniaxially oriented n -C 27 H 56 crystal mat of purity 99.98%, setting the orientation axis parallel or perpendicular to the static magnetic field. On the basis of the change in the lineshape and T 1C value in the different phase A, A′, and H, the molecular motions such as rapid local fluctuation and rotation about the molecular chain axis are discussed in the respective phases.

Phase transitions of odd n-alkanes of high purity - pressure dependence of the transition temperatures determined by high pressure DTA

Abstract Polymorphic transitions of n -pentacosane, n -heptacosane and n -tritriacontane were examined under high pressure up to 500 MPa by high pressure DTA. The samples used were very pure and showed at atmospheric pressure solid-solid transitions, whose number depends on the carbon number in a molecule, beside the rotator transition. Their phase diagrams are presented. As well as the rotator phase, the other high temperature phases showed the upper pressure limit for their existing regions.

Model Study Using Designed Selenopeptides on the Importance of the Catalytic Triad for the Antioxidative Functions of Glutathione Peroxidase

Although the catalytic triad of glutathione peroxidase (GPx) has been well recognized, there was little evidence for the relevance of the interactions among the triad amino acid residues, i.e., selenocysteine (U), glutamine (Q), and tryptophan (W), to the GPx antioxidative functions. Using a designed selenopeptide having an amino acid sequence of GQAUAWG, we demonstrate here that U, Q, and W present at the active site can interact with each other to exert the enzymatic activity. The amino acid sequence was chosen on the basis of the Monte Carlo molecular simulation for various selenopeptides in polarizable continuous water using the SAAP force field (SAAP-MC). Measurement of the GPx-like activity for the selenopeptide obtained by solid-phase peptide synthesis revealed that the antioxidant activity is cooperatively enhanced by the presence of Q and W proximate to U, although the activity was low compared to selenocystine (U2). The effect of Q on the activity was more important than that of W. In addition, the fluorescence spectrometry suggested a close contact between U and W. These experimental observations were supported by SAAP-MC simulation as well as by ab initio calculation. The latter further suggested that the interaction mode among the triad changes depending on the intermediate states.

Freezing-in phenomena of high-temperature structures and their structural determination for henpentacontane and dopentacontane

Abstract Pure henpentacontane (C51) and dopentacontane (C52) were synthesized. Solution-crystallized samples of both n-alkanes clearly showed solid-solid phase transition peaks on their DSC curves before melting, while melt-crystallized samples did not show these peaks. From X-ray and SEM observations, this phenomenon was ascribed to the fact that the high-temperature structures were frozen during crystallization from the melt. By solution crystallization at a temperature range in which the high-temperature phase exists in bulk crystals, single crystals consisting of the high temperature structures were obtained. In X-ray diffraction, the lattice parameters for C51 were determined from Weissenberg photographs, and that of P2 1 a for C52, from oscillation observations. It was noted that the even-odd effect of the number of carbon atoms still works with the high-temperature structures.

Simultaneous determination of the heat capacity and the heat of the transitions for long-chain compounds with a heat-flux type DSC

Abstract The heat capacity method of DSC measurements, for which sapphire was used as a standard, was applied to determine simultaneously the heat of the transitions and the heat capacity for long-chain compounds. A heat-flux DSC was used because of its relatively high sensitivity at slow scanning rates. The data were analyzed with a computer system. Measurements on indium were performed to obtain information about factors affecting the accuracy. Fairly satisfactory results were obtained for the heat of melting and the heat capacity at heating rates of 0.5 and 2 K min −1 . Long-chain symmetrical ketones were also used to test the reliability of the measurement because their thermal behavior is very simple. Solution-grown crystals of henpentacontane and dopentacontane clearly showed high-temperature modifications with a high heat capacity. Analysis by the present method indicates that the heat capacities of these modifications have a relatively large contribution to the overall change in enthalpy from the stable crystal at room temperature to the liquid.

Thermal analysis of long-chain ketones and the extrapolated equilibrium melting for polyethylene

Abstract The thermodynamic quantities of fusion for long-chain symmetrical ketone homologues were studied by DSC measurements. The carbon numbers in the chain measured were from 23 to 63. A heat-flux-type DSC was used because of its relatively high sensitivity at slow scanning rates. In addition to these, the heat capacities of ketones in the solid and liquid states were measured, for which sapphire was used as a standard material. Although the symmetrical ketones have almost the same crystal system and lattice constants as those of odd n-alkane crystals, known as model compounds for polyethylene, ketones have no high-temperature modifications. In view of these points, the purpose of this paper is to examine the linearity of the entropies of fusion, ΔSm(T0m) at the equilibrium melting temperature T0m for n = infinity, calculated using the DSC data measured with ketones, also considering the contributions of another entropy term, R ln n as pointed out by Flory and Vrij in 1963. It is concluded that not only the heats of fusions, ΔHm(T0m), but also ΔSm(T0m) satisfactorily indicate a linearity. Furthermore, it seems reasonable to conclude that the validity of the term R ln n is not clearly shown in these analyzed results.

Detailed thermal study of the rotator phase of pure heptacosane (n‐C27H56) by alternating current calorimetry

The dynamic heat capacity of heptacosane was measured by ac calorimetry. Samples with a homologous purity of 99.9% were used. The characteristic feature of ac calorimetry of excluding latent heat made it possible to study details of the dynamic heat capacity of the rotator phase. In both the heating and cooling processes at a constant rate of 0.5 K/min, slow motion with a correlation time comparable to 1/3 s was observed. At constant temperatures of the rotator phase significant and slow annealing effects were observed. Experimental results showed that the equilibrium state at temperatures equal to or higher than 329.9 K was apparently different from the equilibrium state at temperatures equal to or lower than 329.5 K. It was shown that the observed experimental results could be explained by assuming existence of a parameter which changes only very slowly.

The solubility limit of a solid solutions for the systems of long-chain symmetrical ketones in ann-alkane matrix

The systems of long-chain symmetrical ketones inn-alkane matrices form solid solutions for a low concentration range of the ketone, when the number of carbon atoms n in a ketone molecule Kn is equal to or lower than that in an-alkane molecule. The Kn examined were K25 to K39. The purity of all the samples used was very high. The solubility limit of these solid solutions was determined with DSC measurements. Above the solubility limit (the critical concentration) the systems changed to eutectic mixtures. The relationships for the solubility limit were derived, taking into consideration the lateral two-dimensional and the longitudinal positioning along the chain axes and the correlating energies. The multiplicity of the longitudinal positioning led to a remarkable increase in the solubility limit of the systems of Kn/Cn′ (n′>n). The present result points to the importance of the entropy of mixing and also for the stability of long-chain crystals.