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Dielectric Relaxations in Ferroelectric copolymer VDF(73)/TrFE(27)

Journal of Materials Science & Technology 1995年第1期11卷 31-37页

作者： ZHANG, XY FURUKAWA, TK SCI UNIV TOKYO DEPT CHEMTOKYO 162JAPAN

The characteristics of molecular motion of ferroelectric copolymer VDF(73)/TrFE(27) have been investigated by means of dielectric relaxation. With the rise in crystallinity, the Curie point decreases and the dielectric constant increases in the region of ferroelectric-to-paraelectric phase transition. The frequency spectra of the copolymer below room temperature consist of low-frequency component ascribed to the micro-Brownian motions of molecular segments in the noncrystalline regions and high-frequency component associated with the local motions of small-scale segments. By using the fitting results with one or two cole-cole functions, and calculating with WLF equation acid Arrhenius formula,the glass transition temperature and the activation energy of local relaxation are obtained. the distribution of relaxation time in the molten and ferroelectric phases is located at a Same line, acid complies with Arrhenius rule, which suggests that the elementary motions are of same type for molecular motions provided by the high-temperature process and the low-temperature local mode process.

关键词： FLUORIDE-TRIFLUOROETHYLENE copolymers VINYLIDENE FLUORIDE CURIE TRANSITION co-polymers BEHAVIOR TETRAFLUOROETHYLENE DEPENDENCE

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Structural and functional evidence for two separate oligosaccharide binding sites of Pasteurella multocida hyaluronan synthase

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Advances in Enzyme Research 2013年第4期1卷 97-111页

作者： Floor K. Kooy Hendrik H. Beeftink Michel H. M. Eppink Johannes Tramper Gerrit Eggink Carmen G. Boeriu Bioprocess Engineering Wageningen University and Research Center Wageningen The Netherlands Food and Biobased Research Wageningen University and Research Center Wageningen The Netherlands

Pasteurella multocida hyaluronan synthase (PmHAS) is a bi-functional glycosyltransferase, containing a β1,3-glucuronyltransferase and β1,4-N-acetylglucosaminetransferase domain. PmHAS catalyzes the elongation of hyaluronan (HA) through the sequential addition of single monosaccharides to the non-reducing end of the hyaluronan chain. Research is focused on the relation between the length of the HA oligosaccharide and the single-step elongation kinetics from HA4 up to HA9. It was found that the turnover number kcat increased with length to maximum values of 11 and 14 s-1 for NAc- and UA-transfer, respectively. Interestingly, the specificity constant kcat/KM increased with polymer length from HA5 to HA7 to a value of 44 mM-1s-1, indicating an oligosaccharide binding site with increasing specificity towards a heptasaccharide at the UA domain. The value of kcat/KM remained moderately constant around 8 mM-1s-1 for HA4, HA6, and HA8, indicating a binding site with significantly lower binding specificity at the NAc domain than at the UA domain. These findings are further corroborated by a structural homology model of PmHAS, revealing two distinct sites for binding of oligosaccharides of different sizes, one in each transferase domain. Structural alignment studies between PmHAS and glycosyltransferases of the GT-A fold showed significant similarity in the binding of the UDP-sugars and the orientation of the acceptor substrate. These similarities in substrate orientation in the active site and in essential amino acid residues involved in substrate binding were utilized to localize the two HA oligosaccharide binding sites.

关键词： Pasteurella Hyaluronan Binding Site Polymerization co-polymers

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