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排序方式: 共有58条查询结果,搜索用时 15 毫秒
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13C spin—lattice relaxation times have been measured at 25.15 MHz on poly(but-1-ene) and (but-1-ene)—propylene copolymers, as a function of molecular weight and concentration in 1,2,4-trichlorobenzene, and over the 50°–150°C temperature range. Maximum nuclear Overhauser enhancement was observed for all carbons. Independence of T1 from molecular weight and bulk sample viscosity for solutions less concentrated than 50% w/v, indicates that relaxation is dominated by local conformational rearrangements rather than by overall tumbling of the macromolecule. Relaxation studies on copolymers show that this segmental motion affects only short sequences of the chain. No influence of stereochemical configurations is observed. Relaxation data for backbone carbons was interpreted in terms of a single τ isotropic motional model. Internal rotations of side CH2 and CH3 groups were analysed using the two and three correlation times Woessner models, and show little dependence on an eventual anisotropy of chain motion. Similar temperature dependence (18–21 kJ/mol) is observed for chain and side carbons, and even if internal reorientation of side CH3 groups is much more rapid, it appears interlinked with segmental motion. 相似文献
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Replacing the ({beta}{alpha})-unit 8 of E.coli TIM with its chicken homologue leads to a stable and active hybrid enzyme 总被引:2,自引:0,他引:2
Mainfroid Veronique; Goraj Karine; Rentier-Delrue Francoise; Houbrechts Annick; Noble Martin E.M.; Borchert Torben V.; Wierenga Rik K.; Martial Joseph A. 《Protein engineering, design & selection : PEDS》1993,6(8):893-900
In order to investigate how structural modifications interferewith protein stability, we modified a (ß)-unit inE.coli triosephosphate isomerase (TIM), a typical (ß)-barrelprotein, assuming that the pseudosymmetrical ß-barrelcan be divided into eight successive loop/ß-strand/loop/-helixmotifs. We replaced the eighth (ß)-unit of E.coliTIM with the corresponding chicken (ß)-unit. The substitution,involving the replacement of 10 of the 23 residues of this (ß)-unit, was evaluated first by modelling, then experimentally.Modelling by bomology suggests how the amino add replacementsmight be accommodated in the hybrid E.coli/chicken TIM (ETCM8CHI).Both natural and hybrid recombinant TIMs, overproduced in E.coli,were purified to homogeneity and characterized as to their stabilityand kinetics. Our kinetic studies show that the modificationperformed here leads to an active enzyme. The stability studiesindicate that the stability of ETIM8CHI is comparable to thatof the wild type TIM. 相似文献
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Oracz K Bailly C Gniazdowska A Côme D Corbineau F Bogatek R 《Journal of chemical ecology》2007,33(2):251-264
The aim of this study was to investigate the phytotoxic effect of sunflower on physiological and biochemical processes during
germination of mustard seeds (Sinapis alba L. cv. Nakielska). To exclude the involvement of osmotic stress in seed reaction to phytotoxic compounds, we compared the
effect of 10% (w/v) water extract from sunflower (Helianthus annuus L. cv. Ogrodowy) leaves and 28.4% (w/v) polyethylene glycol (PEG) 8000 solution characterized by an equal Ψ = −1 MPa. We evaluated (1) the amount of hydrogen peroxide (H2O2); (2) activities of antioxidant enzymes: superoxide dismutase, catalase, and glutathione reductase; (3) membrane permeability;
and (4) level of malondialdehyde (MDA). Both, sunflower compounds and PEG solutions inhibited mustard seed germination, but
only phytotoxins caused an increase in the cell membrane permeability, MDA level, H2O2 concentration, and alterations in activities of antioxidant enzymes. Our results demonstrate that despite the activation
of the antioxidant system by sunflower phytotoxins, reactive oxygen species accumulation caused cellular damage, which resulted
in the decrease of germinability and gradual loss of seed vigor. It seems that the negative effect of sunflower on germination
of mustard seeds is mostly because of its toxicity and not to its contribution to osmotic potential. 相似文献
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Jacqueline T. Hecht Alka C. Veerisetty Mohammad G. Hossain Debabrata Patra Frankie Chiu Francoise Coustry Karen L. Posey 《International journal of molecular sciences》2021,22(17)
Pseudoachondroplasia (PSACH), a short limb skeletal dysplasia associated with premature joint degeneration, is caused by misfolding mutations in cartilage oligomeric matrix protein (COMP). Here, we define mutant-COMP-induced stress mechanisms that occur in articular chondrocytes of MT-COMP mice, a murine model of PSACH. The accumulation of mutant-COMP in the ER occurred early in MT-COMP articular chondrocytes and stimulated inflammation (TNFα) at 4 weeks, and articular chondrocyte death increased at 8 weeks while ER stress through CHOP was elevated by 12 weeks. Importantly, blockage of autophagy (pS6), the major mechanism that clears the ER, sustained cellular stress in MT-COMP articular chondrocytes. Degeneration of MT-COMP articular cartilage was similar to that observed in PSACH and was associated with increased MMPs, a family of degradative enzymes. Moreover, chronic cellular stresses stimulated senescence. Senescence-associated secretory phenotype (SASP) may play a role in generating and propagating a pro-degradative environment in the MT-COMP murine joint. The loss of CHOP or resveratrol treatment from birth preserved joint health in MT-COMP mice. Taken together, these results indicate that ER stress/CHOP signaling and autophagy blockage are central to mutant-COMP joint degeneration, and MT-COMP mice joint health can be preserved by decreasing articular chondrocyte stress. Future joint sparing therapeutics for PSACH may include resveratrol. 相似文献
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Fumihiko Tanaka Tsuyoshi Koga Hiroyuki Kojima Francoise M. Winnik 《Canadian Metallurgical Quarterly》2011,29(1)
Collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating and phase diagrams of aqueous PNIPAM solutions with very fiat LCST phase separation line are theoretically studied on the basis of cooperative dehydration (simultaneous dissociation of bound water molecules in a group of correlated sequence), and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol.The downward shift of the cloud-point curves (LCST cononsolvency) with the mole fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends (telechelic PNIPAM) are theoretically and experimentally studied. The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation), and separate from the coil-globule transition line. Associated structures in the solution, such as flower micelles, mesoglobules and higher fractal assembly, are studied by USANS with theoretical modeling of the scattering function. 相似文献