An infra-red spectroscopic study of molecular orientation and conformational changes in poly(ethylene terephthalate)

Polarized infra-red (i.r.) spectroscopy has been used to study the changes in molecular orientation and conformation which occur on drawing poly(ethylene terephthalate) sheets. The i.r. data provided orientation functions 〈P₂(θ)〉_ir where θ is the angle between the molecular chain axis and the draw direction for these uniaxially oriented sheets. Excellent agreement was obtained between the i.r. orientation functions for absorption bands associated with benzene ring mode vibrations, and orientation functions obtained from optical birefringence. Infra-red orientation functions for absorption bands associated with the trans and gauche conformations of the glycol residue, were then combined with i.r. estimates of the trans/gauche concentrations to examine the changes occurring in drawing. For drawing at 80°C up to a draw ratio of about 3·5, there was good agreement with expectations based on the deformation of a rubberlike network.     

Ligand-induced conformational changes in wild-type and mutant yeast pyruvate kinase

A mutant form of pyruvate kinase in which serine 384 has beenmutated to proline has been engineered in the yeast Saccharomycescerevisiae. Residue 384 is located in a helix in a subunit interfaceof the tetrameric enzyme, and the mutation was anticipated toalter the conformation of the helix and hence destabilize theinterface. Previous results indicate that the mutant favoursthe T quarternary conformation over the R conformation, andthis is confirmed by the results presented here. Addition ofphosphoenol-pyruvate (PEP), ADP and fructose-1,6-bisphosphate(Fru 1,6-P₂) singly to the wild-type and mutant enzymes resultsin a significant quenching of tryptophan fluorescence (12–44%),and for Fru-1,6-P₂, a red shift of 15 nm in the emission maximum.Fluorescence titration experiments showed that PEP, ADP andFru-1,6-P₂ induce conformations which have similar ligand-bindingproperties in the wild-type and mutant enzymes. However, theFru-1,6-P₂ induced conformation is demonstrably different fromthose induced by either ADP or PEP. The enzymes differ in theirsusceptibility to trypsin digestion and N-ethylmaleimide inhibition.The thermal stability of the enzyme is unaltered by the mutantion.Far-UV CD spectra show that both enzymes adopt a similar overallsencondary structure in solution. Taken together, the resultssuggest that the Ser384-Pro mutaion causes the enzyme to adopta diffenrent tertiary and/or quaternary structure from the wild-typeenzyme and affects the type and extent of the conformationalchanges induced in the enzyme upon ligand binding. A simplifiedminimal reaction mechanism is proposed in which the R and Tstates differ in both affinity and K_cat. Thus, in terms of themodels of cooperativity and allsoteric interaction, pyruvatekinase is both a K and a V system.     

Infrared spectroscopy study on crystalline and conformational changes in solution-cast syndiotactic polystyrene

Fourier transform infrared (FT-IR) analysis was used to investigate the conformation and the effects on crystals in syndiotactic polystyrene (sPS) subjected to different solvent and/or thermal treatments. It was found that the original helical conformation in sPS cast from a solution of 1,1,2,2-tetrachloroethane could transform to an all-traps planar conformation as the sPS was heated to temperatures above 200 °C. The spectra of sPS showed that the helical conformation chain varied with different casting solvents and the band positions and relative intensities also differed slightly owing to different interactions between the solvent molecule and the δ-type sPS crystalline phase. The solution-casting temperature also had a significant effect on the crystal forms in sPS. Lower temperatures (casting below ∼100 °C) produced helical crystals while the higher temperatures (casting above 120 °C) yielded all-trans crystals. In addition, spectra evidence of different crystals (α-and β-crystals in all-trans conformation) in sPS are discussed and identified in this study.     

Structure characterization of alginate and conformational behaviors of various alkali-metal alginates in solution

Summary The M/G ratio, i. e., number-ratio of D-mannuronate (M) to L-gluronate (G), and diad (MM, GG, MG, etc.) and triad (MMM, MMG, MGM, etc.) frequencies were determined with 100 MHz ¹³C-NMR spectra for two alginate samples, M-rich and G-rich samples. The M/G ratios obtained were in accord with those determined from chemical analysis on hydrolyzed products fairly well. Molecular chain dimensions in solution were investigated by viscosity method for five alkali-metal alginates carrying Li, Na, K, Rb and Cs, and the results were discussed based on ion radius and electronegativity of alkali-metals. Finally, ion-exchange characteristics of these alginates by Ca⁺⁺ ion were interpreted with M/G ratio and electronegativity.     

Folding and conformational studies on SCR1-3 domains of human complement receptor 1

Short consensus repeats SCR3 and SCR1-3 are soluble recombinantproteins, consisting of the third and first three N-terminaldomains of complement receptor 1, respectively, which retainsome anti-complement activity. The conformational stabilitiesand folding/unfolding of SCR3 and SCR1-3 have been studied usingcircular dichroism and equilibrium and pre-equilibrium fluorescencespectroscopy. Denaturation by guanidinium hydrochloride (GdnHCl)is rapid and completely reversible. Reduction of disulphidebridges in the folded proteins by ß-mercaptoethanolleads to an increase in fluorescence intensity. The fluorescenceintensity of the folded proteins is {small tilde}7.5% of thatof the respective unfolded proteins. The data can be approximatedto a two-state transition between native and denatured formsof the proteins. SCR3 has a conformational stability in waterof 12–13 kJ/mol whereas that of SCR1-3 is 19.5–19.9kJ/mol depending upon the technique utilized. The heat capacitychange associated with the unfolding of SCR1-3 was obtainedby a series of GdnHCl unfolding experiments over a range oftemperatures and was found to be 6.6 kJ/K.mol or 33.8 J/K.mol_residue.The refolding process of SCR3 was found to be simple, describedby a single exponential equation, whereas that of SCR1-3 wasfound to be complex and could be fitted to a double exponentialequation indicating the presence of folding intermediates.     

沥青质加氢过程中的结构组成变化

综述了原油沥青质在加氢处理中结构组成变化的研究现状。现有的研究主要针对沥青质加氢过程中H/C原子比随加氢条件变化如何变化,S,N,V,Ni脱除状况,以及加氢处理中沥青质相对分子质量及分子结构的变化,沥青质晶体结构参数的变化。结果表明:加氢处理后沥青质C,H质量分数降低,H/C原子比且随加氢条件变化苛刻而降低。沥青质的S较易脱除,由于N,V,Ni位于沥青质芳环系中,所以难以脱除,且随着反应条件的苛刻,其质量分数增加。反应中沥青质烷基侧链变短、侧链数变少,芳环数减少,芳核缩合度增加,芳香度增加,沥青质平均相对分子质量降低,总环数增加。加氢转化后沥青质芳香层层数M减少,芳香层层间距dm和芳香层堆积高度Lc降低。通过这几方面的研究结果分析,对解决加氢处理中沥青质引起的催化剂生焦及轻质油收率低等不利影响具有重要的理论指导意义。     

Structure of rigid domains and viscoelastic properties of ion-containing polyurethane semicarbazides

By using small-angle X-ray scattering, thermo-mechanical analysis, stress relaxations under constant deformation, etc. the effect of different quaternizing agents on the structure formation in cation-active segmented polyurethanes was investigated. The results obtained have been used for developing a model of a rigid domain, according to which the domain is a complex particle consisting of hard block microassociates. The hard block microassociates result from dipole-dipole, dipole-ion, and ion-ion interactions. The optimum viscoelastic and thermomechanical characteristics of polyurethane ionomers of the given chemical structure are realized when a part of tertiary nitrogen atoms remains unquaternized.     

Structure,molecular orientation,and resultant mechanical properties in core/ sheath poly(lactic acid)/polypropylene composites

We study the coaxial spinning of poly(lactic acid) (PLA) with polypropylene (PP) in a core/sheath configuration. PP_core/PLA_sheath and PLA_core/PP_sheath fibers maintain the high breaking strength that PP and PLA exhibit individually, showing marked improvement in strength over previous reports of PP/PLA blend fibers. Crystalline morphologies are greatly affected by the location within the fiber (i.e., core, sheath, or spun individually), and hence, co-spinning provides a route to tailor the morphology and fiber diameter beyond that available with single component fibers. A new approach to estimate molecular orientation of core sheath fibers based on the tensile response of the fiber is developed, and indicates that co-spinning PP with PLA results in a synergistic effect with increases in the molecular orientation above that which is possible with spinning either PP or PLA individually.     

Deformation-induced conformational changes in stretched samples of amorphous poly(ethylene terephthalate)

The combination of Fourier transform Raman spectroscopy and thermal analysis has been proved to be adequate for the study of the quantitative structural changes which take place in amorphous poly(ethylene terephthalate) on stretching. The two-phase conformational model previously proposed for annealed samples was applied to uniaxially stretched samples. The samples were deformed at different temperatures and speeds. Different conformer contents were found in the stretched sample depending on the stretching conditions. In general, deformation induces a conformational transition of gauche to trans. This behavior was lost at a narrow range of temperatures above the glass transition and at the lowest speeds, where the stretching seemed not to induce conformational changes. This different behavior has been associated to the high mobility of the polymer chains under these conditions, which imposes a different process of deformation marked by a high degree of chain slipping. This kind of deformation is present all along the sample and with the absence of necking in the uniaxial drawing process. © 1996 John Wiley & Sons, Inc.     

国内外烧碱市场变化及走向

回顾了2007年国内外烧碱市场的特点,特别是中国、美国和日本的市场变化情况,预测了今后的发展趋势。     

Using FlAsH to probe conformational changes in a large HEAT repeat protein

We have investigated the use of FlAsH, a small fluorogenic molecule that binds to tetracysteine motifs, to probe folding of the 15-HEAT repeat protein PR65A. PR65A is one of a special class of modular non-globular proteins known as tandem repeat proteins, which are composed of small structural motifs that stack to form elongated, one-dimensional architectures. We were able to introduce linear and bipartite tetracysteine motifs at several sites along the α-helical HEAT array of PR65A without disrupting the structure or stability. When the linear tetracysteine motif CCPGCC was used, FlAsH fluorescence reported globally on the folding of the protein. When the tetracysteine motif was displayed in bipartite mode through the engineering of pairs of cysteines on adjacent HEAT repeats, FlAsH fluorescence became a reporter of local conformation and of oligomerisation. Thus, by designing FlAsH-binding sites at different locations along the repeat array one can interrogate specific properties of PR65A, paving the way for structure-function analysis of this protein both in vitro and in the cell.     

Influence of orientation and ferroelectric domains on the photochemical reactivity of La2Ti2O7

The effects of crystal orientation and ferroelectric domain structure on the photochemical reactivity of La₂Ti₂O₇ have been measured. Electron backscattered diffraction was used to determine the orientations of the surfaces of crystals in a ceramic specimen, piezo-force microscopy was used to determine the domain structure, and the photocathodic reduction of silver from an aqueous silver nitrate solution was used to evaluate the photochemical reactivity. The reactivity is greatest on (001) surfaces (this is the orientation of the layers in this (110)_p layered perovskite structure) while surfaces perpendicular to this orientation have the least reactivity. Complex domain structures were observed within the grains, but they appeared to have no effect on the photocathodic reduction of silver, in contrast to previous observations on other ferroelectrics. La₂Ti₂O₇ is an example of a ferroelectric oxide in which the crystal orientation has a greater influence on the photochemical reactivity than polarization from the internal domain structure.     

Structure and orientation development in biaxial stretching of ultrahigh molecular weight polyethylene gel films

Ultrahigh molecular weight polyethylene (UHMWPE) gel films were prepared by gelation crystallization from decalin solutions. According to wide- and small-angle X-ray scattering (WAXD and SAXS) studies, single crystal mat texture with crystal c-axis oriented normal to film surface generally develops. However, randomly oriented structure can also develop if an external force is applied to gel films during gelation crystallization. Both textures invariably yield high drawability in uniaxial mode with outstanding modulus and strength. Biaxial films, typically 5 × 5, 6 × 6, 8 × 8, and 10 × 10 times the original dimensions, were prepared at 130°C–135°C in a biaxial stretcher. Optical microscopic Investigations reveal the development of interwoven fibrillar structure in all specimens. WAXD and SAXS studies show that lamellar structure transforms to fibrillar texture during biaxial stretching. Crystal orientation is characterized by WAXD pole figure and infrared dichroic methods. Mechanical studies suggest that tensile modulus and strength appear uniform. In comparison with uniaxial deformation, significant improvement in the lateral strength is seen in the biaxially stretched films.     

Applications of silver nanoplates as colorimetric indicators of pH-induced conformational changes in cytochrome c

Silver nanoplates (AgNPs) were used as a colorimetric indicator of pH-induced conformational changes in the protein, cytochorme c (cyt c). Cyt c was covalently bound to the surface of AgNPs, and UV-vis spectroscopy was used to identify changes in the optical properties of the resulting cyt c-AgNP conjugates in colloidal solution over pH, with values ranging from 3.1 to 10.6. Transmission electron microscopy (TEM) results indicated that the morphological changes in cyt c-AgNP conjugates were dependent on pH. Moreover, pH-induced conformational changes in cyt c could be detected by visual inspection of the color changes in cyt c-AgNPs, which occurred as a result of the coupling effect of localized surface plasmon resonance (LSPR) by aggregated AgNPs.     

Hydrodynamic,optical, and conformational properties of some polybenzimidazoles

Molecular hydrodynamics and optical methods have been employed to study the properties of molecules of polybenzimidazoles in 96% H₂SO₄ and to determine their molecular, conformational, and dynamo-optical characteristics. The temperature dependence has been investigated for the intrinsic viscosity. The equilibrium rigidity of molecular chains has been estimated for the examined polybenzimidazoles.     

Essential dynamics of the cellular retinol-binding protein evidence for ligand-induced conformational changes

The cellular retinol-binding protein (CRBP) is an intracellularretinol carrier protein belonging to a family of hydrophobicligand–binding proteins. It transports retinol to specificlocations in the cell where, for instance, it is esterifiedfor storage. Recently solved crystallographic structures ofCRBP homologues with and without bound ligand do not provideevidence for a ligand–induced conformational change. However,it has been shown that there is a difference in binding of holo–CRBPand apo–CRBP to lecithin–retinol acyltransferase.Moreover, proteolysis of holo–CRBP and apo–CRBPyields different products, indicating a difference in structureor dynamics between the two forms. Here, we present the resultsof molecular dynamics simulations of holo–CRBP and apo–CRBP.The simulations show a significant difference in conformation,in agreement with experimental results. The essential dynamicsmethod was used to study differences in dynamics between theapo and holo forms of CRBP, and showed inhibition of essentialmotions upon ligand binding. It also revealed large correlatedmotions of retinol with regions of the protein, pointing toa possible retinol entry/exit site.     

Qualitative prediction of the effects of changes in spinning conditions on spun fiber orientation

A simple model, based on an average elongational rate, correctly predicts qualitatively the effects of changes in all spinning parameters on the orientation of fibers spun from viscoelastic melts. The model may be extended to any extrusion process with an elongational character.     

Arsenic and antimony transporters in eukaryotes

Arsenic and antimony are toxic metalloids, naturally present in the environment and all organisms have developed pathways for their detoxification. The most effective metalloid tolerance systems in eukaryotes include downregulation of metalloid uptake, efflux out of the cell, and complexation with phytochelatin or glutathione followed by sequestration into the vacuole. Understanding of arsenic and antimony transport system is of high importance due to the increasing usage of arsenic-based drugs in the treatment of certain types of cancer and diseases caused by protozoan parasites as well as for the development of bio- and phytoremediation strategies for metalloid polluted areas. However, in contrast to prokaryotes, the knowledge about specific transporters of arsenic and antimony and the mechanisms of metalloid transport in eukaryotes has been very limited for a long time. Here, we review the recent advances in understanding of arsenic and antimony transport pathways in eukaryotes, including a dual role of aquaglyceroporins in uptake and efflux of metalloids, elucidation of arsenic transport mechanism by the yeast Acr3 transporter and its role in arsenic hyperaccumulation in ferns, identification of vacuolar transporters of arsenic-phytochelatin complexes in plants and forms of arsenic substrates recognized by mammalian ABC transporters.