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排序方式: 共有176条查询结果,搜索用时 167 毫秒
1.
A. K. Vidybida 《European biophysics journal : EBJ》1989,16(6):357-361
A theoretical model is proposed to describe the influence of a periodic electric field (PEF) upon a biopolymer. The biopolymer is treated as a classical mechanical system consisting of subsystems (molecular groups) which interact with each other through potential forces. The PEF is treated as a periodic driving force applied to a molecular group. The energy dissipation is considered using the model of fluid (viscous) friction. Arguments for the non-linear character of the friction-velocity dependence caused by the non-Newtonian rheology of a viscous medium are formulated.A forced molecular-group motion is investigated for the situation of a small driving-force period, with oscillations overdamped and a driving force consisting of more than one harmonic. As a result, it is established that the motion always gets to a terminal stage when only a small-scale vibration about some point, X
*, takes place. The terminal motion is preceded by a transient characterized by the presence of a directional velocity component and so by a drift along a potential profile. the drift goes on until a barrier is met which has a sufficiently large steepness (the barrier height is not important). As a result, the point X
* may happen to be remote from the conformation potential local minimum (conformational state). The physical reasons for the drift are described.If we consider the small-scale vibration about X
* in the framework of the hierarchy of scales for intramolecular mobility, it can be regarded as an equilibrium mobility, whereas the drift can be regarded as a functionally important motion, and X
* as a new conformational state which is realizable only in the presence of the PEF. It may be concluded, as the result of a consistent treatment and neglecting the small-scale vibration, that the conformational potential is modified by adding a linear term (in the one-dimensional case). In this connection, the set of conformational states can both deform (deviation of the positions of minima and their relative depth) and rearrange qualitatively (some minima can vanish and/or new minima can appear). In particular, the transition from one conformation to another one may happen due to rearrangement. 相似文献
2.
Carrageenan characterisation by chemical means is long and complex. 13C NMR is undoubtedly the technique providing the most information, but the apparatus is expensive and measurement times are long. For several important problems in carrageenan characterisation, rheological methods provide quantitative information, comparable, for practical purposes, with that obtained by NMR. In particular, the apparent elastic modulus (Gapp) is followed as a function of temperature, during cooling.Small amounts of kappa carrageenan in supposedly pure samples of the iota form have a great impact on the rheology and apparent ion selectivity of the latter. Using the elasticity/temperature relationship, as little as 2% of the kappa form can be detected in samples of iota carrageenan.The fraction of precursor sugars is another important parameter in the gelling of carrageenans. The form of the elasticity/temperature function is strongly dependent on the fraction of precursor sugars.Finally, it is shown that the elasticity/temperature relationship can provide a useful fingerprint of heterogeneous carrageenans. 相似文献
3.
In vitro influence of zinc and magnesium on the deformability of red blood cells artificially hardened by heating 总被引:2,自引:0,他引:2
Collette Dupuy-Fons Jean-Frédéric Brun Claire Mallart Joseph Carvajal Michèle Fussellier Lucette Bardet André Orsetti 《Biological trace element research》1995,47(1-3):247-255
Trace elements have been shown to improve red blood cell (RBC) deformability: zinc in sickle cell disease and magnesium in
an in vitro model of chemically rigidified erythrocytes. In this study, we investigated the effect and the influence of incubation
time of zinc or magnesium on an in vitro model of rigidified RBCs by heating. Erythrocyte rigidity was determined by viscosimetry
at high shear rate by a falling ball viscosimeter MT 90.
In the first part of the study, six normal volunteers participated. Viscosimetry was performed on native blood before and
after heating the sample for 10 min at 50°C. Therefore, increasing concentrations of zinc gluconate (final concentration:
0.5–4 g/L) or isotonic NaCl as control medium were added to the sample. Heating induced a twofold increase in all indices
of RBC rigidity (p<0.05). At all these concentrations of zinc, a highly significant, dose-related fluidifying effect was observed (40–70%):
this effect was immediately obtained and did not change over 60 min. Even at the highest concentration, recovery was not complete.
In the second part of the study, we studied magnesium’s effects on blood. In a first protocol, whole blood was rigidified
by heating at 56°C for 10 min, and the correcting effect of 5 min of incubation at 37°C of RBCs in 150 mmol/L NaCl, MgSO4, magnesium acetate, and magnesium gluconate was investigated. In a second protocol, the same incubation with NaCl and magnesium
salts was made on blood that had not been previously heated. In a third protocol, the correcting effect of magnesium gluconate
on heated red blood cells was tested at four concentrations (75, 150, 225, and 300 mmol/L) over 1 h, for evaluating the effects
of both concentration and time. Erythrocyte rigidity by heating is corrected by the three salts employed in protocol 1 (compared
to sodium). In protocol 2, the deformability of normal (nonheated) red cells is not modified by magnesium. In protocol 3,
no marked modification over 1 h is observed. The correcting effect is not complete for 75 mmol/L Mg, but remains the same
at the three other concentrations.
This study shows that zinc and magnesium at supraphysiological concentration are able to reverse RBC’s rigidification induced
by heating, but that magnesium does not modify the flexibility of normal RBCs. This article suggests that zinc and magnesium
may be studied in vivo as potential pharmacologic tools for improving hemorheologic disturbances. 相似文献
4.
The existence of a yield stress in filamentous fermentation broths has important transport phenomena implications in the design and operation of bioreactors. In this study, the constant shear rate vane method was assessed for directly measuring the yield stress of filamentous Aspergillus niger fermentation broths, as well as model fluids (ketchup, yogurt, and pulp suspensions). The method involved rotating 4-, 6-, and 8-bladed vanes (7.2 cm = height = 15 cm; 4.0 cm = dia. = 6 cm) at speeds of 0.01 to 0.64 rpm in the fluid and plotting the torque as a function of time. Based on visual observations, the consistency of the results with vane type and speed and comparison with previous work on nonbiological samples, it was concluded that the method is an effective and consistent technique for yield stress measurements on filamentous fermentation broths. Based on comparisons with concentric cylinder viscometer results, it was also concluded that the value determined via the vane method was a "static" yield stress (values of up to 28 Pa) which was much greater than the extrapolated (or "dynamic") yield stress determined via the concentric cylinder viscometer. (c) 1992 John Wiley & Sons, Inc. 相似文献
5.
Stéphanie M. C. Bruekers Maarten Jaspers José M. A. Hendriks Nicholas A. Kurniawan Gijsje H. Koenderink Paul H. J. Kouwer 《Cell Adhesion & Migration》2016,10(5):495-504
ABSTRACTThe mechanical and structural properties of the extracellular matrix (ECM) play an important role in regulating cell fate. The natural ECM has a complex fibrillar structure and shows nonlinear mechanical properties, which are both difficult to mimic synthetically. Therefore, systematically testing the influence of ECM properties on cellular behavior is very challenging. In this work we show two different approaches to tune the fibrillar structure and mechanical properties of fibrin hydrogels. Addition of extra thrombin before gelation increases the protein density within the fibrin fibers without significantly altering the mechanical properties of the resulting hydrogel. On the other hand, by forming a composite hydrogel with a synthetic biomimetic polyisocyanide network the protein density within the fibrin fibers decreases, and the mechanics of the composite material can be tuned by the PIC/fibrin mass ratio. The effect of the changes in gel structure and mechanics on cellular behavior are investigated, by studying human mesenchymal stem cell (hMSC) spreading and differentiation on these gels. We find that the trends observed in cell spreading and differentiation cannot be explained by the bulk mechanics of the gels, but correlate to the density of the fibrin fibers the gels are composed of. These findings strongly suggest that the microscopic properties of individual fibers in fibrous networks play an essential role in determining cell behavior. 相似文献
6.
Morten Leth Jepsen Line Hagner Nielsen Anja Boisen Kristoffer Almdal Martin Dufva 《Biopolymers》2019,110(1):e23241
Cell or tissue stretching and strain are present in any in vivo environment, but is difficult to reproduce in vitro. Here, we describe a simple method for casting a thin (about 500 μm) and soft (about 0.3 kPa) hydrogel of gelatin and a method for characterizing the mechanical properties of the hydrogel simply by changing pressure with a water column. The gelatin is crosslinked with mTransglutaminase and the area of the resulting hydrogel can be increased up 13-fold by increasing the radial water pressure. This is far beyond physiological stretches observed in vivo. Actuating the hydrogel with a radial force achieves both information about stiffness, stretchability, and contractability, which are relevant properties for tissue engineering purposes. Cells could be stretched and contracted using the gelatin membrane. Gelatin is a commonly used polymer for hydrogels in tissue engineering, and the discovered reversible stretching is particularly interesting for organ modeling applications. 相似文献
7.
Jose Eduardo U. Rojas Barbara B. Gerbelli Anderson O. Ribeiro Iseli L. Nantes-Cardoso Francesca Giuntini Wendel A. Alves 《Biopolymers》2019,110(2):e23245
In this study, we prepared translucid hydrogels with different concentrations of silk fibroin, extracted from raw silk fibers, and used them as a matrix to incorporate the photosensitizer 5-(4-aminophenyl)-10,15,20-tris-(4-sulphonatophenyl) porphyrin trisodium for application in photodynamic therapy (PDT). The hydrogels obtained were characterized by rheology, spectrophotometry, and scattering techniques to elucidate the factors involved in the formation of the hydrogel, and to characterize the behavior of silk fibroin (SF) after incorporating of the porphyrin to the matrix. The rheology results demonstrated that the SF hydrogels had a shear thinning behavior. In addition, we were able to verify that the structure of the material was able to be recovered over time after shear deformation. The encapsulation of porphyrins in hydrogels leads to the formation of self-assembled peptide nanostructures that prevent porphyrin aggregation, thereby greatly increasing the generation of singlet oxygen. Also, our findings suggest that porphyrin can diffuse out of the hydrogel and permeate the outer skin layers. This evidence suggests that SF hydrogels could be used as porphyrin encapsulation and as a drug carrier for the sustained release of photosensitizers for PDT. 相似文献
8.
Fariña J.I. Santos V.E. Perotti N.I. Casas J.A. Molina O.E. García-Ochoa F. 《World journal of microbiology & biotechnology》1999,15(2):309-316
Scleroglucan production by Sclerotium rolfsii ATCC 201126 has been studied using nitrate or ammonium as nitrogen sources at several concentrations. In all the experiments carried out, both growth and production were modelled by an unstructured kinetic model using logistic and Luedeking–Piret equations for describing growth and production, respectively. The kinetic parameters for growth ( and YXN) and for production ( and ) were obtained by fitting the data to the model using the single-response non-linear regression technique by means of the algorithm of Marquardt coupled to a fourth-order Runge–Kutta algorithm. Biomass and scleroglucan production were higher when nitrate was the nitrogen source. Rheological properties of scleroglucan produced using nitrate as nitrogen source were studied and rheological parameters calculated, revealing similarity between this biopolymer and commercial scleroglucan. 相似文献
9.
高原移居者红细胞滤过指数的变化及其机理 总被引:2,自引:0,他引:2
Xi AQ 《中国应用生理学杂志》2001,17(1):51-54
目的:探讨不同海拔和同一海拔高底氧环境不同血色素范围对高原健康人红细胞流变特性的影响及其可能发生的机制。方法:检测不同海拔高度(2260m、3300m、4080m)对320健康人EFI、SOD、和MDA的影响。结果:随海拔高度的升高,高原健康人EFI和MDA含量明显升高,而红细胞SOD活性明显降低;EFI与MDA呈正相关,而与红细胞SOD活性呈负相关。同一海拔低氧环境Hb增高者,EFI和MDA升高,而红细胞SOD活性降低;随海拔升高,EFI和MDA含量明显升高,而红细胞SOD活性明显降低。结论:不同海拔红细胞流变学和氧自由基代谢差异性的形成,低氧环境起核心作用;而随海拔高度升高和同一海拔低氧环境自由基代谢异常加重者是导致高原健康人红细胞流变学异常的中心环节。 相似文献
10.
Optimization of submerged culture process for the production of mycelial biomass and exo-polysaccharides by Cordyceps militaris C738 总被引:4,自引:0,他引:4
AIMS: The objective of the present study was to determine the optimal culture conditions for mycelial biomass and exo-polysaccharide (EPS) by Cordyceps militaris C738 in submerged culture. METHODS AND RESULTS: The optimal temperatures for mycelial biomass and EPS production were 20 degrees C and 25 degrees C, respectively, and corresponding optimal initial pHs were found to be 9 and 6, respectively. The suggested medium composition for EPS production was as follows: 6% (w/v) sucrose, 1% (w/v) polypeptone, and 0.05% (w/v) K2HPO4. The influence of pH on the fermentation broth rheology, morphology and EPS production of C. militaris C738 was carried out in a 5-l stirred-tank fermenter. The morphological properties were comparatively characterized by pellet roughness and compactness by use of image analyser between the culture conditions with and without pH control. The roughness and compactness of the pellets indicated higher values at pH-stat culture (pH 6.0), suggesting that larger and more compact pellets were desirable for polysaccharide production (0.91 g g(-1) cell d(-1). CONCLUSIONS: Under the optimized culture conditions (with pH control at 6), the maximum concentration of biomass and EPS were 12.7 g l(-1) and 7.3 g l(-1), respectively, in a 5-l stirred-tank fermenter. SIGNIFICANCE AND IMPACT OF THE STUDY: The critical effect of pH on fungal morphology and rheology presented in this study can be widely applied to other mushroom fermentation processes. 相似文献