Time-dependent Structural Changes Research Articles

Changes in the immunoreactivity of Apo A-I during storage

We assayed apolipoprotein (Apo) A-I levels in re-constituted standard sera, and in serum samples and dried blood samples spotted onto filter-paper, taken from 4 healthy adults. An ELISA method was used to study changes in immunoreactivity after storage at different temperatures, for periods of up to 2 yr. For standard Apo A-I sera, immunoreactivity increased significantly with both time and temperature of storage after reconstitution. Immunoreactivity of Apo A-I in sera of healthy adults was stable on storage at −16°C and −70°C for 43 days, at 4°C for 21 days, and at room temperature for only 10 days, after which there was a considerable increase. Dried blood spot Apo A-I immunoreactivity was also stable at −16°C for at least 43 days, but not at 4° C. Storage at room temperature caused significantly increased immunoreactivity from day 2. These changes could result from temperature and time-dependent structural or compositional changes in high density lipoprotein particles leading to exposure of more Apo A-I antigenic sites. The results define the conditions of time and temperature of storage of samples and standards necessary for Apo A-I assays using ELISA methodology.

The viscometric examination of cement pastes with interrupted helical impellers

Synopsis The use of an interrupted helical impeller is known to prevent the occurrence of sedimentation during viscometric examination of cement pastes up to a water/cement ratio of 1·0. This feature is used to advantage in the study of time-dependent structural changes. Tests on high alumina cement pastes show that turbulence is likely to occur at Reynolds numbers exceeding 13 which leads to errors in the quantitative determination of yield stress and plastic viscosity. Guidance on the selection of an impeller for a particular cement paste is given so that flow is non-turbulent and quantitative analysis of the flow curves is justified.

Beta-interferon-induced time-dependent changes in the plasma membrane lipid bilayer of cultured cells.

The time-dependent interferon-induced structural changes in the plasma membrane lipid bilayer have been investigated in cultured cells using spin label electron spin resonance techniques. Treatment of human HeLa-S3 cells in suspension culture with human beta 1 interferon (640 u/ml) for as short a time as 30 min causes an increase in the rigidity of the plasma membrane lipid bilayer (Landsberger, Pfeffer and Tamm, in preparation). The plasma membrane rigidity of interferon-treated cells returns to the level of control cells within 3 to 5 hr, but by 24 hr after beginning of treatment, the rigidity of the plasma membrane is increased again and remains so for at least 2 days. Mouse beta interferon causes both an early (Landsberger, Pfeffer and Tamm, in preparation) and a late increase in membrane rigidity in homologous mouse L-929 cells, but not in heterologous HeLa-S3 cells. Thus, the interferon-induced perturbation of membrane structure is species specific. The late increase in membrane rigidity may have a different underlying mechanism from that of the early increase. While the early and transient change probably is related to signal generation and transmission, the later and persistent change may be an aspect of the phenotype and interferon-treated cells and may reflect changes in the plasma membrane-cytoskeletal complex.

Fibrin formation as a biological assembly process

Fibrin assembly in vivo is initiated by the limited proteolytic action of the enzyme thrombin on the 340,000-dalton soluble protein fibrinogen. In this communication, attention will be focused on the molecular mechanism of fibrin formation. By employing non-rate limiting concentrations of the activating enzyme, it has been possible to separate (in time) the proteolytic steps which initiate gelation from the two subsequent physical steps: polymerization and lateral association, both of which can be directly observed by light scattering. Network formation has been studied by sensitive time-dependent measurement of clot rigidity. More recently, a new negative staining technique developed to study the molecualr structure of fibrinogen in the electron microscope has been applied to follow the time-dependent structural changes characteristic of fibrin assembly. Results of this electron microscope investigation are consistent with the assembly mechanism proposed on the basis of the light scattering kinetic data.

Thermally induced time dependence of mechanical properties in biomedical grade polyurethanes

The time dependence of the mechanical properties of segmented urethanes as well as urethane-urea systems were monitored after the materials had been given a short thermal treatment followed by rapid cooling. Both linear and crosslinked materials were studied but the major focus was on many of the common biomedical grade urethanes. As had been noted in earlier studies on nonmedical segmented urethanes from this laboratory, many of the biomedical grade materials also showed time-dependent changes in mechanical properties that can be directly related to time-dependent changes in the degree of domain structure (microphase separation) that may occur in these segmented copolymers. Interestingly, those systems possessing significant amounts of urea linkage show little or no significant time-dependent changes in structure or properties following thermal treatment. The effect of chemical cross-linking can also influence the domain formation process and its thermal stability. The ramifications of these time dependent effects may have bearing on the biomaterial applications of segmented urethane polymers.

Application of polarization effects in light scattering: a new biophysical tool.

We demonstrate that a newly developed instrument which measures all polarization and intensity information contained in differentially and elastically scattered light has valuable applications in biology. The polarization states of light scattered differentially from suspensions of biological scatters are shown to contain structural information about those systems. The scatterers are discussed in the context of a 16 component matrix which completely characterizes the scattering process. The instrument and method are described in terms of the corresponding matrix algebra. We also discuss the use of the instrument as a device for distinguishing between closely related structural systems and as a tool for following time-dependent structural changes.

Structure and morphology of phosphatidylserine dispersions

1. 1. In contrast to egg phosphatidylcholine, aqueous dispersions of phosphatidylserine exist as a single lamellar liquid crystalline phase up to a water content of approx. 75% with all the water intercalated between the bilayers. In the absence of salt the lamellae of the large multilamelar particles present in dispersions ( c = (g lipid)/(g lipid + g water) > 0.2) are corrugated, particularly the outer lamellae. In the absence of salt planar phosphatidylserine bilayers are less stable than those of neutral phosphatidylcholine. 2. 2. At low lipid concentrations ( c < 0.05) the structural organisation is not yet clearly defined but theoretical X-ray scattering curves calculated for model particles suggest that no extensive multilamellar packing remains. 3. 3. Sonication of phosphatidylserine dispersions produces vesicles varying in size and surrounded by a closed, single bilayer in which the structural organisation of the fundamental lipid bilayer is maintained. In the absence of salt the average vesicle diameter is 16.5 mm (165 Å), about 7–10 nm (70–100 Å) smaller than the average diameter of phosphatidylcholine vesicles. In the presence of salt (0.1 M NaCl, 0.01 M Tris, pH 8.1) the average vesicle diameter is 25 nm (250 Å) identical with that of phosphatidylcholine vesicles. 4. 4. Reducing the double layer repulsion by adding NaCl to unsonicated dispersions ( c > 0.05) leads to the extrusion of water from the interbilayer space and a reduction in the lamellar repeat distance. The multilamellar particles shrink and their lamellae become smooth, indicative of an ordering in the packing of the lamellae. In dilute unsonicated dispersions ( c > 0.05) increasing the ionic strength increases the order and extent of the multilamellar structures. Increasing the ionic strength in sonicated dispersions containing single shelled vesicles leads to aggregation and/or coalescence producing larger single bilayer vesicles as well as multilamellar structures. 5. 5. Time-dependent structural changes in unsonicated phosphatidylserine dispersions in salt-free water are observed which are due to the reorganization of small multilamellar regions, randomly oriented with respect to each other, into more extensive and more ordered multilamellar regions. Aggregation and/or coalescence of single bilayer vesicles occurs in sonicated phosphatidylserine dispersions. This process is temperature dependent and increases with increasing temperature. The structures present in aqueous dispersions after preparation are not in thermodynamic equilibrium. 6. 6. Structural details derived from X-ray diffraction and electron micrographs of free-etched preparations of phosphatidylserine dispersions are consistent with each other. The negative-staining procedure, however, produces artifacts with dispersions of acidic phospholipids.

Strain ratio and volume change during tension and compression creep of thermoplastics

AbstractCreep tests up to 106 seconds in tension and compression were conducted on polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polypropylene (PP). Measurements of longitudinal and lateral strain provided values for tension and compression strain ratio. These increased with time and stress for PMMA and PVC from about 0.38 to 0.44. The results for PP increased from 0.44 to above 0.5 in. tension and showed a decreasing ratio in compression, which suggested a time and stress dependent structural change. Volumetric strain was computed from the linear strains and plotted against stress and axial strain. Except at 102 seconds, the relationships were non‐linear up to volumetric strains of about 0.3% and in the cases of PMMA and PVC there were greater changes of volume in tension than compression.