Molecules Of Different Molecular Weights Research Articles

The accessibility and interconnectivity of the tubular system network in toad skeletal muscle

The tubular (t) system is essential for normal function of skeletal muscle fibre, acting as a conduit for molecules and ions within the cell. However, t system accessibility and interconnectivity have been mainly assessed in fixed cells where the t system no longer fully represents that of the living cell. Here, fluorescent dyes of different diameter were allowed to equilibrate within the t system of intact fibres from toad, mechanically skinned to trap the dyes, and then imaged using confocal microscopy to investigate t system accessibility and interconnectivity. Dual imaging of rhod-2 and a 500 kDa fluorescein dextran identified regions throughout the t system that differed in the accessibility to molecules of different molecular weight. Restrictions within the t system lumen occurred at the junctions of the longitudinal and transverse tubules and also where a transverse tubule split into two tubules to maintain their alignment with Z-lines of adjacent mis-registered sarcomeres. Thus, three types of tubule, transverse, longitudinal and Z, can be identified by their lumenal diameter in this network. The latter we define for the first time as a tubule with a narrow lumen that is responsible for the change in register. Stretch-induced t system vacuolation showed exclusive access of rhod-2 to these structures indicating their origin was the longitudinal tubules. Exposing the sealed t system to highly hypertonic solution reversed vacuolation of longitudinal tubules and also revealed that these tubules are not collapsible. Fluorescence recovery after photobleaching (FRAP) measurements of t system-trapped fluo-5 N showed interconnectivity through the t system along the axis of the fibre. However, diffusion occurred at a rate slower than expected given the known number of longitudinal tubules linking adjacent transverse tubules. This could be explained by the observed narrow opening to the longitudinal tubules from transverse tubules, reducing the effective cross-sectional area in which molecules could move within the t system.

The influence of functionalized MWCNT reinforcement on the thermomechanical properties and morphology of epoxy nanocomposites

The nature of the interphase, often considered the third phase of nanocomposites, has been a central topic in nanocomposites research. In this study, an ionic bonding scheme was employed to graft epoxy-compatible amine terminated linear polypropylene oxide (A-PPO) molecules of different molecular weight to oxidized multi-walled carbon nanotubes (o-MWCNTs), and thereby modify the interphase of MWCNT-epoxy nanocomposites. The amine-cured epoxy systems reinforced with different functionalized MWCNTs demonstrated divergent thermomechanical properties. Fractography revealed distinct fracture behaviour for nanocomposites with long-chain A-PPO modified MWCNTs. The results demonstrate the feasibility of interphase control and performance engineering through the judicious choice of polymer grafts.

Comment on: the Stokes–Einstein equation and the physiological effects of vitreous surgery

Editor, We read with interest the recent editorial on the physiological effects of vitreous surgery, which highlighted the importance of the viscosity of the vitreous fluid on the rate of diffusion of small molecules of physiological importance (Stefansson & Loftsson 2006). From the Stokes–Einstein equation and previous measurements of the viscosity of the human vitreous body (Lee et al. 1992), it was suggested that the rate of diffusion should be 300–2000 times slower before vitrectomy than after. We agree that the rate at which small molecules are distributed in the eye is an important aspect of ocular physiology. However, the published measures of the viscosity of the vitreous were obtained from vitreous gel, while the Stokes–Einstein equation describes the effect of the viscosity on the rate of diffusion in a liquid. David Maurice measured the rate of loss of radioactive protein from the centre of the vitreous body of living rabbit eyes. Using these measures and physical modelling, he concluded that the rate of diffusion of molecules in the intact vitreous body was experimentally indistinguishable from water (Maurice 1959). Dr Maurice’s measurements have since been used to model the distribution of many kinds of molecules in the eye. To make more direct observations of diffusion in the vitreous body at the microscopic scale, we used fluorescence correlation spectroscopy (FCS) (Elson 2004). FCS measures the time spent by single fluorescent molecules in a tiny volume at the focal point of a laser beam. When their molecular weight is known, FCS provides a measure of the viscosity of the medium in which the molecules are moving. We collected data from the diffusion of two fluorescent molecules of different molecular weight, Alexa 488 (~ 0.5 kDa) and recombinant green fluorescent protein [GFP (~ 29 kDa)], in physiological salt solutions, homogenized porcine vitreous or intact porcine vitreous gel. Correlation spectra were collected using a Zeiss Confocor™ (Thornwood, NY, USA) attachment on a LSM510 confocal microscope (Zeiss Confocor™) at a point 100 μm into the solution or gel. The correlation spectra obtained are shown in Fig. 1; mean diffusion times and calculated diffusion coefficients for Alexa 488 and GFP are shown in Tables 1 and ​and2,2, respectively. Fig. 1 Correlation spectra for Alexa 488 (A) and recombinant green fluorescent protein [GFP (B)] in different media. BSS, balanced salt production; PBS, phosphate-buffered saline. Table 1 Mean diffusion times and calculated diffusion coefficients for Alexa 488. Table 2 Mean diffusion times and calculated diffusion coefficients for recombinant green fluorescent protein (GFP). As expected from its greater molecular weight, GFP had a longer diffusion time and lower diffusion coefficient than Alexa 488 under all conditions. However, the diffusion times and calculated diffusion coefficients of each fluorescent molecule measured in saline solutions, homogenized vitreous or vitreous gel were similar. According to the Stokes–Einstein equation, if the microscopic viscosity of the vitreous fluid or the vitreous gel were greater than saline, the calculated diffusion coefficients in the vitreous fluids would have been proportionally smaller in vitreous (gel or liquid) than in saline. Since these values were similar, the microscopic viscosity of liquid or gel vitreous is similar to saline. These measurements and the work of Dr Maurice suggest that the rate of diffusion of a molecule in the vitreous chamber is expected to be similar before and after vitrectomy and in eyes with an intact vitreous body or with extensive vitreous liquefaction. It has been argued that the loss of the gel structure of the vitreous body, as a consequence of vitrectomy or age-related liquefaction, has important effects on the distribution of small molecules in the eye (Stefansson et al. 1990; Harocopos et al. 2004; Stefansson 2006). The available evidence suggests that the increased rate at which molecules are redistributed in the vitreous compartment in the absence of the vitreous gel or after vitreous degeneration is more likely to be because of an increase in fluid circulation than a difference in the rate of diffusion.

Calorimetric Investigation of the Interaction between Lithium Perfluorononanoate and Poly(ethylene glycol) Oligomers in Water

The interaction of lithium perfluorononanoate (LiPFN) with poly(ethylene glycol) (PEG) molecules of different molecular weights (300 < MW < 20000 Da) has been investigated in water at 298.15 and 308.15 K by isothermal titration calorimetry (ITC). Density, viscosity, and conductivity measurements were also performed at 298.15 K. The aggregation process of this surfactant on the PEG polymeric chain was found to be very similar to that exhibited by cesium perfluorooctanoate (CsPFO) and appears to be consistent with the necklace model. ITC titrations indicated that a fully formed LiPFN micellar cluster can be wrapped by a PEG chain having a molecular weight (MW) of approximately 3200 Da, longer than that required by the shorter perfluorooctanoate (MW approximately 2600 Da), and also suggested a stepwise mechanism for the aggregation of successive micelles. Viscosity data indicate that the formation of polymer-surfactant complexes between PEG and LiPFN involves a conformational change of the polymer. The aggregation of preformed micelles of LiPFN or CsPFO or SDS on the PEG polymeric chain always gives rise to further stabilization.

Physicochemical speciation of bioactive trace metals (Cd, Cu, Fe, Ni) in the oligotrophic South China Sea

Bioactive trace elements Cd, Cu, Fe, and Ni in the oligotrophic South China Sea (SCS) waters exhibit distinct chemical affinities and physical size distributions when using cross flow ultrafiltration and ion exchange techniques, even though they display similar nutrient-type distributions. Dissolved Cu (≤ 0.4 μm) concentrations increased from 0.9 nM in surface waters to 3 nM at depths below 500 m, while those of dissolved Ni increased from 2 to 9 nM; those of dissolved Cd, from 0.01 to 0.9 nM; and those of dissolved Fe, from 0.1 to 0.7 nM. The majority of the dissolved Cd (∼ 100%) and Ni (> 80%) concentrations were found in the ≤ 1 kDa size fraction and as cationic labile forms. More than 50% of the total dissolved Cu in surface waters was in the ≤ 1 kDa cationic labile form, while the non-exchangeable fraction that cannot be adsorbed by either cation or anion exchange resins increased from 28% at the surface to 50% below 500 m depth. Similarly, about 60% of total dissolved Fe in surface waters was in the ≤ 1 kDa fraction; the colloidal (1 kDa–0.4 μm) form of Fe was relatively constant throughout the water column, and amounted to 40% at the surface and 20% in deeper waters. Some fractions of the total dissolved Cu and Fe could be adsorbed by both cation and anion exchange resins, suggesting binding to “zwitterionic” molecules with both anionic and cationic binding sites. The cationic labile form of Fe tightly correlates with phosphate and nitrate, total dissolved Ni significantly correlates with silicate, and total dissolved Cd with phosphate, with different slopes in the upper 100 m than below. These correlations suggest a tight coupling of the cycling of these trace metals with those of individual nutrient elements. Biogeochemical and biophysical interactions between metals and organisms are not only through surface complexation to organic molecules of different molecular weights but they are also tightly coupled to colloidal aggregation due to their surfactant activity, properties that should be considered in future studies of oceanic elemental cycles.

Influence of the length of the coating molecules on the nuclear magnetic relaxivity of superparamagnetic colloids

Proton relaxation in superparamagnetic colloids is influenced by the structure of the coating surrounding the ferrimagnetic nanocrystals. Colloidal suspension of superparamagnetic crystals of the same size, but coated with dextran molecules of different molecular weights, were synthesized and characterized by magnetometry and Nuclear Magnetic Relaxation Dispersion (NMRD) profiles, which give the evolution of the water protons with the external magnetic field. The evolution of these NMRD curves with the length of the coating molecule (respectively 35000, 71000 and 200000 g.mol–1) is characterized by an increase of relaxivity at low field. This evolution can be explained by a change of the magnetic anisotropy due to the dipolar coupling between neighbouring superparamagnetic crystals in the particle. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A kinetic investigation of the flocculation of alumina with polyacrylic acid

Using a model colloidal system of alumina and polyacrylic acid (PAA), the kinetics of flocculation was investigated at low polymer concentrations and short durations (on the order of seconds). The polymer-induced flocculation processes obeyed Von Smoluchowski's bimolecular rate equation. Increases in the concentration of the polymer resulted in higher rate constants for the flocculation process. At a fixed concentration (say 50 ppb, parts per billion), the rate constant values showed a maximum value for 250,000 g mol −1 polyacrylic acid. At this polymer concentration, calculations of the surface coverage of alumina by PAA molecules of different molecular weights show that for all the cases the coverage is nearly the same, ∼1×10 −3, but the flocculation response and the rates are significantly different. This trend in flocculation characteristics is attributed to the critical polymer number density requirement for effective flocculation (at least partial charge neutralization and initiation of flocculation). The mechanism governing the flocculation at ultralow concentrations (50 ppb) is the synergistic effect of partial patch neutralization and bridging.

Determining the electrophoretic mobility and translational diffusion coefficients of DNA molecules in free solution.

The free solution mobility of DNA molecules of different molecular weights, the sequence dependence of the mobility, and the diffusion coefficients of small single- and double-stranded DNA (ss- and dsDNA) molecules can be measured accurately by capillary zone electrophoresis, using coated capillaries to minimize the electroosmotic flow (EOF) of the solvent. Very small differences in mobility between various analytes can be quantified if a mobility marker is used to correct for small differences in EOF between successive experiments. Using mobility markers, the molecular weight at which the free solution mobility of dsDNA becomes independent of molecular weight is found to be approximately 170 bp in 40 mM Tris-acetate-EDTA buffer. A DNA fragment containing 170 bp has a contour length of approximately 58 nm, close to the persistence length of DNA under these buffer conditions. Hence, the approach of the free solution mobility of DNA to a plateau value may be associated with the transition from a rod-like to a coil-like conformation in solution. Markers have also been used to determine that the free solution mobilities of ss- and dsDNA oligomers are sequence-dependent. Double-stranded 20-bp oligomers containing runs of three or more adenine residues in a row (A-tracts) migrate somewhat more slowly than 20-mers without A-tracts, suggesting that somewhat larger numbers of counterions are condensed in the ion atmospheres of A-tract DNAs, decreasing their net effective charge. Single-stranded 20-mers with symmetric sequences migrate approximately 1% faster than their double-stranded counterparts, and faster than single-stranded 20-mers containing A(5)- or T(5)-tracts. Interestingly, the average mobility of two complementary single-stranded 20-mers is equal to the mobility of the double-stranded oligomer formed upon annealing. Finally, the stopped migration method has been used to measure the diffusion coefficients of single- and double-stranded oligomers. The diffusion coefficients of ssDNA oligomers containing 20 nucleotides are approximately 50% larger than those of double-stranded DNA oligomers of the same size, reflecting the greater flexibility of ssDNA molecules. The methods used to carry out these experiments are also described in detail.

Loading into and electro-stimulated release of peptides and proteins from chondroitin 4-sulphate hydrogels

Chondroitin 4-sulphate (CS) hydrogels were examined as potential matrices for the electro-controlled delivery of peptides and proteins. A CS hydrogel, cross-linked with ethylene glycol diglycidyl ether, and with a swelling ratio of 20, was used to study the influence of molecular size and shape of guest molecules on loading and release rates. Three positively charged molecules of different molecular weights (vasopressin MW 1084, aprotinin MW 6512 and lysozyme MW 14 400), and one negatively charged protein (bovine serum albumin MW 67 000) were used as model solutes. The hydrogels were loaded by equilibrium swelling. As a result, the three positively charged peptide and proteins were found to be concentrated in the gels, most likely due to ionic attraction between the negative charges in the gel polymeric backbone and the positively charged solutes. No such concentration of solute in the gel was seen for the negatively charged albumin. The latter is presumably loaded passively by diffusing in the water phase of the gel. The loading efficiency (indicated by the loading rate and the total amount of solute loaded in the gel) was found to increase with decreasing molecular size of solute. Electro-stimulated release of the loaded peptide and proteins was followed for 3 h during which an electric field was applied in pulses of 5 V (pulse duration and pulse interval were both of 20 min). The release of lysozyme and aprotinin from CS hydrogels responded to electrical pulses. On the other hand, vasopressin and albumin were largely released by passive diffusion and their release could not be electrically controlled. This work shows that the size of the guest molecule is an important parameter when electrically-stimulated drug release is desired, but further work obviously needs to be carried out with a larger range of molecular weights and conformations.

The light scattering relaxation function of glass-formingmolecules: a general probabilistic approach

Depolarized photon correlation spectra of two fragile glass-formingepoxy molecules of different molecular weights are reported. Thecorrelation function of the simplest molecule showsstretched-exponential behaviour, while deviations from this formhave been observed for the larger molecule, suggesting the presenceof a second, long-time power law. The relaxation dynamics issuitably described for both systems in terms of a probabilisticapproach based on the tool of the limit theorems of probabilitytheory. On the basis of this theory, the presence of the long-timepower law can be related to the existence of intermolecularinteractions, while the stretched-exponential behaviour of simplemolecules can be obtained in the case of negligible long-rangeinteractions.

Relaxation of polymer molecules in networks—the extended aggregate model

Relaxation of a few guest molecules of different molecular weights above Mc which are dissolved in gels of different junction densities are observed by measuring birefringence after a uniaxial step-strain compression. The results are described within the framework of the aggregation model of liquids which is extended so as to include formation of superaggregates. Their appearance should give rise to a hierarchically organized aggregate structure. Our investigation is then based on the assumption that each macroscopic relaxation process can be approximately treated as a collective response of separate Debye processes. The relaxation time distribution is related to the size distribution of aggregates. The description of the relaxation behaviour of an ensemble of guest chains of different lengths dissolved in a gel supports our model. The kinetics shows just the dependence on the chain length as predicted by the reptation model.

LDL-unbound apolipoprotein(a) and carotid atherosclerosis in hemodialysis patients.

High lipoprotein(a) [Lp(a)] plasma concentrations, which are genetically determined by apo(a) size polymorphism, are directly associated with an increased risk for atherosclerosis. Patients with end-stage renal disease (ESRD), who show an enormous prevalence of cardiovascular disease, have elevated plasma concentrations of Lp(a). In recent studies we were able to show that apo(a) size polymorphism is a better predictor for carotid atherosclerosis and coronary artery disease in hemodialysis patients than concentrations of Lp(a) and other lipoproteins. Less than 5% of apo(a) in plasma exists in a low-density lipoprotein (LDL)-unbound form. This "free" apo(a) consists mainly of disintegrated apo(a) molecules of different molecular weight, ranging from about 125 to 360 kDa. LDL-unbound apo(a) molecules are elevated in patients with ESRD. The aim of this study was therefore to investigate whether the LDL-unbound form of apo(a) contributes to the prediction of carotid atherosclerosis in a group of 153 hemodialysis patients. The absolute amount of LDL-unbound apo(a) showed a trend to increasing values with the degree of carotid atherosclerosis, but the correlation of Lp(a) plasma concentrations with atherosclerosis was more pronounced. In multivariate analysis the two variables were related to neither the presence nor the degree of atherosclerosis. Instead, the apo(a) phenotype took the place of Lp(a) and LDL-unbound apo(a). After adjustment for other variables, the odds ratio for carotid atherosclerosis in patients with a low molecular weight apo(a) phenotype was about 5 (p<0.01). This indicates a strong association between the apo(a) phenotype and the prevalence of carotid atherosclerosis. Finally, multivariate regression analysis revealed age, angina pectoris and the apo(a) phenotype as the only significant predictors of the degree of atherosclerosis in these patients. In summary, it seems that LDL-unbound apo(a) levels do not contribute to the prediction of carotid atherosclerosis in hemodialysis patients. However, this does not mean that "free", mainly disintegrated, apo(a) has no atherogenic potential.

Hyaluronan–Phospholipid Interactions

Hyaluronan–phospholipid interactions have been studiedin vitroby negative staining and rotary shadowing electron microscopy. Hyaluronan (HA) molecules of different molecular weights (around 170000; 740000, and 1.9×106Da) were added to phospholipid suspensions (DPPC or egg lecithin) that were in the form of either unilamellar particles or multilamellar vesicles. Suspensions were then gently stirred and incubated at different temperatures from 24 hr up to 7 days. After 24 hr, at temperatures just above the melting point of the phospholipid used, both unilamellar particles and multilamellar vesicles were already shown to change their organization in the presence of HA, giving rise to the formation of (1) huge perforated membrane-like structures lying on the substrate; (2) 12-nm-thick “cylinders” (rollers) with a tendency to aggregate and to form sheets. These structures were seen only in the presence of high-molecular-weight HA, whereas low-molecular-weight HA (170 kDa) induced fragmentation of liposomes and formation of a few short rollers. These data show that phospholipids and HA interact and suggest they may also do soin vivowithin the joint cavity, where both chemical species are present, giving rise to complexes which might exhibit peculiar lubricating and protective properties. It is also proposed that such interactions may not be as efficient in arthritic joints, where HA is degraded to low-molecular-weight fragments.

Chitosans as absorption enhancers for poorly absorbable drugs 2: mechanism of absorption enhancement.

It has recently been shown that the absorption enhancing and toxic effects of chitosans are dependent on their chemical composition. In this study, the mechanisms underlying these effects were investigated at the cellular level. The effects on epithelial cells of chitosans with different chemical composition, absorption enhancing properties and toxicities were studied in Caco-2 monolayers. Chitosan C( 1:31) has a low degree of acetylation (DA) (1%) and a low m.w. (31 kD), and displays dose-dependent absorption enhancement and cytotoxicity; chitosan C(35:170) has a higher DA (35%) and a higher m.w. (170 kD), is less dose-dependent in absorption enhancement, and is not cytotoxic. A third non-toxic chitosan C(49:22) with a high DA (49%), a low m.w. (22 kD), and no influence on epithelial permeability was used as control. C(1:31) and C(35:170) bound tightly to the epithelium. Cellular uptake of the chitosans was not observed. Both chitosans increased apical but not basolateral cell membrane permeability and induced a redistribution of cytoskeletal F-actin and the tight junction protein ZO-1. This resulted in increased paracellular permeability of hydrophilic marker molecules of different molecular weights. Addition of negatively charged heparin inhibited the cellular and the absorption enhancing effects of the chitosans, indicating that these effects are mediated via their positive charges. The onset of the effects of C(35:170) on apical membrane permeability and tight junction structure was much faster than that of C(1:31). C(49:22) did not influence any of the properties of the Caco-2 cell monolayers studied. The binding and absorption enhancing effects of chitosans on epithelial cells are mediated through their positive charges. The interaction of chitosans with the cell membrane results in a structural reorganisation of tight junction-associated proteins which is followed by enhanced transport through the paracellular pathway.

Separation of Gases by Enhanced Upstream Diffusion

Abstract Small amplitude oscillations have been shown to raise the diffusional flux in cylindrical tubes by orders of magnitude. The flux is especially high when the oscillation frequency is tuned to the size of the capillaries and to the properties of the gases involved. The process has been shown to permit separation of molecules of different molecular weight. The separation factor approaches, but is not equal to, the square root of the ratio of the molecular weight of the molecules involved. Hertz showed the separation can be enhanced by diffusion in a convective counterflow; it may then exceed the ratio of the square roots. We have combined oscillations and counterflow to obtain high yield and high separation. The results are encouraging. The separation is 20 to 40 times better than our past results. When applied to a simulated cascade, the savings in equipment and operational costs would be substantial.

Cell wall permeability and its relationship to spore release in Achlya intricata

Dynamically, the process of sporangial emptying in the zoosporic fungus Achlya intricata best fits a model based on the regulated dissipation of an osmotically generated pressure potential. Two testable predictions, derived from this hypothetical discharge mechanism, are (a) that the sporangial cell wall must be capable of maintaining a concentration gradient of some solute to depress the luminal osmotic potential, since the mature sporangium lacks a plasmalemma; and (b) that the emptying process should be inhibited by water stress. The permeability of the sporangial wall was estimated by monitoring the uptake of polyethylene glycol (PEG) molecules of different molecular weights. An aqueous solution of PEG-2000 contained molecules of the critical hydrodynamic volume, corresponding to a maximum pore size of ∼2.0 nm in the sporangial wall. Complete control (and inhibition above a critical solute concentration) over the emptying process was gained using polymers with M r 's in excess of 2000. The pressure potential within the sporangium was estimated, during discharge, to be ∼ + 10 −3 MPa. This value of pressure potential is consistent with that predicted from a mathematical model previously published.

Evasion of alternative complement pathway by Trypanosoma cruzi results from inefficient binding of factor B.

During its differentiation in the insect vector to a stage infective for the mammalian host, Trypanosoma cruzi becomes resistant to lysis by the alternative pathway of complement. To elucidate the mechanism of complement evasion, we studied control of complement activation on the surface of the noninfective epimastigote and the infective culture-derived metacyclic trypomastigote stages (CMT) of T. cruzi. It was found that the predominant form of complement component C3 on epimastigotes is C3b, whereas the majority of C3 on CMT is in the form of the hemolytically inactive fragment iC3b, which cannot participate in C5 convertase formation or lead to deposition of the lytic C5b-9 complex. Our results also showed that C3 binds by a covalent ester linkage to surface molecules of different molecular weight in the epimastigote stage and CMT. Binding studies with purified complement components indicated that CMT do not support efficient formation of an alternative pathway C3 convertase. C3b on the parasite surface fails to bind the amplification component, factor B, rather than showing enhanced binding of the control component, factor H. These results identify the biochemical basis for evasion of complement-mediated killing in T. cruzi and reveal a mechanism for developmental regulation of complement activation.

分子量のそろった高分子の合成法

Synthetic polymers are usually the mixture of molecules of different molecular weights. Since molecular weight and its distribution affect the fundamental properties of polymer materials, one of the most important subjects in synthetic chemistry is to establish facile and versatile procedures for the synthesis of polymers with regulated molecular weight, in view of the great demand for the design of polymer materials of elaborate functions. Up to now, however, the successful examples are limited. This paper describes our recent findings that some metalloporphyrins of aluminum are excellent catalyst of much wider applicability than the reported examples for the synthesis of polymers with regulated molecular weight. The polymer of epoxide or β-lactone, the copolymer of phthalic anhydride-epoxide or CO2-epoxide and their block copolymers are synthesized in the form of regulated molecular weight. The polymer formation takes place on the faces of metalloporphyrin.

Microviscosity of aqueous dextran solutions investigated by square-wave polarography

The polarographic method of measuring microviscosity in aqueous solutions of polymers was applied to dextran solutions (MW 20 000 to 150 000). The diffusing species — T1 + ions, have already been shown to be an excellent microviscosity probe in the investigation of polyethyleneglycol solutions. At low concentrations of dextrans, the microviscosity is much lower than the macroviscosity measured conventionally. The polarographically measured microviscosities, when compared to the macroviscosity, were used to determine several characteristics of the state of dextran molecules of different molecular weight, and their variation with concentration of the polymer. Values of characteristic concentrations, marking dramatic changes of the structure of the solution as well as coil parameters of dextran were thus determined. The experimental critical concentration, limiting the zone of microviscosity, was found to be close to the theoretical value. This, and other parameters not previously available by experiment, contribute to the better understanding of the structure of polymer solutions and permit comparisons between different polymers.

Diffusion of poly- p-phenyleneterephthalamide in concentrated sulphuric acid

The translational diffusion of poly- p-phenyleneterephthalamide molecules of different molecular weights in 96% H 2SO 4 has been investigated with the aid of a new experimental technique. The diffusion coefficients D are not concentration dependent in dilute solutions, and show marked sensitivity to the length of macromolecules. The molecular weights of the specimens M obtained from the diffusion coefficients D in the light of the theory of translational friction of persistent chains are practically equal to those calculated in terms of a Tsvetkov-Klenin hydrodynamic parameter of A 0=3·78 × 10 −10 erg/deg. A slight divergence of the M values in question from those determined by the method of flow birefringence could stem from polydispersity of the specimens and from inconstancy of A 0. In the range of M=(4·−45) × 10 3 examined in this investigation relationships have bee found between the diffusion coefficients D and the intrinsic viscosity [η] and M: D=4·0 × 10 −5 M −0.80, [ η]=2·5 × 10 −5 M 1.7 M ⩽ 10,000) and [ η]=1·7 × 10 −2 M 1.0 ( M>20,000). In the light of a joint analysis of the diffusion and viscosimetric data a conclusion is reached as to the high equilibrium rigidity of poly- p-phenyleneterephthalamide molecules and as to their hydrodynamic properties in 96% H 2SO 4 which approximate to those of another rigid chain polymer, poly- p-benzamide, in the same solvent.