Through pore diameter in the cell wall of Chara corallina.

Determination of pore size of the cell wall of Chara corallina has been made by using the polyethylene glycol (PEG) series as the hydrophilic probing molecules. In these experiments, the polydispersity of commercial preparation of PEGs was allowed for. The mass share (gamma(p)) of polyethylene glycol preparation fractions penetrating through the pores was determined using a cellular 'ghost', i.e. fragments of internodal cell walls filled with a 25% solution of non-penetrating PEG 6000 and tied up at the ends. In water, such a 'ghost' developed a hydrostatic pressure close to the cell turgor which persisted for several days. The determination of gamma(p), for polydisperse polyethylene glycols with different average molecular mass (M) was calculated from the degree of pressure restoration after water was replaced by a 5-10% polymer solution. Pressure was recorded using a dynamometer, which measures, in the quasi-isometric mode, the force necessary for the partial compression of the 'ghost' in its small fragment. By utilizing the data on the distribution of PEG 1000, 1450, 2000, and 3350 fractions over molecular mass (M), it was found that gamma(p), for these polyethylene glycols corresponded to the upper limit of ML=800-1100 D (hydrodynamic radius of molecules, r(h)=0.85-1.05 nm). Thus, the effective diameter of the pores in the cell wall of Chara did not exceed 2.1 nm.     

Allowing for polymer polydispersity--an essential condition for determination of aqueous pore diameters in cell walls and membranes using polymers

A method of allowing for polydispersion of polyethylene glycol (PEG) preparations was developed for the use of these preparations for the osmometrical evaluation of pore diameters with aqueous pores of Chara corallina cell walls as an example. The mass share of polyethylene glycol preparation fractions gamma p penetrating through the pores was determined using cellular "shadows", fragments of internodal cell walls tied up at the ends and filled with a 25% solution of nonpenetrating PEG 6000. When immersed into water, such "shadow" acquired a turgor (hydrostatic) pressure close to the cellular pressure and persistent over long time. The determination of gamma p for polyethylene glycols with different average molecular weights Mw was performed from the degree of pressure restoration after water was replaced by a 5-10% polymer solution. The kinetics of pressure changes was recorded using a mechanotronic dynamometer, which measures, in the quasi-isometric mode, the force necessary for partial compression of the "shadow" in its small fragment. By utilizing the dependence of the overall share of fractions with molecular weights Mi < Mk on Mk (data of [1]), we found that gamma p, for these polyethylene glycols corresponds to the threshold value of Mk = 800-1100 D (hydrodynamic radius of molecules rh = 0.85-1.05 nm). Thus, the effective diameter of the pores in the cell wall of Chara does not exceed 2.1 nm. It was shown that the smoothness of the sigmoid shape of the dependence of ionic channel conductivity on the Mw value of the polymer in the media is largely due to the polydispersion of polymer preparations, particularly, to the reduction in the share of fractions penetrating the channels as Mw is increased. The method normally used to estimate pore diameters in ionic channels which ignores the dispersion of polymer preparations, results in overestimated values.     

Estimation of the radius of the pores formed by the Bacillus thuringiensis Cry1C delta-endotoxin in planar lipid bilayers

Pore formation constitutes a key step in the mode of action of Bacillus thuringiensis delta-endotoxins and various activated Cry toxins have been shown to form ionic channels in receptor-free planar lipid bilayers at high concentrations. Multiple conductance levels have been observed with several toxins, suggesting that the channels result from the multimeric assembly of a variable number of toxin molecules. To test this possibility, the size of the channels formed by Cry1C was estimated with the non-electrolyte exclusion technique and polyethylene glycols of various molecular weights. In symmetrical 300 mM KCl solutions, Cry1C induced channel activity with 15 distinct conductance levels ranging from 21 to 246 pS and distributed in two main conductance populations. Both the smallest and largest conductance levels and the mean conductance values of both populations were systematically reduced in the presence of polyethylene glycols with hydrated radii of up to 1.05 nm, indicating that these solutes can penetrate the pores formed by the toxin. Larger polyethylene glycols had little effect on the conductance levels, indicating that they were excluded from the pores. Our results indicate that Cry1C forms clusters composed of a variable number of channels having a similar pore radius of between 1.0 and 1.3 nm and gating synchronously.     

Xylitol purification by cross-linked glucose isomerase crystals

Xylitol is specifically bound by active, cross-linked glucose isomerase crystals (CLGI). CLGI can be used to purify xylitol or concentrate it from dilute and impure solutions. Bound xylitol can be eluted from CLGI by Ca2 and the material reactivated by Mg2. The binding capacity is 1 mg xylitol per 525 mg CLGI which equals one molecule per active center. CLGI can further be used to purify xylitol and sorbitol from impure mixtures of arabinitol, mannitol, ribitol and monosaccharides. © Rapid Science Ltd. 1998     

Purification, crystallization and preliminary X-ray data of the bacteriophage MS2

Single crystals of the bacteriophage MS2 have been produced by the vapour diffusion technique in the presence of 1.5% polyethylene glycol 6000 and 0.2 M-sodium phosphate buffer (pH 7.4). These are the first bacteriovirus crystals diffracting to high resolution. The crystal space group is C2 with the unit cell parameters a = 467.9 A, b = 289.5 A, c = 275.6 A and beta = 121.8 degrees. The asymmetric unit contains one half of the virion. The maximum resolution limit of the X-ray diffraction data obtained from these crystals was 2.9 A. The purification of the virus material was done by mild procedures exclusively and involved precipitation with polyethylene glycol 6000 and size exclusion chromatography on Sepharose CL-4B.     

Effect of pretreatments and fermentation on pore size in cellulosic materials

Surface area has been proposed as a major factor determining the extent of enzymatic hydrolysis of cellulose. We used cornstalk residue (CR) and Solka Floc BW-300 (SF) as substrates and NaOH (a cellulose swelling agent) and iron sodium tartrate (FeTNa, intercolates between cellulose microfibrils) as pretreatments to study the effect of surface area on extent of fermentation. Micropore sizes (8-130 A) were determined by a solute exclusion technique using glucose, cellobiose, and polyethylene glycols as molecular probes. The pore size distributions follow the logistic model function: I = a/[1+exp(b - cX)] where I is pore volume; X = log D; D is the molecular probe diameter; and a, b, and c are constants. The pore volumes of CR (1.9 mL/g) and SF (1.6 mL/g) are increased to 2.1 mL/g by pretreatment with NaOH. Pretreatment of SF with NaOH and cornstalk residue with FeTNa caused an upward shift in the pore size distribution. Fermentation of untreated CR by rumen microbes resulted in a 46% loss of dry matter while increasing the internal pore size and decreasing the pore volume to 0.9 mL/g. Fermentation of NaOH pretreated CR resulted in a 73% loss of dry matter with little change in pore size, total pore volume, or fiber composition. Fiber analysis indicated that selective utilization of hemicellulose over cellulose in both fermentations was small. The data show that: (1) removal of hemicellulose and lignin increases dry matter disappearance upon fermentation of the remaining material; (2) relative to the size of bacterial cellulases (40-160 A), the pretreatments have little effect on increasing accessibility of surface internal to the cellulose particles; and (3) the micropore changes caused by NaOH or FeTNa treatment do not explain the enchanced fermentation obtained for treated cornstalk residue. These observations infer that external or macropore surface properties may be a significant factor in determining the extent of utilization of the solid substrates by cellulolytic microorganisms.     

Preliminary X-ray crystallographic analysis of Tritrichomonas foetus inosine-5′-monophosphate dehydrogenase

Inosine-5′-monophosphate dehydrogenase (IMPDH) from the protozoan parasite Tritrichomonas foetus has been expressed in E. coli and crystallized. Crystals were grown to 0.1 mm in each dimension in 18 to 72 h using ammonium sulfate and low-molecular-weight polyethylene glycols. The crystals belong to the cubic space group P432 with unit cell edge = 157.25 Å. The enzyme is a homotetramer with each monomer having a molecular weight of 55,534 Da. There is one monomer per asymmetric unit, based on a volume/mass ratio of 2.7 ų/Da and self-rotation analysis. The crystals are adequately stable to allow a complete data set to be collected from a single crystal. Complete native data sets have been collected to 2.3 Å resolution at 4°C using synchrotron radiation. High-quality complete data extending to 3.0 Å resolution have been collected from crystals of four putative derivatives, and the data appear to be isomorphous with that of the native crystals in each case. Efforts to solve the derivatives for use in MIR phasing are underway. © 1995 Wiley-Liss, Inc.     

Effect of some factors on fabrication of poly(L-lactic acid) microporous foams by thermally induced phase separation using N,N-dimethylacetamide as solvent

Biocompatible, highly interconnected microporous poly(L-lactic acid) (PLLA) foams or scaffolds with nano-fibrous structure, containing pores with diameters of 0.1-3.5 μm and fibers with diameters of 300-700 nm scale, were prepared through the thermally induced liquid-liquid phase separation (TIPS) method using N,N'-dimethyl acetamide (DMAc) as solvent. Various foam morphologies were obtained by changing parameters involved in the TIPS process, such as polymer concentration, solvent composition, and quenching temperatures. The morphology of different foams was examined by scanning electron microscopy, whereas the pore size and the pore size distribution were calculated. The results showed that most porous foams presented nano-fibrous structure with interconnected open pores. In the case of using DMAc as solvent, with increasing polymer concentration, either the average pore diameter or the pore size distribution exhibited a maximum value at 0.05 g/mL polymer concentration and quenching temperature of -30°C. It was found that all the pore size distribution fit the F-distribution equation. With increasing the quenching temperature from -30°C to -10°C, the maximum average pore diameter of the foams decreased and the pore size distribution became narrower, whereas the polymer concentration exhibiting the maximum pore size and widest pore size distribution increased from 0.05 g/mL to 0.07 g/mL. In the case of using the mixed solvent of DMAc/DOX (1,4-dioxane) from 9/1 to 7/3 (v/v) there appeared a maximum value of average pore diameter and a widest pore size distribution all at 0.05 g/mL PLLA concentration and quenching temperature of -30°C. The maximum pore size tends to increase with increasing DOX content.     

Tissue reaction to intraperitoneally implanted catheter materials

The limiting factor that introduces long-term complications of intraperitoneal (i.p.) catheters used, for example, with the Programable Implantable Medication System (PIMS) is the encapsulation of the catheter tip with tissues due to tissue reaction. The objective is the development of new catheters for PIMS or other systems for i.p. insulin delivery that allow continued insulin flow. The study is based on two hypotheses: (1) vascularized tissue will grow into a porous end plug mounted at the catheter tip (100–300 μm pore diameter), with sufficient blood supply to carry the insulin to the circulation; (2) use of a narrow pore diameter (25 μm or less) end plug will prevent tissue ingrowth yet allow insulin flow. The biological response to the following materials, all designed for use in catheters, were studied: polyurethane, segmented polyether-urethane, alumina coated on Teflon (proplast regular and micro-pore), pyrolytic carbon, high density polyethylene, ultra high molecular weight polyethylene, hydroxyapatite, bioglass, and expanded Teflon. Some of these materials also are used for several other applications: vascular grafts, in the cardiovascular system, and for dental, orthopaedic, and other purposes. The shape and size of the end plugs made from each of the materials were as similar as possible to minimize size effects. The test materials were implanted i.p. in 12 dogs for a period of 12 weeks. The cylindrical plugs were typically 1.5–2.5 cm long, with an inside diameter of 0.3 cm, and an outside diameter of 0.6 cm. When the explants were retrieved, thin capsules were observed, of varying thickness and blood supply, surrounding the end of the catheters. Pathological evaluation revealed the best large pore material to be proplast regular and the best small pore materials to be carbon and polyurethane.     

Effects of pore size in 3-D fibrous matrix on human trophoblast tissue development

The effects of pore size in a 3-D polyethylene terephthalate (PET) nonwoven fibrous matrix on long-term tissue development of human trophoblast ED27 cells were studied. Thermal compression was used to modify the porosity and pore size of the PET matrix. The pore size distributions in PET matrices were quantified using a liquid extrusion method. Cell metabolic activities, estradiol production, and cell proliferation and differentiation were studied for ED27 cells cultured in the thermally compressed PET matrices with known pore structure characteristics. In general, metabolic activities and proliferation rate were higher initially for cultures grown in the low-porosity (LP) PET matrix (porosity of 0.849, average pore size of 30 microm in diameter) than those in the high-porosity (HP) matrix (porosity of 0.896, average pore size of 39 microm in diameter). However, 17beta-estradiol production and cell differentiation activity in the HP matrix surpassed those in the LP matrix after 12 days. The expression levels of cyclin B1 and p27kip1 in cells revealed progressively decreasing proliferation and increasing differentiation activities for cells grown in PET matrices. Also, difference in pore size controlled the cell spatial organization in the PET matrices and contributed to the tissue development in varying degrees of proliferation and differentiation. It was also found that cells grown on the 2-D surface behaved differently in cell cycle progression and did not show increased differentiation activities after growth had stopped and proliferation activities had lowered to a minimal level. The results from this study suggest that the 3-D cell organization guided by the tissue scaffold is important to tissue formation in vitro.     

Characterization and evaluation of hydrophobically modified chitosan scaffolds: Towards design of enzyme immobilized flow-through electrodes

Chitosan scaffolds were fabricated by application of thermally induced phase separation from aqueous solutions of unmodified chitosan and hydrophobically modified chitosan polymer. The final pore structure, in terms of diameter and geometry, were correlated to freezing temperature and freezing time for both the unmodified and hydrophobically modified chitosan polymer. Results showed that the resulting pore structure is strongly dependent upon the freezing temperature and less dependant upon the freezing time. For scaffolds produced from unmodified chitosan, the pore size decreased as expected with decreasing freezing temperature from ?5 °C to ?10 °C. However, an inconsistency in this trend was observed as the freezing temperature was decreased to ?20 °C. Combined analysis of pore size distribution and average pore diameter suggested that the freezing process was mainly mass transfer dominated at ?5 °C and ?10 °C, but principally heat transfer dominated at ?20 °C. In comparison, the scaffolds produced from hydrophobically modified chitosan (butyl-chitosan) followed the expected trend of decreasing mean pore diameter with decreased freezing temperatures throughout the entire temperature range. The scaffolds produced from the unmodified chitosan were more stable and rigid, and possessed average pore diameters that were generally smaller than those fabricated from the hydrophobically modified chitosan. The generally larger pores in the butyl-modified chitosan scaffolds might be explained by increased phase separation rates due to the introduced hydrophobicity of the chitosan polymer. Among the scaffolds fabricated from the butyl-modified chitosan, those produced at ?20 °C yielded the most uniform pore structure, the smallest average pore diameters, and the least temporal broadening of pore size distribution.     

Assembly of Staphylococcus aureus leukocidin into a pore-forming ring-shaped oligomer on human polymorphonuclear leukocytes and rabbit erythrocytes.

Staphylococcal leukocidin consists of two separate proteins, LukS and LukF, which cooperatively lyse human and rabbit polymorphonuclear leukocytes and rabbit erythrocytes. Here we studied the pore-forming properties of leukocidin and the molecular architecture of the leukocidin pore. (1) Leukocidin caused an efflux of potassium ions from rabbit erythrocytes and swelling of the cells before hemolysis. However, ultimate lysis of the toxin-treated swollen erythrocytes did not occur when polyethylene glycols with hydrodynamic diameters of > or = 2.1 nm were present in the extracellular space. (2) Electron microscopy showed the presence of a ring-shaped structure with outer and inner diameters of 9 and 3 nm, respectively, on leukocidin-treated human polymorphonuclear leukocytes and rabbit erythrocytes. (3) Ring-shaped structures of the same dimensions were isolated from the target cells, and they contained LukS and LukF in a molar ratio of 1:1. (4) A single ring-shaped toxin complex had a molecular size of 205 kDa. These results indicated that LukS and LukF assemble into a ring-shaped oligomer of approximately 200 kDa on the target cells, forming a membrane pore with a functional diameter of approximately 2 nm.     

Characterization of Streptococcus agalactiae CAMP factor as a pore-forming toxin

A recombinant form of CAMP factor of Streptococcus agalactiae has been expressed as glutathione S-transferase-CAMP fusion protein in Escherichia coli. After thrombin cleavage of the fusion protein, the recombinant CAMP factor exhibited hemolytic activity comparable with that of the native form. Osmotic protection experiments with polyethylene glycols show that CAMP factor forms discrete transmembrane pores with a diameter upward of 1.6 nm on susceptible membranes; electron microscopy reveals circular membrane lesions of heterogeneous size, up to 12-15 nm in diameter. Liposome permeabilization studies show that pore formation is a highly cooperative process, which suggests that it involves the oligomerization of CAMP factor. Chemical cross-linking experiments also support an oligomeric mode of action.     

The stimulating effect of glycols and their polymers on the tarsal receptors of blowflies

The rejection thresholds of Phormia regina Meigen for twenty-four glycols have been determined. A definite relationship between the concentration of the test material and the distribution of thresholds has been noted regularly in samples of flies selected at random from a population of known age which had been reared under standard conditions. The scattering of thresholds is normal with respect to the logarithm of concentration. Recalculation of the data of other workers reveals the same sort of relationship with other species of insects and the minnow Phoxinus. The underlying reason for the phenomenon is not known. The glycols in common with other series of homologous alipbatic compounds are rejected at logarithmically decreasing concentrations as the chain length is increased. In general the straight chain diols are more stimulating than the corresponding polyethylene and polypropylene glycols. This difference is related in some manner to the presence of ether linkages in the latter. Polypropylene glycols, with chains of three carbon atoms between the ether linkages are more stimulating than polyethylene glycols, where the spacing is -O-C-C-O-. Unipolymers are more stimulating than mixtures of homologues with the same average molecular weights. Polyethylene glycol 1540 is the largest molecule of measured molecular weight known to stimulate chemoreceptors. The introduction of a second terminal hydroxyl group into the straight hydrocarbon chain reduces the stimulating effect. Alcohols corresponding to the first three diols average about four times as stimulating as the latter while those corresponding to the higher diols average more than one hundred times as stimulating.     

Differences in apparent pore sizes of low and high voltage-activated Ca2+ channels

Pore size is of considerable interest in voltage-gated Ca(2+) channels because they exemplify a fundamental ability of certain ion channels: to display large pore diameter, but also great selectivity for their ion of choice. We determined the pore size of several voltage-dependent Ca(2+) channels of known molecular composition with large organic cations as probes. T-type channels supported by the Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 subunits; L-type channels encoded by the Ca(V)1.2, beta(1), and alpha(2)delta(1) subunits; and R-type channels encoded by the Ca(V)2.3 and beta(3) subunits were each studied using a Xenopus oocyte expression system. The weak permeabilities to organic cations were resolved by looking at inward tails generated upon repolarization after a large depolarizing pulse. Large inward NH(4)(+) currents and sizable methylammonium and dimethylammonium currents were observed in all of the channels tested, whereas trimethylammonium permeated only through L- and R-type channels, and tetramethylammonium currents were observed only in L-type channels. Thus, our experiments revealed an unexpected heterogeneity in pore size among different Ca(2+) channels, with L-type channels having the largest pore (effective diameter = 6.2 A), T-type channels having the tiniest pore (effective diameter = 5.1 A), and R-type channels having a pore size intermediate between these extremes. These findings ran counter to first-order expectations for these channels based simply on their degree of selectivity among inorganic cations or on the bulkiness of their acidic side chains at the locus of selectivity.     

Decrease in intestinal permeability to polyethylene glycol 1000 during development in the pig

Changes in intestinal permeability during postnatal development in the pig were investigated by using different-sized polyethylene glycols in the Mr 766-1338 range (polyethylene glycol 1000) as permeability probes. Pigs of varying age, newborn (Oh), 36-45 h old and 22-28 days old, were gavage fed polyethylene glycol 1000 together with the macromolecular markers bovine serum albumin, ovalbumin or FITC-labelled dextran 70,000. The 4-h blood serum concentrations of the different markers were determined and taken as an estimate of their intestinal transmission. In the newborn pigs, high serum levels of polyethylene glycols were obtained, concomitant with high serum levels of bovine serum albumin and FITC-dextran. After intestinal macromolecular closure in the 36-45 h-old pigs, lower serum polyethylene glycol levels were found, especially of those with a Mr greater than 1100 Da. In the 22-28 days-old pigs, polyethylene glycol levels were reduced to one-tenth or less of those in the 36-45 h-old pigs, with the levels decreasing markedly with increasing molecular size. These results show that there is a correlation between the intestinal permeability of polyethylene glycols, especially those larger than 1100, and macromolecules in the newborn pig around intestinal closure, suggesting that such polyethylene glycols traverse the gut by the macromolecular route. During later development, further intestinal maturation results in a markedly reduced permeability to polyethylene glycol 1000.     

Manipulation of cell volume and membrane pore comparison following single cell permeabilization with 60- and 600-ns electric pulses

Intense nanosecond-duration electric pulses (nsEP) open stable nanopores in the cell membrane, followed by cell volume changes due to water uptake or expulsion, as regulated by the osmolality balance of pore-impermeable solutes inside and outside the cell. The size of pores opened by either fifty 60-ns EP (~13 kV/cm) or five, 600-ns EP (~6 kV/cm) in GH3 cells was estimated by isoosmotic replacement of bath NaCl with polyethylene glycols and sugars. Such replacement reduced cell swelling or resulted in transient or sustained cell shrinking in response to EP. depending on the availability of pores permeable to the test solute. Unexpectedly, solute substitutions showed that for the same integral area of pores opened by 60- and 600-ns treatments (as estimated by cell volume changes), the pore sizes were similar. However, the 600-ns exposure triggered significantly higher cell uptake of propidium. We concluded that 600-ns EP opened a greater number of larger (propidium-permeable pores), but the fraction of the larger pores in the entire pore population was insufficient to contribute to cell volume changes. For both the 60- and 600-ns exposures, cell volume changes were determined by pores smaller than 0.9 nm in diameter; however, the diameter increased with increasing the nsEP intensity.     

Hemoglobin precipitation by polyethylene glycols leads to underestimation of membrane pore sizes

The size of pores formed in the plasma membrane by various substances is frequently determined using polyethylene glycols as osmotic protectants. In this work, we have found that the size of pores formed by saponin in the red blood cell membrane determined by hemolysis versus molecular weight of polyethylene glycol was different to that estimated by light dispersion of cell suspensions. After complete swelling of cells induced by saponin in semiisotonic salt media containing 150 mOsm PEG-4000 or PEG-3000, a significant increase in the light absorbance at 640 nm was developed resulting from the formation of hemoglobin precipitates. Easily sedimenting aggregates were also formed when the supernatant of lysed cells was added to the equiosmotic solutions of polyethylene glycols with molecular weight higher than 1000. We suggest that the real size of large pores could be underestimated due to the phenomenon of hemoglobin precipitation by polyethylene glycols.     

Structural basis and dynamics of the fiber-to-crystal transition of sickle cell hemoglobin

The kinetics of the assembly of structurally distinct, polymeric aggregates constituting the fiber-to-crystal transition of sickle cell hemoglobin in slowly stirred, deoxygenated solutions has been studied with the use of electron microscopy as a function of pH, as a function of the crystal structures of mutant forms of human deoxyhemoglobins employed as nucleating seeds, and as a function of hemoglobin S chemically modified at the Cys F9 (beta 93) position. The temporal order of appearance of fibers of approximately 210 A diameter, bundles of aligned fibers, macrofibers of greater than or equal to 650 A diameter, and microcrystals is observed. Microscopic fragments of end-stage crystals formed under slowly stirred conditions and introduced as nucleating seeds enhance the rate of crystallization only when added prior to the formation of large bundles of aligned fibers, while microscopic seed crystals added after the formation of bundles of aligned fibers do not alter the rate of crystallization. Over the pH range 6.3 to 7.1, the presence of macrofibers does not influence modulation of the kinetics of the transition with seed crystal fragments. Microscopic seed crystals of deoxyhemoglobin S and deoxyhemoglobin C formed under acidic conditions (pH less than 6.5) have a comparable influence on the kinetics of the fiber-to-crystal transition to that of end-stage crystals. Microscopic seed crystals of deoxyhemoglobin C formed under alkaline conditions (pH greater than 6.5) enhance the formation of macrofibers but do not alter the rate of crystallization. Under conditions associated with enhanced formation of macrofibers, metastable microscopic crystals having axial periodicities of approximately 64 A and approximately 210 A are observed in the intermediate phase of the transition, while end-stage crystals have axial unit cell dimensions identical to those of deoxyhemoglobin S crystallized from polyethylene glycol solutions of pH less than 6.5. Although the metastable crystals may arise from fragments of macrofibers, it is shown that they cannot be transformed directly into end-stage crystals under slowly stirred conditions without undergoing dissolution. These results stipulate that the pathway of the fiber-to-crystal transition proceeds according to the reaction: (Formula: see text) wherein the rate-limiting step is the alignment of fibers into large bundles, and macrofibers are not an intermediate of the fiber-to-crystal transition.(ABSTRACT TRUNCATED AT 400 WORDS)     

A soft poration of planar bilayer lipid membranes from dipalmitoylphosphatidylcholine at the temperature of the phase transition from the liquid crystalline to the gel state

A method of soft poration of lipid bilayer was suggested, which is based on the structural rearrangement of lipid bilayer formed from disaturated phospholipids on the phase transition from liquid crystalline state to the gel. As opposed to the widely used method of electropbration, this method allows one to obtain a lipid pore population without application of high electric field. In the case of soft poration, the electric field does not exceed the physiological level of 10-100 mV. It was shown that, in planar bilayer lipid membranes formed from dipalmitoylphosphatidylcholine in water solution of 1 M LiCl, there appear up to 10 lipid pores in 1 min per 1 mm of membrane surface with an average conductivity of a pore of 31 +/- 13 nS. The average pore radius estimated using soluble polyethylene glycols ranged between 1.05-1.63 nm. Monovalent cation conductivity of a single lipid pore on soft poration was shown to decrease in the order Li+ > or = Na+ > K+ = Rb+ > or = Cs+. This order coincides with that observed by Marra and Israilashvili for dipalmitoylphosphatidylcholine-water interbilayer where the repulsive hydration force contribution is significant.