| 1. |
- Almgren, Mats, et al.
(author)
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Cryotransmission electron microscopy of thin vitrified samples.
- 1996
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In: Current Opinion in Colloid & Interface Science. - 1359-0294 .- 1879-0399. ; 1:2, s. 270-278
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Journal article (peer-reviewed)abstract
- During the past few years cryotransmission electron microscopy (EM) of vitrified thin samples has gained acceptance as a standard method in the arsenal of the colloid and interface scientist. The seemingly direct visualization of fluid colloidal structures during the use of cryotransmission EM is both convincing and reliable to the scientist who nowadays has an increasing awareness of the limitations and pitfalls of instrumentation. Notable recent observations include branched threadlike micelles, faceted particles (cubosomes) of a dispersed cubic phase and transitions of certain structures from globular micelles via bilayers to reversed structures. These transitions may be caused by changes of compos ition, temperature, pH, or salt concentration.
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| 2. |
- Andersson, Mikael, et al.
(author)
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Effect of bilayer phase transitions on vesicle structure and its influence on the kinetics of viologen reduction.
- 1995
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In: Journal of physical chemistry. - 0022-3654. ; 99:39, s. 14531-14538
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Journal article (other academic/artistic)abstract
- Vesicles were prepared from pure phosphatidylcholines (PC), phosphatidic acids (PA), or dioctadecyldimethylamonium bromide (DODAB). The aggregate structure was examined above and below the gel-to-liquid crystalline transition temperature (T-m) by means of cryo-transmission electron microscopy. The redox reaction between a membrane bound viologen and dithionite was studied in the different lipid systems. It was found that the presence of faceted vesicles or open bilayer fragments, below T-m, lead to double-exponential reduction kinetics.
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| 3. |
- Basanez, G, et al.
(author)
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Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles : a cryo-transmission electron microscopy study of liposome fusion.
- 1997
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In: Biophysical Journal. - 0006-3495 .- 1542-0086. ; 72:6, s. 2630-2637
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Journal article (peer-reviewed)abstract
- Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the ''honeycomb structure'') that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure (''stalk'') would act as a structural intermediate in membrane fusion, Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.
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| 4. |
- Beugin, S, et al.
(author)
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New sterically stabilized vesicles based on nonionic surfactant, cholesterol, and poly(ethylene glycol)-cholesterol conjugates.
- 1998
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In: Biophysical Journal. - 0006-3495 .- 1542-0086. ; 74:6, s. 3198-3210
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Journal article (other academic/artistic)abstract
- Monomethoxypoly(ethylene glycol) cholesteryl carbonates (M-PEG-Chol) with polymer chain molecular weights of 1000 (M-PEG1000-Chol) and 2000 (M-PEG2000-Chol) have been newly synthesized and characterized. Their aggregation behavior in mixture with diglycerol hexadecyl ether (C(16)G(2)) and cholesterol has been examined by cryotransmission electron microscopy, high-performance gel exclusion chromatography, and quasielastic light scattering. Nonaggregated, stable, unilamellar vesicles were obtained at low polymer levels with optimal shape and size homogeneity at cholesteryl conjugate/ lipids ratios of 10 mol% M-PEG1000-Chol or 5 mol% M-PEG2000-Chol, corresponding to the theoretically predicted brush conformational state of the PEG chains. At 20 mol% M-PEG1000-Chol or 10 mol% M-PEG2000-Chol, the saturation threshold of the C(16)G(2)/cholesterol membrane in polymer is exceeded, and open disk-shaped aggregates are seen in coexistence with closed vesicles. Higher levels up to 30 mol% lead to the complete solubilization of the vesicles into disk-like structures of decreasing size with increasing PEG content. This study underlines the bivalent role of M-PEG-Chol derivatives: while behaving as solubilizing surfactants, they provide an efficient steric barrier, preventing the vesicles from aggregation and fusion over a period of at least 2 weeks.
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| 5. |
- Edwards, Katarina, et al.
(author)
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Aggregate Structure in Dilute Aqueous Dispersions of Oleic Acid/Sodium Oleate and Oleic Acid/Sodium Oleate/Egg Phosphatidylcholine
- 1995
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 11:7, s. 2429-2434
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Journal article (peer-reviewed)abstract
- Cryo-transmission electron microscopy was used to investigate the aggregate structure in dilute aqueous samples of oleic acid, as a function of pH. At pH 10.7, where the fatty acid is almost completely deprotonated, the micrographs show spherical or cylindrical micelles depending on the concentration. Upon a decrease of the pH to values just above 9, formation of unilamellar vesicles is induced. With decreasing pH the vesicles show an increasing tendency to aggregate. At pH between 8 and 7, large clusters of aggregated vesicles coexist with dispersed nonlamellar, presumably inverted hexagonal structures. Further decrease in pH results in a complete transition into nonlamellar liquid-crystalline structures and finally to the formation of oil droplets. Addition of high concentrations of oleic acid to small unilamellar lecithin vesicles induces, at pH 7.4 and lower, clustering and formation of particles with nonlamellar structure. At high pH, on the other hand, oleic acid acts like a conventional cationic surfactant. With increasing fatty acid: lipid molar ratio both significant vesicle growth and finally lipid solubilization into mixed micelles are observed.
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| 6. |
- Edwards, Katarina, et al.
(author)
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Effect of polyethyleneglycol-phospholipids on aggregate structure in preparations of small unilamellar liposomes
- 1997
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In: Biophysical Journal. - 0006-3495 .- 1542-0086. ; 73:1, s. 258-266
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Journal article (other academic/artistic)abstract
- Phospholipids with covalently attached poly(ethylene glycol) (PEG lipids) are commonly used for the preparation of long circulating liposomes. Although it is well known that lipid/PEG-lipid mixed micelles may form above a certain critical concentration of PEG-lipid, little is known about the effects of PEG-lipids on liposome structure and leakage at submicellar concentrations. In this study we have used cryogenic transmission electron microscopy to investigate the effect of PEG(2000)-PE on aggregate structure in preparations of liposomes with different membrane compositions. The results reveal a number of important aggregate structures not documented before. The micrographs show that enclosure of PEG-PE induces the formation of open bilayer discs at concentrations well below those where mixed micelles begin to form. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the phospholipid chain length, whereas phospholipid saturation or the presence of cholesterol has little or no effect. The presence of cholesterol does, however, affect the shape of the mixed micelles formed at high concentrations of PEG-lipid. Threadlike micelles form in the absence of cholesterol but adapt a globular shape when cholesterol is present.
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| 7. |
- Hammarström, Leif, et al.
(author)
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Cryo-TEM Evidence : Sonication of Dihexadecyl Phosphate Does Not Produce Closed Bilayers with Smooth Curvature
- 1995
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 11:2, s. 408-410
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Journal article (other academic/artistic)abstract
- Dihexadecyl phosphate (DHP) is commonly used for formation of model membranes. In this report cryo-transmission electron microscopy (cryo-TEM) pictures are presented. They clearly show that after cooling to room temperature, dispersions of DHP sonicated at 80 degrees C are dominated by open and folded bilayer fragments, rather than by vesicles with smooth curvature. Quantitative results of an earlier kinetic investigation of viologen reduction in DHP were reproduced using these dispersions.
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| 8. |
- Johnsson, Markus, et al.
(author)
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Effect of PEO-PPO-PEO Triblock Copolymers on Structure and Stability of Phosphatidylcholine Liposomes
- 1999
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 15:19, s. 6314-6325
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Journal article (peer-reviewed)abstract
- The interactions of five poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (PEO-PPO-PEO), Pluronic, copolymers and phosphatidylcholine liposomes of varying composition have been studied. Structural studies were performed by means of cryo-transmission electron microscopy (c-TEM) and reveal that inclusion of low amounts (similar to 2 mol %) of Pluronics gives rise to significant morphological changes of the liposome preparations. Pluronics with large poly(oxyethylene) (PEO) blocks, such as F127, F108, and F87, induce the formation of bilayer disks, whereas those with comparably short PEO blocks, P105 and P85, tend to to promote a reduction in the liposome size. Inclusion of cholesterol in the liposomal preparations reduces the incorporation of copolymers in the lipid bilayer and thus reduces, and in some cases even abolishes, the morphological changes observed in the absence of cholesterol. The effect of the copolymers on liposome permeability was also investigated. All investigated copolymers were found to increase the leakage of carboxyfluorescein from preformed liposomes. This was true also in the case of cholesterol-containing Liposomes despite the fact that no change in the liposome structure could be observed by means of c-TEM. The magnitude of the induced leakage was found to correlate well with the hydrophobicity, as measured by the cmc, of the respective Pluronic. By raising the temperature or decreasing the solvency of the copolymer in the solution, the effect of the copolymer on liposome leakage was found to increase significantly.
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| 9. |
- Johnsson, Markus, et al.
(author)
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Optimization of drug loading procedures and characterization of liposomal formulations of two novel agents intended for Boron Neutron Capture Therapy (BNCT)
- 1999
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In: Journal of liposome research. - : Informa UK Limited. - 0898-2104 .- 1532-2394. ; 9:1, s. 53-79
-
Journal article (peer-reviewed)abstract
- The characterization of two liposomal formulations of boronated DNA-interacting agents has been performed. It is shown that the two boronated drugs, WSA-Water Soluble Acridine and WSP-Water Soluble Phenantridine, can be encapsulated within unilamellar sterically stabilized liposomes with high drug-to-lipid ratios (up to 0.50:1 (mol:mol)), using transmembrane pH gradients. The steric stabilization of the liposomes was accomplished by the addition of DSPE-PEG(2000) (PEG-lipid) to DSPC/Cho lipid mixtures and the composition used was DSPC:Cho:DSPE-PEG 55:40:5 (mol%). The loading of the drugs resulted in drug precipitation in the liposomal aqueous core as observed by cryo-transmission electron microscopy (c-TEM). Moreover, it is shown that when pH gradients across the bilayer were used for remote loading of WSP or when ammonium sulfate gradients were used for remote loading of WSA, the formation of small bilayer fragments (discs) was induced. We present compelling evidence that the formation of discs is a consequence of precipitate growth in the liposomal interior. The precipitate growth causes some of the liposomes to rupture resulting in the above mentioned disc-formation and a substantial decrease in trapping efficiency. The in vitro stability of the drug loaded liposomes was excellent, both in buffer and in 25% human serum. For most of the formulations, the release of the drugs was below or around 10% after 24 hours at 37 degrees C. Furthermore, the influence of initial internal pH and internal buffering capacity on release properties of WSA and WSP were investigated. It is shown that the release profiles of the drugs can be controlled, to a large extent, by varying the composition of the internal liposomal aqueous phase.
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| 10. |
- Parmar, Manjeet M, et al.
(author)
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Incorporation of bacterial membrane proteins into liposomes : factors influencing protein reconstitution
- 1999
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In: Biochimica et Biophysica Acta - Biomembranes. - 0005-2736 .- 1879-2642. ; 1421:1, s. 77-90
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Journal article (peer-reviewed)abstract
- Meningococcal and gonococcal outer membrane proteins were reconstituted into liposomes using detergent-mediated dialysis. The detergents octyl glucopyranoside (OGP), sodium cholate and Empigen BB were compared with respect to efficiency of detergent removal and protein incorporation. The rate of OGP removal was greater than for cholate during dialysis. Isopycnic density gradient centrifugation studies showed that liposomes were not formed and hence no protein incorporation occurred during dialysis from an Empigen BB containing reconstitution mixture. Cholate-mediated reconstitution yielded proteoliposomes with only 75% of the protein associated with the vesicles whereas all of the protein was reconstituted into the lipid bilayer during OGP-mediated reconstitution. Essentially complete protein incorporation was achieved with an initial protein-to-lipid ratio of 0.01:1 (w/w) in the reconstitution mixture; however, at higher initial protein-to-lipid ratios (0.02:1) only 75% protein incorporation was achieved. Reconstituted proteoliposomes were observed as large (>300 nm), multilamellar structures using cryo-electron microscopy. Size reduction of these proteoliposomes by extrusion did not result in significant loss of protein or lipid. Extruded proteoliposomes were unilamellar vesicles with mean diameter of about 100 nm.
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