1,721,081 research outputs found
Peculiarities in the aggregative behaviour of gangliosides, glucosidic surfactant of biological origin
No full textGlucosidic surfactants play an important role in living matter. An interesting class of them are the gangliosides which have a double-tail hydrophobic part, Like phospholipids, and a ramified oligosaccharide chain as polar head. The large hydrophobic moiety ensures very low CMC values, of the order of 10(-8) M. Phospholipids have a small head group compared with their hydrophobic: part. Therefore they form mainly bilayer-type structures in water. Gangliosides, instead, may form either micelles or bilayers depending on the length and conformation of the oligosaccharide chain. For long chains, micelles are formed with a large aggregation number and a long lifetime (many hours), while vesicles are found for short sugar chains. The presence of a large head group makes vesicle mechanical properties rather peculiar. For instance, the ganglioside GM3, which has the longest sugar chain compatible with bilayer-type structures, forms vesicles spontaneously in solution without supply of external energy. This means that the bending rigidity of the GM3 bilayer is very low, allowing very large thermal undulations. GM3 vesicles, made of a single surfactant, are then found to be in thermodynamic equilibrium with a small amount of large bilayer and multilamellar structures. Frustration of the single-component vesicle system is released if a second amphiphile, of the same type of the ganglioside GM3 but with a larger head group, is added. In this case, spontaneous-curvature readjustements of the two monolayers via demixing can give rise to a spontaneous curvature of the bilayer, energetically favouring vesicles over other structures. It is, in fact, observed experimentally that by adding increasing proportions of GM1 the large aggregates gradually diminish in number, until a pure vesicle solution is obtained for a GM1 to GM3 ratio of 35 to 65
Self aggregation of glyco-lipids in water: vesicle to micelle transition
No full textAn experimental study of the spontaneous aggregation geometry of sialic acid containing glyco-lipids, the gangliosides, in water solution is presented. Interestingly enough, it is possible to follow the transition from the energetically ''Frustrated'' bilayer type structures of pure GM3 solutions to lower energy vesicular states and then to mixed micellar solutions by progressively adding the second ganglioside GMI to the initial GM3 solution
Self-aggregation properties of gangliosides, glycosidic surfactants of biological origin
No full textA comparative study of the aggregation properties of a class of glycosidic surfactants, all having the same hydrophobic part but different saccharidic headgroups, is presented. Although the surfactants, gangliosides, are of biological origin, the reported data can be of general use. Gangliosides are double-tail surfactant like phospholipids, but have a ramified oligosaccharide chain headgroup. They may form either micelles or bilayers depending on the length and conformation of the oligosaccharide chain. For long chains, micelles are formed with a large aggregation number and a long lifetime (many hours), while vesicles are formed for shell sugar chains. Their critical micelle concentration is very low of the order of 10(-8) M, and the micellar dimension is independent of the ionic strength of the solution, even if gangliosides are ionic surfactants due to the presence of sialic acids. Steric effects of the headgroups are very important. For instance the ganglioside GM3 forms vesicles spontaneously in solution with a low bending rigidity of the bilayer. Data are interpreted in terms of the existing geometrical models of self-assembly
The effect of temperature on interacting micelles of gangliosides in water
No full textMicellar solutions of the ganglioside GM1 are studied as a function of temperature in the concentration range 0.003-0.30 g CM-3 in water with light and x-ray scattering. In a narrow temperature range around 50-degrees-C, the micelles decrease their aggregation number from 300 to 235. The solution structuring, which is due to the electrostatic interactions among micelles, is found to be strongly affected by the micellar change. For instance, at high concentration the isotropic micellar solution at 25-degrees-C transforms into a primitive cubic phase at 60-degrees-C
Cubic phases of gangliosides in water: possible role of the conformational bistability of the headgroup
No full textAn accurate X-ray scattering study of the self-association properties in water of the ganglioside GM1, a biological amphiphile similar to phospholipids but with an oligosaccharide headgroup, is described. The phase diagram displays a large cubic region, for concentrations between 31% and 55% by weight, enclosed by isotropic micellar and lamellar phases. The peculiar feature of the GM1 molecule to present a conformational bistability, recently observed in ganglioside micelles, can be well recognized in the cubic phase region, suggesting that GM1-water behaves like a pseudoternary system, with an additional degree of freedom, automatically readjusting the mole fraction of the two conformers in order to optimize the molecular packing
Bistable molecular selfassembly
No full textSelf-assembled aggregates made up of molecules taking different geometrical conformational states are discussed. Besides the well-known order-disorder conformational transition involving the hydrophobic tails, the review focuses on the geometrical variations occurring in the head group region and on the related changes of the whole assembly. These effects are particularly relevant in amphiphile molecules carrying bulky and flexible headgroups like the glycolipids. Possible biological relevance of the headgroup bistability is discusse
Tightly packed lipid lamellae with large conformational flexibility in the interfacial region may exhibit multiple periodicity in their repeat distance. A theoretical analysis and X-ray verification
No full textA temperature-dependent irreversible variation of the average aggregation number of GM1 micelles, with no change in the chemical structure of the molecule, has been observed by light and x-ray scattering. GM1 is an amphiphilic molecule of biological origin, similar to phospholipids but with an extended headgroup, made up of many sugar units. A simple model has been developed to describe the experimental results. It assumes that the polar headgroups of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure and displaying preferential interactions with the surrounding heads once inserted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle surface, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups different amounts of an amphiphilic spacer have been inserted in GM1 micelles. The thermal hysteresis phenomenon was still observed on mixed micelles until a molar ratio GM1/spacer similar to 1/3 was reached, corresponding to the critical concentration calculated from the mean-field theory of dilute magnetic alloys in a 2D lattice; The observed behaviour, then, appears as a critical phenomenon of topological nature happening in a confined two-dimensional system, that is the micelle surfac
Thermal fluctuations of small vesicles: observation by dynamic light scattering
No full textThe laser light scattering technique can be used in a nonconventional fashion to study dynamic properties of vesicles which are too small to be observed by microscopy. In fact, in suitable experimental conditions, the correlation function of the scattered light contains a contribution from bilayer fluctuations, besides the usual diffusion one. Characteristic fluctuation times have been determined for single-component phospholipid vesicles of 60 nm radius, prepared by extrusion. The addition of small amounts of a glycolipid (to 2% mole fraction), induces a significative increase in the fluctuation times (of the order of 20%) but still does not affect the diffusive motion, indicating a softening of the membrane. Being so sensitive, this technique is quite promising both for the study of membrane properties in the presence of defects and for applications to biology and pharmacology
Phase transition at the surface of mixed micelles of the ganglioside GM1 and dodecylphosphocholine
No full textA temperature-dependent irreversible variation of the average aggregation number of GM1 micelles, with no change in the chemical structure of the molecule, has been observed by light and x-ray scattering. GM1 is an amphiphilic molecule of biological origin, similar to phospholipids but with an extended headgroup, made up of many sugar units. A simple model has been developed to describe the experimental results. It assumes that the polar headgroups of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure and displaying preferential interactions with the surrounding heads once inserted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle surface, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups different amounts of an amphiphilic spacer have been inserted in GM1 micelles. The thermal hysteresis phenomenon was still observed on mixed micelles until a molar ratio GM1/spacer similar to 1/3 was reached, corresponding to the critical concentration calculated from the mean-field theory of dilute magnetic alloys in a 2D lattice; The observed behaviour, then, appears as a critical phenomenon of topological nature happening in a confined two-dimensional system, that is the micelle surfac
Physical aspects of non-ideal mixing of amphiphilic molecules in solution
No full textWe review the interesting physical effects arising when bifunctional molecules, consisting of two moieties, one polar and the other apolar, are dissolved in a simple solvent. The peculiarity of such amphiphilic molecules is that they generate by themselves confined structures in which they are embedded and of which they dictate the topology according to their aggregative properties. Individual molecules are still free to redistribute within or across the aggregates. When two or more types of amphiphilic molecule are mixed together, the strong coupling between local interactions and the aggregate topology gives experimental access to a variety of non-ideal mixing effects. The couplings between the lateral distribution and the geometrical features of amphiphile assemblies are discussed separately for monolayers, bilayers, multilayers, and micelles. Concentration and temperature effects are taken into account. The case of non-ideal mixing of chemically identical molecules with mutually interchanging conformations is also discussed. It is found that bistability and thermal hysteresis phenomena can be important. Theoretical arguments are supported by a variety of experimental observations made of a class of amphiphilic molecules, the gangliosides, which are particularly suitable subjects for investigation as regards such non-ideality effects. Gangliosides, with a double-tail hydrophobic part, like phospholipids, and a bulky saccharidic headgroup, show relevant like-like and like-unlike interactions, and easily form large aggregates whose topology is quite sensitive to monomer redistribution
- …
