E. Del Favero

Cubic phases of gangliosides in water: Possible role of the conformational bistability of the headgroup

An 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.

Collective phenomena in confined micellar systems of gangliosides

In the present paper, we investigate in deeper detail some features of the bistable behavior observed in micellar aggregates of gangliosides. In these systems, a collective transition takes place connected to different possible conformations of the glycosidic headgroup and resulting in different aggregation numbers for different thermal hystories. Light scattering experiments performed on mixed micelles indicate that it is the aggregated structure which dictates the conformation a ganglioside monomer assumes, among the allowed ones. Moreover, density measurements confirm the results previously obtained by DSC and show that the conformation of the hydrophobic tails at a given temperature is coupled to that of the headgroup. NMR measurements are consistent with previous results and, on a more local scale, indicate that the hydration of the ganglioside molecule is affected by the conformational transition.

Thermal fluctuations of small vesicles: observation by dynamic light scattering

The 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.

Selfaggregation of glyco-lipids in water: vesicle to micelle transition

Abstract An 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 GM1 to the initial GM3 solution.

A calorimetric study of thermal hysteresis effects in ganglioside micelles

Light, X-ray and neutron scattering measurements clearly showed strong temperature-related variations in micellar mean aggregation number, accompanied by dramatic thermal hysteresis effects, for a variety of micelle-forming gangliosides. Gangliosides, sialic-acid-containing glycosphingolipids, are amphiphilic molecules of biological origin with a ceramide (a double-tailed hydrophobic part, like phospholipids) and an oligosaccharide chain as the headgroup. When temperature is varied in the range 30–55 °C (below the critical temperature T c = 55 °C) strong hysteresis effects are observed, with the micellar size depending on the thermal history of the sample. This thermotropic behaviour of ganglioside micelles has been attributed to the existence of different conformations of the bulky flexible saccharidic headgroup, each conformation having a slightly different geometric hindrance, and also to the temperature related change of the equilibrium between conformers in the micelle. In this paper, careful calorimetric measurements are presented which show that the collective properties of the hydrophobic core of ganglioside micelles also depend on the thermal history of the sample below the critical temperature T c. The hydrophobic chains of the core undergo an order-disorder transition, like phospholipids bilayers, but at a temperature which depends on the micellar size. Measurements refer to the GM1 ganglioside, both in the natural composition and with a well-characterized hydrophobic part, namely C18 and C20 sphingosines.

The diffusive dynamics of water confined in ganglioside micelles

Abstract We have investigated by QENS the dynamics of water associated to gangliosides. The dependence of the QENS line-broadening versus Q indicates that proton diffusion is restricted when investigated over scale lengths of about 6–8 A; at smaller distances the diffusivity parameters are similar to those of pure water at a lower temperature.

Directional K+ channel insertion in a single phospholipid bilayer: Neutron reflectometry and electrophysiology in the joint exploration of a model membrane functional platform

We investigated the insertion of small potassium (K+) channel proteins (KcvMA-1D and KcvNTS) into model membranes and the lipid-protein structural interference, combining neutron reflectometry and electrophysiology. Neutron reflectometry experiments showed how the transverse structure and mechanical properties of the bilayer were modified, upon insertion of the proteins in single model-membranes, either supported on solid substrate or floating. Parallel electrophysiology experiments were performed on the same channels reconstituted in free-standing planar lipid bilayers, of both typical composition and matched to the neutron reflectometry experiment, assessing their electrical features. Functional and structural results converge in detecting that the proteins, conical in shape, insert with a directionality, cytosolic side first. Our work addresses the powerful combination of the two experimental approaches. We show here that membrane structure spectroscopy and ion channel electrophysiology can become synergistic tools in the analysis of structural-functional properties of biomimetic complex environment.