Claudine Guthmann

Study of the transverse diffusion of spin labelled phospholipids in biological membranes. I. Human red blood cells

Spin labeled analogs of phosphatidylcholine were used to study the transverse diffusion (flip-flop) of phospholipids in the erythrocyte membrane. The nitroxide spin label was placed either on the beta acyl chain or on the choline group. These labeled phosphatidylcholine molecules were incorporated into the membrane by incubation of the red cells at 22 degrees C with sonicated spin-labed phosphatidylcholine vesicles from which all traces of free fatty acids and lyso derivatives were carefully removed by bovine serum albumin treatment. This incorporation did not provide any change in the morphology of the cell as indicated by scanning electron microscopy. When spin-labeled phosphatidylcholine, having a nitroxide on the beta chain but near the polar head-group, was incorporated into the erythrocyte membrane, ascorbate treatment at 0 degrees C allows selective reduction of the signal coming from the outer layer of the membrane. When the label was on the polar head-group, the inner content of the erythrocyte rapidly reduced the label facing the cytoplasm, thus creaging a spontaneous anisotropy of the labeling. The anisotropic distribution of spin-labeled phosphatidylcholine in the erythrocyte membrane was found to be stable at 22 and 37 degrees C for more than 4 h. It is therefore concluded that the rate of outside-inside and inside-outside transition is so slow that the anisotropic distribution of the phospholipids in the erythrocyte membrane can be maintained during cell life.

Macroscopic 2D Wigner islands

In this paper we present new versatile 2D macroscopic Wigner islands useful to investigate the various behaviors observed in mesoscopic confined systems. Our Wigner islands consist of electrostatically interacting charged balls with millimeter size. We have experimentally determined the ground configurations for systems of N confined particles (N = 1-30) and checked the influence of the confinement and interacting potentials. The results obtained are compared with the published numerical results.

Single file diffusion in macroscopic Wigner rings.

The single file diffusion in a circular channel of millimetric charged balls is studied. The evolution in time of the mean square displacement is shown to be subdiffusive, but slower than the powerlike t1/2 behavior observed in circular colloidal systems or predicted in one-dimensional infinite systems.

Single file diffusion enhancement in a fluctuating modulated quasi-1D channel

We show that the diffusion of a single file of particles moving in a fluctuating modulated quasi-1D channel is enhanced with respect to the one in a bald pipe. This effect, induced by the fluctuations of the modulation, is favoured by the incommensurability between the channel potential modulation and the moving file periodicity. This phenomenon could be of importance in order to optimize the critical current in superconductors, in particular in the case where mobile vortices move in 1D channels designed by patterns of pinning sites.

Local symmetries and order-disorder transitions in small macroscopic Wigner islands

The influence of local order on the disordering scenario of small Wigner islands is discussed. A first disordering step is put in evidence by the time correlation functions and is linked to individual excitations resulting in configuration transitions, which are very sensitive to the local symmetries. This is followed by two other transitions, corresponding to orthoradial and radial diffusion, for which both individual and collective excitations play a significant role. Finally, we show that, contrary to large systems, the focus that is commonly made on collective excitations for such small systems through the Lindemann criterion has to be made carefully in order to clearly identify the relative contributions in the whole disordering process.

Determination of the interactions in confined macroscopic Wigner islands: theory and experiments

Abstract.Macroscopic Wigner islands present an interesting complementary approach to explore the properties of two-dimensional confined particles systems. In this work, we characterize theoretically and experimentally the interaction between their basic components, viz., conducting spheres lying on the bottom electrode of a plane condenser. We show that the interaction energy can be approximately described by a decaying exponential as well as by a modified Bessel function of the second kind. In particular, this implies that the interactions in this system, whose characteristics are easily controllable, are the same as those between vortices in type-II superconductors.

Enhancement of mobilities in a pinned multidomain crystal

Mobility properties inside and around degenerate domains of an elastic lattice partially pinned on a square array of traps are explored by means of a fully controllable model system of macroscopic particles. We focus on the different configurations obtained for filling ratios equal to 1 or 2 when the pinning strength is lowered. These theoretically expected but never observed configurations are degenerated, which implies the existence of a multidomain crystal. We show that the distinction between trapped and untrapped particles that is made in the case of strong pinning is not relevant for such a weaker pinning. Indeed, one ought to distinguish between particles inside or around the domains associated to positional degeneracies. The possible consequences on the depinning dynamics of the lattice are discussed.