Carbohydrate-carbohydrate interactions are rarely considered in biologically relevant situations such as cell recognition and adhesion. One Ca²⁺-mediated homotypic interaction between two Lewis^x determinants (Le^x) has been proposed to drive cell adhesion in murine embryogenesis. Here, we confirm the existence of this specific interaction by reporting the first direct quantitative measurements in an environment akin to that provided by membranes. The adhesion between giant vesicles functionalized with Le^x was obtained by micropipette aspiration and contact angle measurements. This interaction is below the thermal energy, and cell-cell adhesion will require a large number of molecules, as illustrated by the Le^x concentration peak observed at the cell membranes during the morula stage of the embryo. This adhesion is ultralow and therefore difficult to measure. Such small interactions explain why the concept of specific interactions between carbohydrates is often neglected.

Working with pure lipidic systems (giant unilamellar vesicles, 10-150 microm in diameter) as models for biological membranes, we have considered possible structures of the contact area of two adherent membranes by investigating the diffusion of fluorescent lipid analogues from one vesicle to another. Two bilayers in close contact can almost be seen as a lamellar structure in equilibrium. This is the usual configuration of two adherent vesicles, in which the interbilayer distance is estimated to be 3 nm. We have increased the attraction between the membranes by either adding depletion forces or by using a trick, inspired from the interaction between nucleic bases in nucleosides (herein adenosine and thymidine). The nucleosides were attached to the polar head of amphiphilic molecules that behave like phospholipids and were incorporated in the model membrane. The extra attraction between two membranes, resulting from base pairing, strongly decreased the interbilayer distance down to about 1 nm. This change of the water content induced lipid rearrangements, which could also be viewed in terms of a phase transition at low water content. These rearrangements were not observed in the case of depletion forces. We conclude that the introduction of an additional attractive force in the system modifies the equilibrium state, leading to a drastic change in the membrane behavior, which will tentatively be related to hemifusion.

Two highly hydrophobic Lewis X glycolipids 2 and 3 were prepared. The glycoconjugate 2 was constructed in the following way: pentaerythrytol was used as a distributor on which three racemic phytol hydrophobic chains and a triethyleneglycol spacer β-glycosylated with the pentasaccharide Gal (β 1−4)[Fuc (α 1−3)] GlcNAc (β 1−3) Gal (β 1−4) Glc were anchored. The glycoconjugate 3 was constructed in a similar way, the sugar moiety being the trisaccharide Gal (β 1−4)[Fuc (α 1−3)] GlcNAc, the so-called Lewis X determinant. A triethyleneglycol spacer was used in order to introduce the mobility required for the study of single carbohydrate−carbohydrate interactions. Three phytyl chains increase the hydrophobicity of the lipid moiety compared to the natural ceramide glycolipid 1. These glycolipids display a liquid-expanded behaviour with a high compressibility in monolayer studies. These properties associated with a low solubility in water make them good candidates for the study of the interaction between two Lewis X functionalized vesicles.

Several hydrophobically alpha- and alpha,omega-end-capped poly(ethylene oxide) polymers were studied by light scattering below and above their critical association concentration, in order to understand their association mechanisms. In the case of monofunctionalized PEO, a one-step closed association model well fits the experimental data, with a limit number of aggregation of about 30, consistent with other experimental data and a theoretical approach. In the case of difunctionalized PEO, a good description of the experimental data is obtained by assuming a two-step association process: at low concentration, the formation of "flower-like" micelles is well described by a closed association model, whereas at higher concentration, the progressive bridging of these "flowers" can be modeled by an open association.

We report the results of a study on aqueous solutions of hydrophobically modified polyelectrolytes (charge-neutral diblock copolymers) by different techniques which have been specifically chosen for demonstrating unambiguously the existence of micelles. Self-diffusion measurements by fluorescence recovery after photobleaching (FRAPP) and pulsed NMR spectroscopy measure the micellar sizes which are in agreement with light scattering data on dilute solutions. Cryo-transmission electron microscopy (Cryo-TEM) pictures confirm the existence of the micellar cores in contrast with homopolyelectrolyte solutions where no features are observed.