A comparative structural study in monolayers of GPI fragments and their binary mixtures

Zwitterionic Character and Lipid Composition Determine the Behaviour of Glycosylphosphatidylinositol Fragments in Monolayers

Glycosylphosphatidylinositols (GPIs) are complex glycolipids found in free form or anchoring proteins to the outer leaflet of the cell membrane in eukaryotes. GPIs have been associated with the formation of lipid rafts and protein sorting on membranes. The presence of a conserved glycan core with cell-specific modifications together with lipid remodelling during biosynthesis suggest that the properties of the glycolipids are being fine-tuned. We synthesized a series of GPI fragments and evaluated the interactions and arrangement of these glycolipids in monolayers as a 2-D membrane model. GIXD and IRRAS analyses showed the need of N-acetylglucosamine deacetylation for the formation of hydrogen bonds to obtain highly structured domains in the monolayers and an effect of the unsaturated lipids in formation and localization of the glycolipids within or between membrane microdomains. These results contribute to understand the role of these glycolipids and their modifications in the organization of membranes.

Strong Headgroup Interactions Drive Highly Directional Growth and Unusual Phase Co-Existence in Self-Assembled Phenolic Films

Self-assembled materials as surface coatings are used to confer functional properties to substrates, but such properties are highly dependent on molecular organization that can be controlled through tailoring the noncovalent interactions. For monomolecular films, it is well-known that strong, dipolar interactions can oppose line tension generating noncircular domain growth. While many surfactant films exhibit liquid crystalline arrangement of the alkyl chains, there are relatively few reports of crystalline headgroups. Here, we report the self-assembly of phenolic surfactants where the combination of hydrogen bonding and π-stacking leads to a herringbone arrangement of the headgroups, generating a molecular super-lattice that can be observed using grazing incidence X-ray diffraction; such an arrangement has been previously proposed for related phenolic systems but never experimentally observed. We also investigated using pH to modulate the intermolecular interactions and the response of the system in terms of molecular organization. The first hydroxyl deprotonation does not appear to impact the structure but has significant impact on the domain size and morphology. Higher pH generates both strong directional domain growth and a loss of the molecular lattice structure, attributed to a second deprotonation. In contrast, a shorter chain surfactant, lauryl gallate, forms a liquid expanded phase that can contract upon deprotonation. In the condensed phase, the deprotonation kinetics are unusually slow for which an internal charge re-organization is proposed. The slow kinetics leads to the co-existence of three distinct phases for a single component system over relatively long timescales and provides evidence of a liquid-mediated polymorphic transformation process in two-dimensional, soft-matter films. This work has implications for understanding the long-range ordering in aromatic self-assembled structures and the mechanisms underlying Langmuir monolayer polymorphism.

Relationship between structure and molecular interactions in monolayers of specially designed aminolipids

Artificial cationic lipids are already recognized as highly efficient gene therapy tools. Here, we focus on another potential use of aminolipids, in their electrically-uncharged state, for the formation of covalently cross-linked, one-molecule-thin films at interfaces. Such films are envisioned for future (bio-)materials applications. To this end, Langmuir monolayers of structurally different aminolipids are comprehensively characterized with the help of highly sensitive surface characterization techniques. Pressure-area isotherms, Brewster angle microscopy, grazing-incidence X-ray diffraction and infrared reflection-absorption spectrometry experiments provide a detailed, comparative molecular picture of the formed monolayers. This physico-chemical study highlights the relationship between chemical structures and intermolecular interactions, which can serve as a basis for the rational design of cross-linked thin films with precisely controlled properties.

Headgroup-Ordered Monolayers of Uncharged Glycolipids Exhibit Selective Interactions with Ions

Selective interactions of ions with charge-neutral saccharides can have far-reaching consequences in biological and wet-technological contexts but have so far been observed only indirectly. Here, we directly quantify by total-reflection X-ray fluorescence the preferential accumulation of ions near uncharged saccharide surfaces in the form of glycolipid Langmuir monolayers at air/water interfaces exhibiting different levels of structural ordering. Selective interactions with ions from the aqueous subphase are observed for monolayers featuring crystalline ordering of the saccharide headgroups, as determined by grazing-incidence X-ray diffraction. The attracted ion species depend on the structural motifs displayed by the ordered saccharide layer. Our results may constitute a basis to understand the salt-specific swelling of wood materials and various phenomena in membrane biophysics.

Structural organization and phase behaviour of meta-substituted dioctadecylaminobenzoquinones at the air/water interface

The structural organization and phase behaviour of an amphiphilic zwitterionic quinonemonoimine at the air/water interface are presented. Brewster angle microscopy reveals multiple co-existing phases are observed over the entire isotherm while grazing incidence X-ray diffraction (GIXD) shows that these comprise both tilted, untilted and multilayer structures with crystalline headgroups. Despite the heterogeneity, the phase transitions are highly reversible over multiple cycles.

Synthesis of Galactosylated Glycosylphosphatidylinositol Derivatives from Trypanosoma brucei

Trypanosoma brucei uses variant surface glycoproteins (VSGs) to evade the host immune system and ensure parasitic longevity in animals and humans. VSGs are attached to the cell membrane by complex glycosylphosphatidylinositol anchors (GPI). Distinguishing structural feature of VSG GPIs are multiple α- and β-galactosides attached to the conserved GPI core structure. T. brucei GPIs have been associated with macrophage activation and alleviation of parasitemia during infection, acting as disease onset delaying antigens. Literature reports that link structural modifications in the GPIs to changes in biological activity are contradictory. We have established a synthetic route to prepare structurally overlapping GPI derivatives bearing different T. brucei characteristic structural modifications. The GPI collection will be used to assess the effect of galactosylation and phosphorylation on T. brucei GPI immunomodulatory activity, and to perform an epitope mapping of this complex glycolipid as potential diagnostic marker for Trypanosomiasis. A strategy for the synthesis of a complete α-tetragalactoside using the 2-naphthylmethyl protecting group and for subsequent attachment of GPI fragments to peptides is presented.