Thermodynamic Properties and Swelling Behavior of Glycolipid Monolayers at Interfaces

Interplay of Trans- and Cis-Interactions of Glycolipids in Membrane Adhesion

Glycolipids mediate stable membrane adhesion of potential biological relevance. In this article, we investigate the trans- and cis-interactions of glycolipids in molecular dynamics simulations and relate these interactions to the glycolipid-induced average separations of membranes obtained from neutron scattering experiments. We find that the cis-interactions between glycolipids in the same membrane leaflet tend to strengthen the trans-interactions between glycolipids in apposing leaflets. The trans-interactions of the glycolipids in our simulations require local membrane separations that are significantly smaller than the average membrane separations in the neutron scattering experiments, which indicates an important role of membrane shape fluctuations in glycolipid trans-binding. Simulations at the experimentally measured average membrane separations provide a molecular picture of the interplay between glycolipid attraction and steric repulsion of the fluctuating membranes probed in the experiments.

Weak carbohydrate-carbohydrate interactions in membrane adhesion are fuzzy and generic

Carbohydrates such as the trisaccharide motif LeX are key constituents of cell surfaces. Despite intense research, the interactions between carbohydrates of apposing cells or membranes are not well understood. In this article, we investigate carbohydrate-carbohydrate interactions in membrane adhesion as well as in solution with extensive atomistic molecular dynamics simulations that exceed the simulation times of previous studies by orders of magnitude. For LeX, we obtain association constants of soluble carbohydrates, adhesion energies of lipid-anchored carbohydrates, and maximally sustained forces of carbohydrate complexes in membrane adhesion that are in good agreement with experimental results in the literature. Our simulations thus appear to provide a realistic, detailed picture of LeX-LeX interactions in solution and during membrane adhesion. In this picture, the LeX-LeX interactions are fuzzy, i.e. LeX pairs interact in a large variety of short-lived, bound conformations. For the synthetic tetrasaccharide Lac 2, which is composed of two lactose units, we observe similarly fuzzy interactions and obtain association constants of both soluble and lipid-anchored variants that are comparable to the corresponding association constants of LeX. The fuzzy, weak carbohydrate-carbohydrate interactions quantified in our simulations thus appear to be a generic feature of small, neutral carbohydrates such as LeX and Lac 2.

Regioselective oxidation of unprotected 1,4 linked glucans

Palladium-catalyzed alcohol oxidation allows the chemo- and regioselective modification of unprotected 1,4 linked glucans. This is demonstrated in the two-step bisfunctionalization of 1,4 linked glucans up to the 7-mer. Introduction of an anomeric azide is followed by a highly regioselective mono-oxidation of the terminal C3-OH functionality. The resulting orthogonal bis-functionalized oligosaccharides are a viable alternative to PEG-spacers as demonstrated in the conjugation of a cysteine mutant of 4-oxalocrotonate tautomerase with biotin.

Thermomechanic-electrical coupling in phospholipid monolayers near the critical point

Lipid monolayers have been shown to represent a powerful tool in studying mechanical and thermodynamic properties of lipid membranes as well as their interaction with proteins. Using Einstein's theory of fluctuations we here demonstrate that an experimentally derived linear relationship both between transition entropy S and area A as well as between transition entropy and charge q implies a linear relationships between compressibility κT, heat capacity cπ, thermal expansion coefficient αT, and electric capacity CT. We demonstrate that these couplings have strong predictive power as they allow calculating electrical and thermal properties from mechanical measurements. The precision of the prediction increases as the critical point TC is approached.

Modulation of intermembrane interaction and bending rigidity of biomembrane models via carbohydrates investigated by specular and off-specular neutron scattering

We designed artificial models of biological membranes by deposition of synthetic glycolipid membrane multilayers on planar silicon substrates. In contrast to commonly used phospholipid membranes, this offers the unique possibility to study the influence of membrane-bound saccharide chains (cell glycocalix) on the membrane mechanics. Taking advantage of the planar sample geometry, we carried out specular and off-specular neutron scattering experiments to identify out-of-plane and in-plane scattering vector components. By considering the effects of finite sample sizes, we were able to simulate the measured two-dimensional reciprocal space maps within the framework of smectic liquid-crystal theory. The results obtained both at controlled humidity and in bulk water clearly indicate that a subtle change in the molecular chemistry of the saccharides strongly influences intermembrane interactions and membrane bending rigidities.

Anisotropic aggregation and phase transition in Langmuir monolayers of methyl/ethyl esters of 2,3-dihydroxy fatty acids

We studied interfacial properties of a series of methyl and ethyl esters of enantioenriched syn-2,3-dihydroxy fatty acids with different chain lengths at the air-water interface, using a Langmuir type film balance and a Brewster angle microscope (BAM). After analyzing their surface pressure (Pi)-area (A) isotherms, we inferred that these molecules existed as an E conformation in the liquid-expanded (LE) phase of monolayers, and the E conformation of molecules changed into a Z conformation during the LE-LC transition in a monolayer. BAM images evidenced the formation of elongated LC aggregates. This is possibly induced by the intermolecular hydrogen bonds, leading to the anisotropic growth of LC domains, on the basis of the FT-IR spectroscopy data. The enthalpy change of the LE-LC phase transition is considered to result from the three types of intermolecular interactions at the air-water interface during compression of these amphiphiles. These findings are discussed in terms of various physical factors that influenced intermolecular interactions and macroscopic aggregations of these amphiphiles.

In-Plane Structures of Synthetic Oligolactose Lipid Monolayers-Impact of Saccharide Chain Length

Grazing incidence X-ray diffraction (GIXD) was used to investigate the in-plane structure of monolayers of synthetic glycolipids with oligolactose head groups (Lac N, with N = number of lactose units) at the air/water interface. The Lac 1 monolayer exhibits three sharp diffraction peaks. One peak can be deconvoluted into two diffraction peaks, which suggests that alkyl chains in the monolayer form an orthorhombic lattice. On the other hand, the other two peaks are related to bulk crystalline materials residing at the interface. Another weak peak from the head group correlation, either in a monolayer or in bulk crystals, can also be observed in some experiments. The scattering patterns of the Lac 2 monolayer indicate that the alkyl chains order in an orthorhombic lattice, where a shift in the hydrophilic/hydrophobic balance seems to fluidize the film. Alkyl chains in the Lac 3 monolayer also seem to assume an orthorhombic lattice; however, a weak diffraction peak from the correlation between trilactose head groups can also be observed. The estimated lattice dimensions are compared systematically to those of bulk dispersions, as well as to the viscoelastic properties of the monolayer. The obtained results strongly suggest that the in-plane structure of synthetic glycolipid monolayers can provide a well-defined basis to understand the impact of the chemical structure on the cooperativity and function of the glycocalix of cellular surfaces.

Hydrophilic/Hydrophobic balance determines morphology of glycolipids with oligolactose headgroups

The morphology of synthetic glycolipids with lactose oligomers (Lac N, the number of lactose units, N = 1, 2, 3) was studied in lamellar phase. By a systematic combination of differential scanning calorimetry and small- and wide-angle x-ray scattering experiments, the effects of hydrophilic/hydrophobic balance on their thermotropic phase behaviors were discussed. The dispersion of Lac 1 exhibited a crystalline-fluid phase transition, dominated by the strong van der Waals interaction between dihexadecyl chains. In the case of Lac 2, the hydrophilic/hydrophobic balance between the headgroup and the alkyl chains is shifted to the hydrophilic side, resulting in a gel-fluid phase transition with a decreased transition temperature and phase transition enthalpy. Different from the first two systems, the differential scanning calorimetry trace of Lac 3 showed much less remarkable peaks. The small- and wide-angle x-ray diffraction patterns did not reveal any transition in the chain ordering, suggesting that the correlation between the hexasaccharide headgroups is so strong that the melting of the alkyl chains was not allowed. Such dominant effects of the hydrophilic/hydrophobic balance on the morphology of Lac N lipids can be attributed to the small sterical mismatch between the alkyl chains and the linear, cylindrical oligolactose groups.