Atomic Force Microscopy Characterization of Palmitoylceramide and Cholesterol Effects on Phospholipid Bilayers: A Topographic and Nanomechanical Study

Supported planar bilayers (SPBs) on mica substrates have been studied at 23 °C under atomic force microscopy (AFM)-based surface topography and force spectroscopy with two main objectives: (i) to characterize palmitoylceramide (pCer)-induced gel (L_β) domains in binary mixtures with either its sphingolipid relative palmitoylsphingomyelin (pSM) or the glycerophospholipid dipalmitoylphosphorylcholine (DPPC) and (ii) to evaluate effects of incorporating cholesterol (Chol) into the previous mixtures in terms of Cer and Chol cooperation for the generation of lamellar gel (L_β) phases of ternary composition. Binary phospholipid/pCer mixtures at <i>X</i>_pCer < 0.33 promote the generation of laterally segregated micron-sized pCer-rich domains. Their analysis at different phospholipid/pCer ratios, by means of domain thickness, roughness, and mechanical resistance to tip piercing, reveals unvarying AFM-derived features over increasing pCer concentrations. These results suggest that the domains grow in size with increasing pCer concentrations while keeping a constant phospholipid/pCer stoichiometry. Moreover, the data show important differences between pCer interactions with pSM or DPPC. Gel domains generated in pSM/pCer bilayers are thinner than the pSM-rich surrounding phase, while the opposite is observed in DPPC/pCer mixtures. Furthermore, a higher breakthrough force is observed for pSM/pCer as compared to DPPC/pCer domains, which can be associated with the preferential pCer interaction with its sphingolipid relative pSM. Cholesterol incorporation into both binary mixtures at a high Chol and pCer ratio abolishes any phospholipid/pCer binary domains. Bilayers with properties different from any of the pure or binary samples are observed instead. The data support no displacement of Chol by pCer or vice versa under these conditions, but rather a preferential interaction between the two hydrophobic lipids

Solvation and Hydration of the Ceramide Headgroup in a Non-Polar Solution

The microscopic hydration of the ceramide headgroup has been determined using a combination of experimentalboth NMR and neutron diffraction techniques and computational techniquesempirical potential structure refinement (EPSR) and molecular dynamics (MD). The addition of water to ceramide in chloroform solutions disrupts the chloroform solvation of the ceramide headgroup, and the water forms distinct pockets of density. Specifically, water is observed to preferentially hydrate the two hydroxyl groups and the carbonyl oxygen over the amide NH motif. Further assessment of the location and orientation of the water molecules bound to the ceramide headgroup makes it clear that the strongly solvated carbonyl moiety of the amide bond creates an anchor from which water molecules can bridge via hydrogen bonding interactions to the hydroxyl groups. Moreover, a significant difference in the hydration of the two hydroxyl groups indicates that water molecules are associated with the headgroup in such a way that they bridge between the carbonyl motif and the nearest neighbor hydroxyl group

Fluorescent Polyene Ceramide Analogues as Membrane Probes

Three ceramide analogues have been synthesized, with sphingosine-like chains containing five conjugated double bonds. Pentaene I has an <i>N-</i>palmitoyl acyl chain, while the other two pentaenes contain also a doxyl radical, respectively, at C5 (Penta5dox) and at C16 (Penta16dox) positions of the <i>N</i>-acyl chain. Pentaene I maximum excitation and emission wavelengths in a phospholipid bilayer are 353 and 478 nm, respectively. Pentaene I does not segregate from the other lipids in the way natural ceramide does, but rather mixes with them in a selective way according to the lipid phases involved. Fluorescence confocal microscopy studies show that when lipid domains in different physical states coexist, Pentaene I emission is higher in gel than in fluid domains, and in liquid-ordered than in liquid-disordered areas. Electron paramagnetic resonance of the pentaene doxyl probes confirms that these molecules are sensitive to the physical state of the bilayer. Calorimetric and fluorescence quenching experiments suggest that the lipids under study orient themselves in lipid bilayers with their polar moieties located at the lipid–water interface. The doxyl radical in the <i>N</i>-acyl chain quenches the fluorescence of the pentaene group when in close proximity. Because of this property, Penta16dox can detect gel–fluid transitions in phospholipids. The availability of probes for lipids in the gel phase is important in view of novel evidence for the existence of gel microdomains in cell membranes