Molecular effects of a nanocrystalline quartz support upon planar lipid bilayers

How to control interactions of cellulose-based biomaterials with skin: the role of acidity in the contact area

Being able to control the interactions of biomaterials with living tissues and skin is highly desirable for many biomedical applications. This is particularly the case for cellulose-based materials which provide one of the most versatile platforms for tissue engineering due to their strength, biocompatibility and abundance. Achieving such control, however, requires detailed molecular-level knowledge of the dominant interaction mechanisms. Here, we employed both biased and unbiased atomic-scale molecular dynamics simulations to explore how cellulose crystals interact with model stratum corneum bilayers, ternary mixtures of ceramides, cholesterol, and free fatty acids. Our findings show that acidity in the contact area directly affects binding between cellulose and the stratum corneum bilayer: Protonation of free fatty acids in the bilayer promotes attractive cellulose-bilayer interactions. We identified two major factors that control the cellulose-skin interactions: (i) the electrostatic repulsion between a cellulose crystal and the charged (anionic due to deprotonated fatty acids) surface of a stratum corneum bilayer and (ii) the cellulose-stratum corneum hydrogen bonding. When less than half of the fatty acids in the bilayer are protonated, the first factor dominates and there is no binding to skin. At a larger degree of fatty acid protonation the cellulose-stratum corneum hydrogen bonding prevails yielding a tight binding. Remarkably, we found that ceramide molecules are the key component in hydrogen bonding with cellulose. Overall, our findings highlight the critical role of fatty acid protonation in biomaterial-stratum corneum interactions and can be used for optimizing the surface properties of cellulose-based materials aimed at biomedical applications such as wound dressings.

Controlled On-Off Switching of Tight-Binding Hydrogen Bonds between Model Cell Membranes and Acetylated Cellulose Surfaces

Controlling interactions between cellulose-based materials and membranes of living cells is critical in medicine and biotechnology in, for example, wound dressing, tissue engineering, hemodialysis membranes, and drug transport. Cellulose acetylation is a widely used approach to tuning those interactions. Surprisingly, however, detailed interactions of acetylated cellulose and membranes have thus far not been characterized. Using atomistic molecular dynamics (MD) simulations, we show that the key to such control is hydrogen bonds: by tuning the number of hydrogen bonds between tissue (cell membranes) and cellulose, binding can be controlled in a precise manner. We demonstrate that the acetylation of each hydroxymethyl group reduces the free energy of cellulose-membrane binding by an order of magnitude as compared to that of pristine cellulose. Remarkably, this acetylation-induced weakening does not occur gradually and is characterized by a sharp threshold in the degree of substitution, beyond which the microscopic character of lipid-cellulose interactions changes drastically. When the degree of substitution does not exceed 0.125, the cellulose-lipid interactions are mainly driven by hydrogen bonding between cellulose's hydroxyl groups and phosphate groups of lipid molecules. This results in the tight binding of a cellulose crystal and a lipid bilayer. Larger degrees of substitution (here, 0.25 and 0.5) prevent hydrogen bonding, leading to rather weak and unstable cellulose-bilayer binding. In this case, the lipid-cellulose binding is controlled by the interactions of lipid choline groups with hydroxyl(hydroxymethyl) groups and carbonyl groups of acetyl moieties of acetylated cellulose.

Molecular-Level Insight into the Interaction of Phospholipid Bilayers with Cellulose

Molecular-level insight into the interactions of phospholipid molecules with cellulose is crucial for the development of novel cellulose-based materials for wound dressing. Here we employ the state-of-the-art computer simulations to unlock for the first time the molecular mechanisms behind such interactions. To this end, we performed a series of atomic-scale molecular dynamics simulations of phospholipid bilayers on a crystalline cellulose support at various hydration levels of the bilayer leaflets next to the cellulose surface. Our findings clearly demonstrate the existence of strong interactions between polar lipid head groups and the hydrophilic surface of a cellulose crystal. We identified two major types of interactions between phospholipid molecules and cellulose chains: (i) direct attractive interactions between lipid choline groups and oxygens of hydroxyl (hydroxymethyl) groups of cellulose and (ii) hydrogen bonding between phosphate groups of lipids and cellulose's hydroxymethyl/hydroxyl groups. When the hydration level of the interfacial bilayer/support region is low, these interactions lead to a pronounced asymmetry in the properties of the opposite bilayer leaflets. In particular, the mass density profiles of the proximal leaflets are split into two peaks and lipid head groups become more horizontally oriented with respect to the bilayer surface. Furthermore, the lateral mobility of lipids in the leaflets next to the cellulose surface is found to slow down considerably. Most of these cellulose-induced effects are likely due to hydrogen bonding between lipid phosphate groups and hydroxymethyl/hydroxyl groups of cellulose: the lipid phosphate groups are pulled toward the water/lipid interface due to the formation of hydrogen bonds. Overall, our findings shed light on the molecular details of the interactions between phospholipid bilayers and cellulose nanocrystals and can be used for identifying possible strategies for improving the properties of cellulose-based dressing materials via, e.g., chemical modification of their surface.

Nanostructural determination of a lipid bilayer tethered to a gold substrate

Tethered lipid bilayer membranes (tBLM) are planar membranes composed of free lipids and molecules tethered to a solid planar substrate providing a useful model of biological membranes for a wide range of biophysical studies and biotechnological applications. The properties of the tBLM depend on the free lipids and on the chemistry of the tethering molecules. We present a nanoscale characterization of a tBLM composed of deuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (d-DMPC) free lipids, benzyl disulfide undecaethylene glycol phytanol (DLP) tethering molecules, and benzyl disulfiide tetraethylene glycol polar spacer molecules (PSM) used to control the areal density of tethering molecules through coadsorption. The use of selected isotopic substitution provides a way to distinguish the conformation and location of the tethered lipids from the free lipids and to elucidate how the two components influence the structure of the tBLM. These findings provide useful information to optimise the insertion of transmembrane proteins into the tethered bilayer system.

Conditions for liposome adsorption and bilayer formation on BSA passivated solid supports

Planar solid supported lipid membranes that include an intervening bovine serum albumen (BSA) cushion can greatly reduce undesirable interactions between reconstituted membrane proteins and the underlying substrate. These hetero-self-assemblies reduce frictional coupling by shielding reconstituted membrane proteins from the strong surface charge of the underlying substrate, thereby preventing them from strongly sticking to the substrate themselves. The motivation for this work is to describe the conditions necessary for liposome adsorption and bilayer formation on these hetero-self-assemblies. Described here are experiments that show that the state of BSA is critically important to whether a lipid bilayer is formed or intact liposomes are adsorbed to the BSA passivated surface. It is shown that a smooth layer of native BSA will readily promote lipid bilayer formation while BSA that has been denatured either chemically or by heat will not. Atomic force microscopy (AFM) and fluorescence microscopy was used to characterize the surfaces of native, heat denatured, and chemically reduced BSA. The mobility of several zwitterionic and negatively charged lipid combinations has been measured using fluorescence recovery after photobleaching (FRAP). From these measurements diffusion constants and percent recoveries have been determined and tabulated. The effect of high concentrations of beta-mercaptoethanol (β-ME) on liposome formation as well as bilayer formation was also explored.

Lipid bilayer templated gold nanoparticles nanoring formation using zirconium ion coordination chemistry

We used positively charged lipids to prepare lipid bilayer assemblies (LBAs) upon which we assembled negatively charged gold nanoparticles (AuNPs). Treatment of the assembly with zirconium chloride resulted in the formation of nanorings of the diameters inversely related to the zirconium ion concentration. The nanorings were attributed to the zirconium ion coordinated AuNPs formed during the lipid bilayer budding process promoted by the acid effect of zirconium chloride. Nanoring organization was also dependent on the fluidity of lipid bilayers, an indication of LBA-assisted nanomaterials organization. We suggest that such bioorganic-inorganic hybrid assemblies coupled to unique topological and morphological variations might be useful as stimuli-responsive sensors or storage compartments for proteins or drugs.

Single particle tracking reveals corralling of a transmembrane protein in a double-cushioned lipid bilayer assembly

A predominate question associated with supported bilayer assemblies containing proteins is whether or not the proteins remain active after incorporation. The major cause for concern is that strong interactions with solid supports can render the protein inactive. To address this question, a large transmembrane protein, the serotonin receptor, 5HT(3A), has been incorporated into several supported membrane bilayer assemblies of increasing complexity. The 5HT(3A) receptor has large extracellular domains on both sides of the membrane, which could cause strong interactions. The bilayer assemblies include a simple POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) supported planar bilayer, a “single-cushion” POPC bilayer with a PEG (poly(ethylene glycol)) layer between membrane and support, and a “double-cushion” POPC bilayer with both a PEG layer and a layer of BSA (bovine serum albumin). Single-cushion systems are designed to lift the bilayer from the surface, and double-cushion systems are designed to both lift the membrane and passivate the solid support. As in previously reported work, protein mobilities measured by ensemble fluorescence recovery after photobleaching (FRAP) are quite low, especially in the double-cushion system. But single-particle tracking of fluorescent 5HT(3A) molecules shows that individual proteins in the double-cushion system have quite high local mobilities but are spatially confined within small corralling domains (<r(C)2> 450 nm). Comparisons with the simple POPC membrane and the single-cushion POPC−PEG membrane reveal that BSA both serves to minimize interactions with the solid support and creates the corrals that reduce the long-range (ensemble averaged) mobility of large transmembrane proteins. These results suggest that in double-cushion assemblies proteins with large extra-membrane domains may remain active and unperturbed despite low bulk diffusion constants.