Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Structure of voltage-dependent anion channel-tethered bilayer lipid membranes determined using neutron reflectivity

Neutron reflectivity (NR) has emerged as a powerful technique to study the structure and behavior of membrane proteins at planar lipid interfaces. Integral membrane proteins (IMPs) remain a significant challenge for NR owing to the difficulty of forming complete bilayers with sufficient protein density for scattering techniques. One strategy to achieve high protein density on a solid substrate is the capture of detergent-stabilized, affinity-tagged IMPs on a nitrilotriacetic acid (NTA)-functionalized self-assembled monolayer (SAM), followed by reconstitution into the lipids of interest. Such protein-tethered bilayer lipid membranes (ptBLMs) have the notable advantage of a uniform IMP orientation on the substrate. Here, NR is used to provide a structural characterization of the ptBLM process from formation of the SAM to capture of the detergent-stabilized IMP and lipid reconstitution. The mitochondrial outer-membrane voltage-dependent anion channel (VDAC), which controls the exchange of bioenergetic metabolites between mitochondria and the cytosol, was used as a model β-barrel IMP. Molecular dynamics simulations were used for comparison with the experimental results and to inform the parameters of the physical models describing the NR data. The detailed structure of the SAM is shown to depend on the density of the NTA chelating groups. The relative content of detergent and protein in surface-immobilized, detergent-stabilized VDAC is measured, while the reconstituted lipid bilayer is shown to be complete to within a few percent, using the known atomic structure of VDAC. Finally, excess lipid above the reconstituted bilayer, which is of consequence for more indirect structural and functional studies, is shown to be present.

The role of human monoacylglycerol lipase (hMAGL) binding pocket in breakup of unsaturated phospholipid membranes

Human monoacylglycerol lipase (hMAGL) plays a key role in homeostatic tuning of the endocannabinoid signaling system and supports aggressive tumorogenesis, making this enzyme a promising therapeutic target. hMAGL features a membrane-associated lid domain that regulates entry of endocannabinoid lipid substrates into the hydrophobic channel accessing the active site, likely from the membrane bilayer. The present work applied simultaneous surface plasmon resonance and electrochemical impedance spectroscopy measurements to show that, in absence of the substrate, hMAGL can remove phospholipid molecules from the membrane and, thereby, disintegrate pre-formed, intact, tethered phospholipid bilayer membrane mimetics (tBLMs) composed of unsaturated phosphatidylcholines. To probe the mechanism of hMAGL-induced on tBLMs compromise, we investigated the effect of wild type and mutant hMAGLs and hMAGL rendered catalytically inactive, as a function of concentration and in the presence of chemically distinct active-site inhibitors. Our data show that hMAGL's lid domain and hydrophobic substrate-binding pocket play important roles in hMAGL-induced bilayer lipid mobilization, whereas hydrolytic activity of the enzyme does not appear to be a factor.

Investigating organic multilayers by spectroscopic ellipsometry: specific and non-specific interactions of polyhistidine with NTA self-assembled monolayers

BACKGROUND

A versatile strategy for protein-surface coupling in biochips exploits the affinity for polyhistidine of the nitrilotriacetic acid (NTA) group loaded with Ni(II). Methods based on optical reflectivity measurements such as spectroscopic ellipsometry (SE) allow for label-free, non-invasive monitoring of molecule adsorption/desorption at surfaces.

RESULTS

This paper describes a SE study about the interaction of hexahistidine (His6) on gold substrates functionalized with a thiolate self-assembled monolayer bearing the NTA end group. By systematically applying the difference spectra method, which emphasizes the small changes of the ellipsometry spectral response upon the nanoscale thickening/thinning of the molecular film, we characterized different steps of the process such as the NTA-functionalization of Au, the adsorption of the His6 layer and its eventual displacement after reaction with competitive ligands. The films were investigated in liquid, and ex situ in ambient air. The SE investigation has been complemented by AFM measurements based on nanolithography methods (nanografting mode).

CONCLUSION

Our approach to the SE data, exploiting the full spectroscopic potential of the method and basic optical models, was able to provide a picture of the variation of the film thickness along the process. The combination of δΔ i +1 ,i (λ), δΨ i +1 ,i (λ) (layer-addition mode) and δΔ(†) i ', i +1(λ), δΨ(†) i ', i +1(λ) (layer-removal mode) difference spectra allowed us to clearly disentangle the adsorption of His6 on the Ni-free NTA layer, due to non specific interactions, from the formation of a neatly thicker His6 film induced by the Ni(II)-loading of the NTA SAM.

Grafted biomembranes containing membrane proteins--the case of the leucine transporter

Here, we bind the sodium dependent amino acid transporter on nitrilotriacetic acid/polyethylene glycol functionalized gold sensors in detergents and perform a detergent-lipid exchange with phosphatidylcholine. We characterize the LeuT structure in the adsorbed film by magnetic contrast neutron reflection using the predicted model from molecular dynamic simulations.

Dithiol-based modification of poly(dopamine): enabling protein resistance via short-chain ethylene oxide oligomers

A facile strategy to modify surface properties of poly(dopamine)-coated substrates.

Membrane protein resistance of oligo(ethylene oxide) self-assembled monolayers

As part of an effort to develop biointerfaces for structure-function studies of integral membrane proteins (IMPs) a series of oligo(ethylene oxide) self-assembled monolayers (OEO-SAMs) were evaluated for their resistance to protein adsorption (RPA) of IMPs on Au and Pt. Spectroscopic ellipsometry (SE) was used to determine SAM thicknesses and compare the RPA of HS(CH2)3O(CH2CH2O)6CH3 (1), HS(CH2)3O(CH2CH2O)6H (2), [HS(CH2)3]2CHO(CH2CH2O)6CH3 (3) and [HS(CH2)3]2CHO(CH2CH2O)6H (4), assembled from water. For both substrates, SAM thicknesses for 1 to 4 were found to be comparable indicating SAMs with similar surface coverages and OEO chain order and packing densities. Fibrinogen (Fb), a soluble plasma protein, and rhodopsin (Rd), an integral membrane G-protein coupled receptor, adsorbed to the SAMs of 1, as expected from previous reports, but not to the hydroxy-terminated SAMs of 2 and 4. The methoxy-terminated SAMs of 3 were resistant to Fb but, surprisingly, not to Rd. The stark difference between the adsorption of Rd to the SAMs of 3 and 4 clearly indicate that a hydroxy-terminus of the OEO chain is essential for high RPA of IMPs. The similar thicknesses and high RPA of the SAMs of 2 and 4 show the conditions of protein resistance (screening the underlying substrate, packing densities, SAM order, and conformational mobility of the OEO chains) defined from previous studies on Au are applicable to Pt. In addition, the SAMs of 4, exhibiting the highest resistance to Fb and Rd, were placed in contact with undiluted fetal bovine serum for 2h. Low protein adsorption (≈12.4ng/cm(2)), obtained under these more challenging conditions, denote a high potential of the SAMs of 4 for various applications requiring the suppression of non-specific protein adsorption.