DPPTE Thiolipid Self-Assembled Monolayer: A Critical Assay

Abstract

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au-S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 ± 0.4 °C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

Plasmonic Resonant Nanoantennas Induce Changes in the Shape and the Intensity of Infrared Spectra of Phospholipids

“…1 mM solution of DPPTE was prepared by dissolving the powder in chloroform and the gold substrates (AuNPs, gold layer and Au nanoantennas, separately) were incubated into the lipid solution at room temperature (25 °C) for 18 h. After 18 h of immersion, the gold substrates were carefully rinsed with chloroform (~99.9% grade) to remove physically adsorbed phospholipids from the surface, and then dried under a gentle argon flow. The immobilization procedure used here was similar to that one reported by Raffaella Lettieri et al [ 21 ].…”

“…The RMS roughness increases from approximately 0.36 ± 0.01 to 1.5 ± 0.3 nm with the appearance of a nanotextured surface after chloroform rinsing, thus confirming the deposition of DPPTE lipid. The corresponding cross sectional profile of the nanotextured surface showed the presence of domain with an average height of 7 nm, suggesting the presence of DPPTE bilayer with 3.26 nm in lipid height in all trans configuration [ 21 ]. A previous study showed that DPPTE phospholipids incubated for 18 h in ethanol solution, formed a monolayer on the gold surface [ 21 ], whose thickness (23.5 ± 0.5 Å) is significantly lower than the lipid film obtained in this study by immersion of the gold layer substrate in 1 mM DPPTE chloroform solution for 18 h. The difference in the thickness can be due to the difference in the sample preparation.…”

“…The corresponding cross sectional profile of the nanotextured surface showed the presence of domain with an average height of 7 nm, suggesting the presence of DPPTE bilayer with 3.26 nm in lipid height in all trans configuration [ 21 ]. A previous study showed that DPPTE phospholipids incubated for 18 h in ethanol solution, formed a monolayer on the gold surface [ 21 ], whose thickness (23.5 ± 0.5 Å) is significantly lower than the lipid film obtained in this study by immersion of the gold layer substrate in 1 mM DPPTE chloroform solution for 18 h. The difference in the thickness can be due to the difference in the sample preparation. Also, it was previously shown that it is possible to modulate the properties of the DPPTE SAM by selecting the experimental incubation conditions [ 22 , 23 , 24 ].…”

“…In previous study on the phase behavior of DPPTE lipid by means of infrared reflection absorption spectroscopy (IR-RAS), the lipid layer supported on a gold surface after 18 h incubation at room temperature in ethanol solution demonstrated the presence of domain structures of two phases, the liquid-expanded and liquid-condensed phases [ 21 ]. The phase transition temperature determined for the DPPTE lipid using fluorescence anisotropy and liposomes made from the phospholipid in phosphate buffer solution was about 41.1 ± 0.4 °C [ 21 ], and 45 °C for crystalline DPPTE measured by calorimetry [ 44 ]. It is noted, that the experiments reported have been made at different experimental conditions, especially concerning the lipid layer creation.…”

Behavior of a Peptide During a Langmuir–Blodgett Compression Isotherm: A Molecular Dynamics Simulation Study

“…The Langmuir–Blodgett (LB) technique is a simple and low-cost method for the production of layered structures at the air/water interface, the ordering of which can be controlled by mechanical forces. , The LB method is typically applied to amphiphilic compounds, detergents, and lipids, − whose conformational and aggregation behavior at the air/water interface has been deeply characterized by experimental methods and molecular dynamics (MD) simulations. − Detailed analyses concerning the conformation and the different phases that molecules experience at the air/water interface during an LB isotherm are usually reported by considering amphiphilic molecules like lipids and fatty acid. However, extensive experimental research has also been carried out on Langmuir films formed by “unusual” molecules, like polymers, DNA, liquid crystals, proteins, carbon nanotubes, and inorganic nanostructures. ,, …”