Abstract:A novel concept is introduced for the oriented incorporation of membrane proteins into solid supported lipid bilayers. Recombinant cytochrome c oxidase solubilized in detergent was immobilized on a chemically modified gold surface via the affinity of its histidine-tag to a nickel-chelating nitrilo-triacetic acid (NTA) surface. The oriented protein monolayer was reconstituted into the lipid environment by detergent substitution. The individual steps of the surface modification, including (1) chemical modification of the gold support, (2) adsorption of the protein, and (3) reconstitution of the lipid bilayer, were followed in situ by means of surface-enhanced infrared absorption spectroscopy (SEIRAS) and accompanied by normalmode analysis. The high surface sensitivity of SEIRAS allows for the identification of each chemical reaction process within the monolayer at the molecular level. Finally, full functionality of the surface-tethered cytochrome c oxidase was demonstrated by cyclic voltammetry after binding of the natural electron donor cytochrome c.

Tethered lipid bilayers (tBLMs) were obtained by the fusion of liposomes from diphytanoylphosphatidylcholine (DPhyPC) with self-assembled monolayers (SAMs) of a newly designed archaea analogue thiolipid, 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-d,l-α-lipoic acid ester (DPTL) on template stripped gold (TSG) films from silicon wafer as a template. SAMs, as characterized by reflection absorption infrared spectroscopy (RAIRS), show a mixture of different conformations of the tetraethylene segment in air, which appears to rearrange into the fully extended conformation when the SAM is immersed into an aqueous electrolyte solution, as deduced from thickness measurements by surface plasmon resonance spectroscopy (SPR). The fusion of liposomes was followed by SPR, quartz crystal microbalance (QCM), and fluorescence microscopy. Highly resistive tBLMs were obtained, as demonstrated by electrochemical impedance spectroscopy (EIS) results, which are equivalent to those for the BLM. This large resistivity is attributed to the ultraflat surface of TSG, as well as to the distinctive architecture of the newly designed molecule. The roughness of the TSG obtained from mica and silicon wafer as template was determined by AFM and compared to that of a Au(111) surface on mica. The largest roughness features of TSG are shown to be 0.5−1 nm, which is small compared to the vertical dimension of the DPTL molecules. This is regarded to be crucial for the self-assembly process, particularly in the case of amphiphilic molecules.

A new concept of solid-supported tethered bilayer lipid membrane (tBLM) for the functional incorporation of membrane proteins is introduced. The incorporated protein itself acts as the tethering molecule resulting in a versatile system in which the protein determines the characteristics of the submembraneous space. This architecture is achieved through a metal chelating surface, to which histidine-tagged (His-tagged) membrane proteins are able to bind in a reversible manner. The tethered bilayer lipid membrane is generated by substitution of protein-bound detergent molecules with lipids using in-situ dialysis or adsorption. The system is characterized by surface plasmon resonance, quartz crystal microbalance, and electrochemical impedance spectroscopy. His-tagged cytochrome c oxidase (CcO) is used as a model protein in this study. However, the new system should be applicable to all recombinant membrane proteins bearing a terminal His-tag. In particular, combination of surface immobilization and membrane reconstitution opens new prospects for the investigation of functional membrane proteins by various surface-sensitive techniques under a defined electric field.

Membrane-bound cytochrome c oxidase was attached to an electrode via a His-tag linker and studied by surface enhanced resonance Raman spectroscopy, demonstrating intact redox site structures and electron transfer between the electrode and the immobilized enzyme.

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