Abstract
Purple membranes (PM) of the bacteria Halobacterium salinarum are a unique natural membrane where bacteriorhodopsin (BR) can convert photon energy and pump protons. Elucidating the electronic properties of biomembranes is critical for revealing biological mechanisms and developing new devices. We report here the electric properties of PMs studied by using multi-functional electric force microscopy (EFM) at the nanoscale. The topography, surface potential, and dielectric capacity of PMs were imaged and quantitatively measured in parallel. Two orientations of PMs were identified by EFM because of its high resolution in differentiating electrical characteristics. The extracellular (EC) sides were more negative than the cytoplasmic (CP) side by 8 mV. The direction of potential difference may facilitate movement of protons across the membrane and thus play important roles in proton pumping. Unlike the side-dependent surface potentials observed in PM, the EFM capacitive response was independent of the side and was measured to be at a dC/dz value of ~5.25 nF/m. Furthermore, by modification of PM with de novo peptides based on peptide-protein interaction, directional oriented PM assembly on silicon substrate was obtained for technical devices. This work develops a new method for studying membrane nanoelectronics and exploring the bioelectric application at the nanoscale.
Highlights
Investigations on the electric properties of bio-membranes are vital important to reveal their biological functions and mechanisms
The Purple membrane (PM) samples were deposited on highly doped silicon under ambient condition for the nanocharacterization with electric force microscopy (EFM) measurements
The PM samples were deposited on highly doped silicon under ambient conditions for the EFM measurements
Summary
Investigations on the electric properties of bio-membranes are vital important to reveal their biological functions and mechanisms. BR acts as a light-driven, voltage-sensitive proton pump in the PM and serves as an ideal model system to study protein-rich biological membranes at the nanoscale [3]. BR converts the energy of single photons into large structural changes to pump protons from the CP side to the EC side across purple membrane directionally, thereby creating an electrochemical gradient used by the ATPases to energize the cellular processes [5]. The function of PM as a light-driven proton pump requires that BR undergoes a cyclic photoreaction, resulting in the release and uptake of protons on the opposite sides of the membrane [6]
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