Lipid Bilayers as Soft Dielectrics and Heme-binding Peptides as Conductors

Electron transport is critical for sustaining life processes in biological systems, but the structure-function relationships that govern the electronic properties of proteins and lipids are not fully understood. In this talk, I will discuss electron transport in two classes of soft materials: heme-binding peptides and lipid bilayers. In the first part, we examine the role of amino acid sequence on electron transport in heme-binding peptides inspired by cytochrome bc1 using a combination of molecular electronics experiments and simulations. Self-assembled monolayers (SAMs) of sequence-defined peptides capable of forming helical secondary structures are characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Following monolayer formation, the electronic properties (I-V curves) are characterized using a soft-contact liquid-metal electrode technique based on eutectic gallium-indium alloys (EGaIn). Remarkably, we observe a 1000-fold increase in current density across SAM junctions upon addition of heme, while maintaining a constant junction thickness. These findings show that amino acid sequence dictates heme-mediated enhancements in electron transport. In the second part, we investigate the dielectric properties of lipid bilayers using the EGaIn method. Lipid assemblies, which serve as naturally occurring charge-separating structures in biology, provide inspiration for bio-derived soft electronic materials. Polymer-supported lipid bilayers are formed on gold electrodes via vesicle fusion and exhibit sub-nm roughness and high uniformity, as confirmed by AFM. Our results show that the capacitance of lipid bilayers can be tuned by molecular structure (lipid tail length), composition (addition of cholesterol), and assembly conditions (temperature). Measurements on dehydrated bilayers reveal that densely packed lipid films retain their bilayer architecture and act as smooth, stable soft dielectrics. Together, these studies highlight how self-assembled peptides and lipids provide model systems for understanding electron transport in biologically inspired soft materials.