Introducing Solid-State Biomolecular Protonics—Lateral Proton Conduction Across Biological Membranes

Nature is full of various charge transfer circuits that are at the heart of our existence, such as in our aerobic respiration system or the plant’s photosystem. While the focus of this symposium is Biomolecular Electronics, meaning following electron transport (ET) across biomaterials, proton transport (PT) is equivalent in its importance, and in some cases, such as in the mentioned examples, the role of the ET chain reaction is to facilitate PT. As one can imagine, the mechanisms behind ET and PT are different, having different aspects within the biomolecule that are important for the transport mechanism. Moreover, from an experimental point of view, in electronic devices, it might be hard to distinguish between PT and ET. Here, we introduce a new experimental approach to investigate lateral PT across membranes, which is based on measuring long-range lateral proton conduction via a few layers of lipid bilayers in a solid-state-like environment, i.e., without having bulk water surrounding the membrane. This configuration enables focusing on the surface of the membrane while decoupling it from bulk water. Hence, by controlling the relative humidity of the environment, we can directly explore the role of water in the lateral PT process. We show that proton conduction is highly dependent on the membrane composition, where we explore the role of the head group, the level of tail saturation, and the role of the membrane phase and fluidity. The measured PT as a function of temperature shows an inverse temperature dependency, which we explain by the desorption/adsorption of water molecules into the solid membrane platform. We explain our findings by discussing the role of percolating hydrogen bonding within the membrane structure in a Grotthuss-like mechanism. Our new understanding of the specific role of the membrane interface in supporting lateral long-range PT is not only providing new information related to this fundamental biochemical and biophysical process but also very important to any use of membranes in various applications, from biomedical applications to the use of proton-conductive membranes in energy-related applications.