Functionalised interfaces mimicking photosynthesis

Abstract

Nature has developed different types of photosynthetic molecular assemblies in bacteria, algae and plants. However, in each case the conversion of light into the universal energy currency ATP and the reduction equivalent NADPH is the starting point of anabolic metabolism. We are working on systems where designed interfaces are carrying out these reactions by coupled photoelectrochemistry. In a first step towards mimicking process of ATP regeneration in an artificial membrane we have combined the stability of a solid-supported tethered lipid membrane [1] based on archaebacterial lipids with the light-to-electrochemical energy converting protein, bacteriorhodopsin (bR) [2]. The tethered membrane system is formed onto gold substrates by a gold-sulfur self-assembly process. The membrane spanning lipid contains both a 4nm lipid region as well as a 4nm hydrophilic, oligoethylene glycol region which forms an ionic reservoir between the gold electrode and the lipid membrane Using electrical and electrochemical techniques we have shown that bR can be successfully incorporated and oriented into such tethered lipid membranes. This was seen as a directed proton flux across the lipid membrane into the ionic reservoir in the presence of light. Results obtained were similar to those obtained with Black Lipid Membrane experiments [3]. The current system combines relatively high photoresponses with the stability of tethered archaebacterial lipid membranes. This type of artificial lipid membrane has the potential to act as a matrix for other membrane proteins. An additional advantage of the ionic reservoir is that it allows the incorporation of membrane proteins with large extracellular regions. Incorporation of F0F1-ATPase, which can use the pH gradient to synthesise ATP is one next step in the production of an artificial leaf. The possibility to use these anchor molecules to immobilise redoxcouples like methylviologen and Flavoenzymes [4] to reduce electroenzymatic NADP+ and electron donor, pigment, acceptor multilayer assemblies to supply the electricity are discussed.