From the salt marsh to the ID-card: Bacteriorhodopsin as a functional material in nanobiotechnology

Abstract

Bacteriorhodopsin (BR) is an evolutionary highly optimized photochromic retinal protein, which is found in extremely halophilic bacteria, e.g., in salt marshes. We demonstrated that starting from the wildtype as a blueprint by means of gene technology and biotechnology a versatile material for optical information recording can be developed. BR is structurally related to the visual pigment rhodopsin. It is the key molecule in the halobacterial photosynthetic system — an alternative to the chlorophyll-dependent photosynthesis. Its biological function ist that of a light-driven proton pump. In the halobacterial cell — which are found e.g. in salt marshes — it converts light energy into chemical energy, i.e. a proton gradient over the cell membrane, which finally supplies ATP to the cell. The photochromic properties of BR are very attractive compared to those of known organic photochromic compounds, in particular as far as longevity under exposure to oxygen and light is concerned. This is one of the reasons why we try to utilized this evolutionary optimized biomaterial for technical applications in particular in optical data storage and processing. As the biological function of BR is optimized for energy conversion, the physical properties of BR need to be tuned to turn this molecule into a material which matches the requirements of optical applications in data storage and processing. Gene technology is a powerful tool for the controlled modification of physical properties of a biomolecule like BR. In technical applications water needs to be omitted. However, the function of biomaterials strictly depends on the presence of water. Membrane proteins are much less dependent on the presence of water which makes them good candidates for technical applications. We showed that BR can be processed into dry polymeric films where its function is preserved. In a field test where ID-cards comprising BR-based inks as security elements it has been demonstrated that biomaterials may be integrated in active form as functional components into conventional technical applications. Conventional nanomaterials supply properties to a product, biomaterials supply functions.