PEGylation of membrane proteins like bacteriorhodopsin as a tool to increase their stability toward ethanol

Protection of biological compounds, for example, enzymes, viruses, or even whole cells, against degradation is very important for many applications. Embedding of such compounds into polymer matrices is a straightforward common method. However, in biotechnology and medicine there is a great interest to prepare micro- and nanosized shells around the biocomponents in order to protect them and having only a minor increase in size. The PEGylation of biological macromolecules has gained attention because degradation by proteolytic enzymes is significantly retarded and, in turn, their bioavailability is enhanced. We found that PEGylation is also a powerful tool to protect biomaterials from degradation by small organic solvent molecules, in particular, ethanol. Methoxy-polyethylene glycol (MPEG) modified BR survives exposure to significant concentrations of ethanol, up to 30%, and preserves its photochromism, whereas unmodified PM is instantaneously denatured at such concentrations. This is useful for potential technical applications of BR but is of relevance for many other applications where biomaterials and, in particular, biomembranes may be exposed to solvents.