Plastic-Protein Hybrid Materials

Enzymatic films for bioactive surfaces

We encounter them every day in laundry detergent, dishwashing liquid, or shower gel: surfactants – surface-active substances. Surfactants belong to a category of molecules called amphiphiles, molecular hermaphrodites consisting of a water-loving (hydrophilic) “head” and a water-hating (hydrophobic) “tail”. Most surfacants are small amphiphilic molecules. However, an international research team working with Roeland J. M. Nolte, University of Nijmegen, has now built “giant amphiphiles”, hybrid molecules made of proteins and polymers. These new molecules are not just meant to clean better, they could find uses in biochips as well.

What’s so special about amphiphiles? In aqueous solutions, they organize themselves so that the hydrophobic tails have as little contact with the water as possible. This leads to structures such as micelles, vesicles, or films on the surface of the water (with the amphiphiles’ heads in the water and their tails in the air).

The researchers chose to use the protein streptavidin as the hydrophilic head for the construction of their giant amphiphiles. Streptavidin is made of four identical substructures that are set opposite each other in pairs. Each substructure has a binding site for biotin, a small molecule that is classified among the vitamins. This is what the Dutch researchers use to attach their hydrophobic tail; first they attach biotin molecules to polystyrene, and then they couple two biotinylated polystyrene chains to two neighboring binding sites on the streptovidin. The two opposite binding sites are left open. Just like their smaller cousins, the giant amphiphiles form films on the surface of water.

Next the empty binding sites on the streptovidin come into play; the researchers attach enzymes or other functional proteins, again by using biotin molecules. For example, Nolte and his colleagues tried this with horseradish peroxidase. The catalytic activity of the peroxidase is retained, even when it is coupled to the film.

All of this results in a polymer film with densely packed functional enzymes hanging from it. “Such a film is useful as a biosensor, or as a catalyst,” explains Nolte. “Because of their dimensions and their amphiphilic character, plastic-protein hybrids are predestined for lab-on-chip technology.”