Wrapping molecules in organoclay improves their stability.

Most proteins are too sensitive for many technological applications where they might become useful, including biotechnological reactors, or biosensors. Many researchers have tried to improve their stability by genetic manipulation, immobilisation or coupling with molecular stabilisers. Now, Stephen Mann and his group at the University of Bristol, UK, are suggesting a new and surprisingly versatile pathway to more robust proteins: coating the molecules in organoclay.

Mann assigns part of the motivation behind this work to a biomimetic approach. 'We wanted to test how small an object we could replicate "in stone",' he says. Using a synthetic, clay-like material where organic molecules are covalently attached to a talc-like core, gives him the opportunity to adapt the construction for different purposes. In earlier work, his group wrapped layers of clay around lipid templates, resulting in extended organoclay tubules.¹ Now they have succeeded in extending the approach to proteins.²

One avenue Mann's group has pursued in the protein work is to use an inorganic clay framework modified with propylamino groups. By simply protonating these groups, the researchers can pull apart the layer structure of the clay and obtain individual layers, which avidly bind to protein molecules such as myoglobin or glucose oxidase, resulting in discrete nanoparticles. Using standard spectroscopic methods they could demonstrate that each of the particles contains one protein molecule, which retains its native structure and biochemical function. At high temperatures (85^oC), the clay wrapping protects the protein from thermal unfolding.

Taking the procedure one step further, the researchers managed to convert the dispersed bioinorganic particles into a self-assembling solid-state system. All they had to do was to replace about half of the aminopropyl groups with hydrophobic hexadecyl groups, making the outsides of the nanoparticles sticky. The modified procedure resulted in a network where the clay-wrapped proteins are separated by spaces of 4-5nm, which is similar to the particle radius and sufficient to allow the access of substrate molecules.

This new, highly adjustable wrapping technique should be useful in applications where proteins have to be stabilised and/or immobilised, such as large-scale enzyme catalysis or biosensors.

Michael Gross