Encapsulation of Fluorescent Polyelectrolytes into Biologically Active Protein Cages
Our research has demonstrated that natural protein cages can encapsulate fluorescent polyelectrolytes to form new optically active synthetic/biological composites
Initially, we used viral capsid proteins from the Cowpea Chlorotic Mottle Virus as a cage to encapsulate a fluorescent polymer (PSS-Rhodamine). We showed that the capsid proteins formed cages around these polymers to obtain optically active virus-like particles. In fact, encapsulated PSS-Rhodamine virus-like particles are 500 times more resistant to fluorescent quenching than the free polymer.
We then demonstrated that under the right conditions the polymer can allow for tuning of the hybrid-composites' optical properties. By using a polyanionic semiconducting polymer [poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene), MPS-PPV], we set up a system in which the polymer conformation is determined by the ionic strength of its solution. The polymer conformation drives the formation of either tubular or spherical virus-like structures, each of which leads to different optical properties for the hybrid. The results illustrate the synergy of this hybrid composite: polymer conformation drives the structure of the composite material, which in turn modifies the polymer optical properties. We are currently developing other synthetic/biological hybrid materials with designed functionality.
MPS-PPV has also been encapsulated inside protein nanocapsules called "vaults", as demonstrated by several spectroscopic studies. Small angle X ray scattering (SAXS) provides further proof of this encapsulation. The MPS-PPV study provides the groundwork for the use of vaults in encapsulation and delivery applications. Currently, we are engineering a lead-binding protein into vault cages to sequester lead ions, and exploring the application of this versatile technique to other metal-binding proteins. These engineered protein cages have applications in heavy metal poisoning and PET or MRI imaging.