Constrained Polymer synthesis, using the P22 capsid as a template
The use of protein-polymer composites for materials and medical applications aims to take advantage of the exquisite monodispersity and bioactivity of biomacromolecules while imparting new materials properties via polymer conjugation. By conjugating polymers to the biomolecule, the composite material can exhibit improved retention and lowered immunogenicity, in addition to materials properties such as light, pH, and thermal responsiveness.
Viral capsids provide an ideal template for the synthesis of these hybrid polymer materials. Internal polymerization dramatically increases the surface area and through polymer chain interaction (covalent or non-covalent) changes in the physical properties of the protein cage (thermal stability, stiffness) can be affected. Polymerization on the exterior of the protein cage provides a ‘polymer-brush’-like architecture where the interaction between particles can be designed to facilitate non-covalent interactions as a means to initiate hierarchical assembly or to screen interactions of these protein cage-polymer composites.
We are exploring four complementary types of polymer syntheses (Fig 3). These are: (1) atom transfer radical polymerization (ATRP), (2) azide-alkyne [3+2] ‘click’ polymerization; (3) coordination polymerization; (4) genetically programmed polypeptide synthesis. ATRP provides a mild, very rapid, and selective means of generating polymer/copolymer chains covalently attached to the protein interface. “Click chemistry” when coupled to the protein is an effective and gentle means to grow a variety of functional polymers in a step-wise manner . Coordination polymers can be constructed inside the protein cage via metal-chelate interactions (self assembly of MOF-like structures). Also, incorporation of metal chelating monomers into either the ATRP or ‘click’ synthesis allows us to explore a combination of these polymerization approaches. Incorporation and encapsulation of designed polypeptides on the interior of the P22 (described below - Project 3) is an exciting parallel to the chemical polymer syntheses.
We have shown that the formation of crosslinked polymer networks, inside our protein cages, using azide-alkyne click chemistry, results in a confined hyperbranched polymer scaffold . This polymer network alters the physical properties of the protein cage and, with appropriate design of the monomers, the polymer side-chains act as attachment points for functional molecules of interest. When Gd-chelates are appended to the polymer, the construct can be used as a contrast agent with vastly improved relaxivity per particle (see MRI contrast agent section) . Alternatively, if the monomer is based on a coordination complex (with polymerizable sites on the ligand), the resulting coordination polymer (metallopolymer) incorporates the metal centers as part of the polymer . This approach to coordination polymer formation allows for the incorporation of metal complexes into the protein-polymer composite that are stable in aqueous conditions, but may not be readily formed under protein compatible conditions.
Using ATRP allows us to form the desired polymer in a continuous process and has the advantage of fast syntheses. This method results in products with low polydispersity, but is also promiscuous with respect to the range of monomers and functional groups that can be included in the polymer. When modified metal chelators (imidazole, pyridine, iminodiacetic acid, catechol…) are incorporated into the acrylate or acrylamide monomers the resulting polymer can be doped with appropriate metals forming extensively cross-linked structures similar to those found in biological tissues. If the monomer bears a primary amine, or other addressable group, then coordination complexes or other molecules of interest (drugs, fluorophores, light harvesting chromophores, reactive metal coordination complexes…) can be appended to the polymer introducing new desired functionalities. In addition to development of highly promising MRI contrast agents we are currently exploring functionalized polymer packaging inside P22 as a means to affect reactive site proximity (coupled light harvesting and H2 production) and probing the effects of molecular crowding on reactivity.
1. M. J. Abedin, L. Liepold, P. Suci, M. Young and T. Douglas, "." Journal of the American Chemical Society, 2009, 131, 4346-4354.
2. L. O. Liepold, M. J. Abedin, E. D. Buckhouse, J. A. Frank, M. J. Young and T. Douglas, "." Nano Letters, 2009, 9, 4520-4526.
3. J. Lucon, M. J. Abedin, M. Uchida, L. Liepold, C. C. Jolley, M. Young and T. Douglas, "." Chemical Communications, 2010, 46, 264-266.
Synthesis, Protein Chemistry, Inorganic, Chemical Biology, Bioinorganic, Biochemistry