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Final Report Summary - PROTEINATRP (Protein-based ATRP catalysts: From Nanoreactors to ATRPases)

Atom transfer radical polymerization (ATRP) is one of the most important current synthesis methods for the preparation of well-defined polymers. Polymers prepared by ATRP find applications in nanotechnology, biomedicine, and advanced materials. However, conventional ATRP catalysts are based on complexes of transition metals and have some drawbacks, as they can be toxic and difficult to remove from the polymeric product. Moreover, they can interfere with the polymer’s intended applications. Therefore, enhancing the catalytic performance of conventional ATRP catalysts, as well as replacing these catalysts with environmentally benign catalysts is of great importance. Within this project, two approaches to achieve these goals were followed. In the first one, copper-based catalysts were conjugated to proteins, more specifically to the inside of the protein cage thermosome (THS) and to the surface of fluorescent proteins. The THS was used as a nanoreactor for ATRP of the monomers N-isopropylacrylamide (NIPAAm) and poly(poly(ethylene glycol) methyl ether acrylate (PEGA). Polymerizations within the THS resulted in polymers with a narrower distribution of molecular weights compared to polymerizations with a catalyst that was conjugated to a globular protein. THS is a nanoreactor with pores that are gated by proteinaceous lids. THS could be established as an ATP-responsive nanoreactor, giving opportunity to modulate the activity of encapsulated catalysts by ATP and its analogues. The conformational changes of the THS were not only investigated with conventional biochemical assays, but also with nanomechanical cantilever arrays.

Protein-catalyst conjugates provide an efficient way of removing the copper catalyst quantitatively from the polymer product, as the biomolecule serves as a biochemical handle that allows filtering the protein and therefore the catalyst from the polymer solution. Using enhanced yellow fluorescent protein, the copper concentration of polymer solutions could be reduced to the background level of copper in tap water.

While working on protein-catalyst conjugates we discovered that some native, non-modified proteins can catalyze ATRP. Biocatalysts can alleviate the environmental and toxicity problems associated with conventional ATRP catalysts, as proteins are prime examples for environmentally friendly and non-toxic catalysts. The ATRPase activity of the heme enzyme horseradish peroxidase (HRP) and the heme protein hemoglobin (Hb) was investigated in great detail. The polymerizations followed first order kinetics and yielded bromine-terminated polymers with relative low molecular weight distributions (down to polydispersity indices (PDIs) of 1.44 for polyNIPAAM and below 1.2 for polyPEGA). Moreover, the molecular weight of polyPEGA increased with conversion. The proteins were stable under the reaction conditions, as evidenced by a multitude of biochemical and spectroscopical analytical methods. Moreover, the redoxchemistry of Hb during ATRP was determined by UV/vis spectroscopy, giving first mechanistic insights into the novel enzymatic activity of the protein.

The Marie Curie European Reintegration Grant supported the fellow in a crucial period of his academic career in which he established an independent research group and laid the scientific foundation for his research profile on biocatalysis in polymer chemistry. Based on these accomplishments he was awarded a SNSF-Professorship by the Swiss National Science Foundation and became Professor of Macromolecular Chemistry at the Adolphe Merkle Institute, University of Fribourg, Switzerland.

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