Unnatural Amino Acid Spurs Polymer Growth

An unnatural amino acid containing a polymerization initiator allows the easy formation of protein-polymer bioconjugates, according to a paper in the Journal of the American Chemical Society (DOI: 10.1021/ja104493d). Ryan A. Mehl of Franklin & Marshall College, in Lancaster, Pa., and coworkers designed the amino acid 4-(2ʹ-bromoisobutyramido)­phenylalanine as an initiator for atom-transfer radical polymerization (ATRP). The molecule is genetically installed at defined sites in a protein by using a specially evolved transfer RNA that recognizes the amber stop codon, a three-nucleotide “word” that usually stops DNA transcription. In addition to initiating the polymerization reaction, the amino acid serves as a linker between the protein and the growing polymer. The researchers incorporated the amino acid ATRP initiator into green fluorescent protein (GFP). By combining a GFP that contains an initiator with 2,2ʹ-bipyridine, copper ions, and a monomer—oligo(ethylene oxide) monomethyl ether methacrylate—they were able to efficiently produce a polyethylene glycol-GFP bioconjugate. “We have had success in moving the initiator amino acid around to different positions in different proteins,” Mehl says. “This will allow us to fabricate materials that have protein-polymers of different protein or polymer type organized in space.”