Incorporation of unnatural amino acids (UAAs) bearing bioorthogonal reactive groups into proteins could be a powerful tool for developing novel protein-based biomaterials with innovative and controlled performance. Bombyx mori silk fibroin is one of naturally derived protein materials extensively studied for biomaterials development due to its mechanical strength and biocompatibility. We recently reported the in vivo incorporation of UAAs, 4-substituted analogues of phenylalanine including 4-azidophenylalanine (AzPhe), into silk fibroin by expanding the repertoire of amino acids for protein biosynthesis in silk glands of B. mori using transgenic techniques. We demonstrated that azide groups in AzPhe incorporated into silk fibroin can be selectively modified by bioorthogonal azide-alkyne cycloaddition reactions (click chemistry). However, the incorporation of AzPhe into silk fibroin required a special feeding condition, which led to the limited production of silk fibroin. Here we report more efficient production of an AzPhe-incorporated silk fibroin (termed AzidoSilk) and its modification by click chemistry in varied material forms (thread, film, and porous sponge). Using this methodology, photolithographic micropatterning of fluorescent molecules directly onto silk fibroin film was achieved and should further expand the availability of silk-based biomaterials for cell culture substrates, drug delivery, tissue scaffolds, implantable devices, and so on.
Keywords: azide; click chemistry; phenylalanyl-tRNA synthetase; photopatterning; silk fibroin; unnatural amino acid.