Expanding the Structural Diversity of Protein Building Blocks with Noncanonical Amino Acids Biosynthesized from Aromatic Thiols
Incorporation of structurally novel noncanonical amino acids (ncAAs) into proteins is valuable for both scientific and biomedical applications. To expand the structural diversity of available ncAAs and to reduce the burden of chemically synthesizing them, we have developed a general and simple biosynthetic method for genetically encoding novel ncAAs into recombinant proteins by feeding cells with economical commercially available or synthetically accessible aromatic thiols. We demonstrate that nearly 50 ncAAs with a diverse array of structures can be biosynthesized from these simple small-molecule precursors by hijacking the cysteine biosynthetic enzymes, and the resulting ncAAs can subsequently be incorporated into proteins via an expanded genetic code. Moreover, we demonstrate that bioorthogonal reactive groups such as aromatic azides and aromatic ketones can be incorporated into green fluorescent protein or a therapeutic antibody with high yields, allowing for subsequent chemical conjugation.
Reaction of [18F]4-fluorobenzenediazonium cations with cysteine or the cysteinyl group: Preparation of 18F-labeled S-aryl-cysteine and a radiolabeled peptide
A reaction route for the preparation of no-carrier-added (n.c.a.) [18F]S-4-fluorophenylcysteine 7 via the [18F]-4-fluorobenzenediazonium ion 4 is described. The key step in this radiosynthesis is the reaction of 4 with cysteine forming [18F]4-fluorophenyldiazocysteine 6, which is subsequently converted into 7 by irradiation with 366 nm light. 4 was synthesized by reacting 1,4-dinitrobenzene 1 with [18F]-fluoride in acetonitrile in a PEEK-capillary in a microwave oven. After dilution of the reaction mixture with methanol, the resulting [18F]4-fluoro-1-nitrobenzene 2 was submitted to reduction by means of H2 with Pd on C catalyst. The resulting [18F]4-fluoroaniline 3 was purified by HPLC and diazotized to 4. The preparation of 4 was optimized with regard to yield and purity. The radiochemical yield of 6 was > 90% (based on 3) while after UV irradiation and HPLC purification 45% of 7 (based on 3) was obtained (yields corrected for decay). The suitability of this method for labeling peptides with fluorine-18 was demonstrated by application to the tripeptide, glutathione (GSH; γ-L-glutamyl-L-cysteinyglycine) 8. Copyright