A strategy for last-step F-18 fluorination of bioconjugated peptides is reported that exploits an “Achilles heel” in the substrate specificity of the fluorinase enzyme. An acetylene functionality at the C-2 position of the adenosine substrate projects from the active site into the solvent. The fluorinase catalyzes a transhalogenation of 5′-chlorodeoxy-2-ethynyladenosine (CID EA) to 5′-fluorodeoxy-2-ethynyladenosine (FDEA). Extending a polyethylene glycol linker from the terminus of the acetylene allows the presentation of bioconjugation cargo to the enzyme for F-18 labelling. The method uses an aqueous solution ((H2O)-O-18) of [F-18]fluoride generated by the cyclotron and has the capacity to isotopically label peptides of choice for positron emission tomography (PET).