to the protein solution and the reaction was shaken at 37 u00b0 c for 2 hours an additional 165 mg of imidazole was added to push the reaction to completion after 5 hours of total reaction time lcms analysis revealed full conversion to isohistidine calculated mass 26749 26749 found the protein solution was passed through a pd10 column equilibrated with the same buffer 2 5 ml for each of two pd10 columns this protein solution was used immediately in the next reaction to 5 0 ml of the sbl 156his iso prepared above 0 71 mg ml ph 8 0 sodium phosphate buffer 50 mm was added 4 7 mg of pmsf solution in 200 u03bcl mecn the reaction was shaken at room temperature for 20 minutes and then analyzed directly by lcms full conversion to the sbl 156his iso pmsf adduct was observed calculated mass 26903 found 26905 small molecules were removed using pd10 columns equilibrated with the same buffer a 500 u03bcl aliquot of the sbl 156his iso pmsf adduct prepared above 0 51 mg ml ph 8 0 sodium phosphate buffer 50 mm was added to a plastic tube along with 40 u03bcl of 1 0 m naoh the reaction was rotated at 4 u00b0 c for two hours lcms analysis of the reaction mixture revealed full conversion to sbl 156his iso 221dha calculated mass 26731 found 26733 recently biosynthetic incorporation of selenocysteine derivatives into peptides and proteins and conversion to dha has been reported these strategies however rely on peroxide induced oxidative elimination that compromises sensitive side chains such as methionine met dehydroalanine dha is a unique chemical handle for such modifications and the present methods allow access to dha without.