Methods for producing modified polypeptides containing amino acid analogues are disclosed, as well as compositions comprising purified dihydrofolate reductase polypeptides, produced by the methods of the invention. In certain aspects, methionine residues of the compositions are replaced with homoallyglycine, homoproparglycine, norvaline, norleucine, cis-crotyiglycine, trans-crotylglycine, 2-aminoheptanoic acid, 2-butynyiglycine and allyglycine.