Site-specific isotopically-labeled proteins, amino acids, and biochemical precursors therefor

Site-specific isotopically-labeled valine, leucine, and isoleucine and biosynthetic precursors for these amino acids are provided. The amino acids are labeled with ¹³C or ¹⁴C at the methyl group carbon atom(s) most remote from the carboxyl group. Also disclosed are the biochemical precursors of these labeled amino acids, 2-keto-4-(ⁿC)butyric acid and 2-keto-3-(ⁿC-methyl)-4-(ⁿC)-butyric acid in which n, at each occurrence, is 13 or 14. Also disclosed are proteins, protein fragments, and polypeptides containing these site-specifically isotopically labeled amino acids, and methods for preparing the biochemical precursors, the amino acids, and the proteins, protein fragments, and polypeptides.