Solid-phase synthesis of peptides containing the spin-labeled 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC).

2,2,6,6-Tetramethylpiperidine-1-oxyl-4-amino4-carboxylic acid (TOAC) is a nitroxide spin-labeled, achiral Calpha-tetrasubstituted amino acid recently shown to be not only an effective beta-turn and 3(10)/alpha-helix promoter in peptides, but also an excellent rigid electron paramagnetic resonance probe and fluorescence quencher. Here, we demonstrate that TOAC can be effectively incorporated into internal positions of peptide sequences using Fmoc chemistry and solid-phase synthesis in an automated apparatus.