Synthesis of unnatural amino acids functionalized with sterically shielded pyrroline nitroxides.

A series of unnatural amino acids functionalized with sterically shielded pyrroline nitroxides were synthesized. Their reduction by ascorbate/glutathione indicates that L-cysteine functionalized with gem-diethylpyrroline nitroxide is reduced at the slowest rate and is comparable to that measured for the most resistant to reduction pyrroline and pyrrolidine nitroxides.