nitronyl nitroxide α-amino acids, we report here the synthesis of a novel, conformationally constrained, quaternary, chiral residue, Aic(CN), and its subsequent conversion into a blue-colored, imidazolinyl nitronyl nitroxide-bearing α-amino acid, Aic(NN). Deprotection and peptide coupling reactions were examined. The configurational assignment of Aic(NN) was achieved by X-ray crystallographic analysis of