A Quaternary Nitronyl Nitroxide α-Amino Acid: Synthesis, Configurational and Conformational Assignments, and Physicochemical Properties

To expand the presently extremely limited repertoire of known nitronyl nitroxide -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 -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 an appropriate tripeptide. This latter investigation, accompanied by an IR absorption conformational analysis, strongly supports the view that the Aic(NN) residue is an efficient peptide turn former. Numerous spectroscopic and magnetic properties of its derivatives and the tripeptide are also described. Of particular relevance for future applications are its UV/Vis absorption, NMR, and EPR signatures.