ESR characterization of hexameric, helical peptides using double TOAC spin labeling

The unprecedented, unnatural alpha-amino acid 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) is used to examine the structure of hexameric sequences. It allows the study of Ala-rich peptides containing C(alpha)-tetrasubstituted residues without worrying over signal overlap that frequently hinders nuclear magnetic resonance investigations. C(alpha)-tetrasubstituted amino acids were found to prefer the helix, but do not 'lock in' the helical conformation.