The spin label amino acid TOAC and its uses in studies of peptides: chemical, physicochemical, spectroscopic, and conformational aspects

We review work on the paramagnetic amino acid 2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid, TOAC, and its applications in studies of peptides and peptide synthesis. TOAC was the first spin label probe incorporated in peptides by means of a peptide bond. In view of the rigid character of this cyclic molecule and its attachment to the peptide backbone via a peptide bond, TOAC incorporation has been very useful to analyze backbone dynamics and peptide secondary structure. Many of these studies were performed making use of EPR spectroscopy, but other physical techniques, such as X-ray crystallography, CD, fluorescence, NMR, and FT-IR, have been employed. The use of double-labeled synthetic peptides has allowed the investigation of their secondary structure. A large number of studies have focused on the interaction of peptides, both synthetic and biologically active, with membranes. In the latter case, work has been reported on ligands and fragments of GPCR, host defense peptides, phospholamban, and β-amyloid. EPR studies of macroscopically aligned samples have provided information on the orientation of peptides in membranes. More recent studies have focused on peptide–protein and peptide–nucleic acid interactions. Moreover, TOAC has been shown to be a valuable probe for paramagnetic relaxation enhancement NMR studies of the interaction of labeled peptides with proteins. The growth of the number of TOAC-related publications suggests that this unnatural amino acid will find increasing applications in the future

Peptide Foldamers: from Spectroscopic Studies to Applications.

Peptide foldamers are synthetic oligopeptides which attain a few, specific, constrained conformations in solution. Here, we review our contributions to the study of the structural features of several foldamers, comprising C\u3b1-tetrasubstituted aminoacids, by spectroscopic techniques and, in particular, by a combined approach employing time-resolved energy transfer (FRET) experiments and molecular modeling to determine interprobe distances and orientations. Our data show that, for rigid systems, the commonly used assumption of random orientation of donor and acceptor is unjustified, and that in these cases a correct evaluation of the orientation factor is mandatory for meaningful structural determinations. Finally, we illustrate some applications of peptide foldamers in studies on the kinetics of protein folding and on the realization of peptide-based molecular devices.