Peptoids that are oligomers of N-substituted glycines represent a class of peptide mimics with great potential in areas ranging from medicinal chemistry to biomaterial science. Controlling the equilibria between the cis and trans conformations of their backbone amides is the major hurdle to overcome for the construction of discrete folded structures, particularly for the development of all-cis polyproline type I (PPI) helices, as tools for modulating biological functions. The prominent role of backbone to side chain electronic interactions (n → π*) and side chains bulkiness in promoting cis-amides was essentially investigated with peptoid aromatic side chains, among which the chiral 1-naphthylethyl (1npe) group yielded the best results. We have explored for the first time the possibility to achieve similar performances with a sterically hindered α-chiral aliphatic side chain. Herein, we report on the synthesis and detailed conformational analysis of a series of (S)-N-(1-tert-butylethyl)glycine (Ns1tbe) peptoid homo-oligomers. The X-ray crystal structure of an Ns1tbe pentamer revealed an all-cis PPI helix, and the CD curves of the Ns1tbe oligomers also resemble those of PPI peptide helices. Interestingly, the CD data reported here are the first for any conformationally homogeneous helical peptoids containing only α-chiral aliphatic side chains. Finally we also synthesized and analyzed two mixed oligomers composed of NtBu and Ns1tbe monomers. Strikingly, the solid state structure of the mixed oligomer Ac-(tBu)-(s1tbe)-(tBu)-COOtBu, the longest to be solved for any linear peptoid, revealed a PPI helix of great regularity despite the presence of only 50% of chiral side chain in the sequence.

The typical 2-Cys peroxiredoxins are thiol-peroxidases involved in the physiology of hydrogen peroxide not only as a toxic but also as a signaling molecule. Coordination of these functions depends on the sulfinylation of the catalytic Cys, a modification reversed by ATP-dependent sulfiredoxin, which specifically reduces the sulfinic acid group of overoxidized 2-Cys peroxiredoxins into a sulfenic acid. Sulfiredoxin was originally proposed to operate by covalent catalysis, with formation of a peroxiredoxin-sulfiredoxin intermediate linked by a thiosulfinate bond between the catalytic Cys of both partners, a hypothesis rejected by a study of the human enzyme. To settle the argument, we investigated the catalytic mechanism of Saccharomyces cerevisiae sulfiredoxin, by the characterization of the nature and kinetics of formation of the protein species formed between sulfiredoxin and its substrate in the presence of ATP, using mutants of the non-essential Cys residues of both proteins. We observed the formation of a dithiothreitol-reducible peroxiredoxin-sulfiredoxin species using SDS-PAGE and Western blot analysis, and its mass was shown to correspond to a thiosulfinate complex by high resolution mass spectrometry coupled to liquid chromatography. We next measured indirectly and directly a rate constant of formation of the thiosulfinate species of ϳ2 min ؊1 , for both wild-type and mutant sulfiredoxins, at least equal to the steady-state rate constant of the reaction, with a stoichiometry of 1:1 relative to peroxiredoxin. Taken altogether, our results strongly argue in favor of the formation of a covalent thiosulfinate peroxiredoxin-sulfiredoxin species as an intermediate on the catalytic pathway.