A tendril perversion in a helical oligomer: trapping and characterizing a mobile screw-sense reversal

Michael Tomsett, Irene Maffucci, Bryden Le Bailly, Liam Byrne, Stefan M. Bijvoets, Maria Giovanna Lizio, James Raftery, Craig P. Butts, Simon Webb, Alessandro Contini, Jonathan Clayden

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    Abstract

    Helical oligomers of achiral monomers adopt domains of uniform screw sense which are occasionally interrupted by screw-sense reversals. These rare, elusive, and fast-moving features that have eluded detailed characterization. We now describe the structure and habits of a screw-sense reversal trapped within a fragment of helical oligoamide foldamer of the achiral quaternary amino acid 2-aminoisobutyric acid (Aib). The reversal was enforced by compelling the amide oligomer to adopt a right-handed screw sense at one end and a left-handed screw sense at the other. The trapped reversal was characterized by X-ray crystallography, and its dynamic properties were monitored by NMR and circular dichroism, and modelled computationally. Raman spectroscopy indicated that a predominantly helical architecture was maintained despite the reversal. NMR and computational results indicated a stepwise shift from one screw sense to another on moving along the helical chain, indicating that in solution the reversal is not localised at a specific location, but is free to migrate across a number of residues. Analogous unconstrained screw-sense reversals that are free to move within a helical structure are likely to provide the mechanism by which comparable helical polymers and foldamers undergo screw-sense inversion.
    Original languageEnglish
    JournalChemical Science
    Early online date25 Jan 2017
    DOIs
    Publication statusPublished - 2017

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