title: Tuning the guest-binding ability of a helically folded capsule by in situ modification of the aromatic oligoamide backbone.
abstract: Starting from a previously described aromatic oligoamide helically folded capsule that binds tartaric acid with high affinity and diastereoselectivity, we demonstrate the feasibility of the direct in situ modification of the helix backbone, which results in a conformational change that reduces its affinity for guests by two orders of magnitude. Specifically, ring contraction of the central pyridazine unit into a pyrrole in the full helical sequence was investigated by using electrochemical and chemical processes. The sequence containing the pyrrole was synthesized independently in a convergent manner to ascertain its structure. The conformation of the pyrrolic folded capsule was elucidated in the solid state by X-ray crystallography and in solution by using (1)H and (13)C NMR spectroscopy. Solution studies revealed an unanticipated solvent-dependent equilibrium between the anti-anti and syn-syn conformations of the pyrrole ring with respect to its two adjacent pyridine units. Titrations of the pyrrole-containing sequence monitored by (1)H NMR spectroscopy confirmed the expected drop in affinity for tartaric acid and malic acid that arises from the conformation change in the backbone that follows the replacement of the pyridazine by a pyrrole. The reduction of the pyridazine to a pyrrole was characterized by cyclic voltammetry both on the entire sequence and on a shorter precursor. The lower cathodic potential of the precursor made its preparative-scale electroreduction possible. Direct in situ modification of the pyridazine within the entire capsule sequence was achieved chemically by using zinc in acetic acid.
authors.affiliation: Université de Bordeaux, CBMN, UMR 5248, Institut Européen de Chimie Biologie, 2 rue Escarpit 33607 Pessac (France); CNRS, CBMN, UMR5248 (France).
authors.forename: Guillaume
authors.initials: G
authors.lastname: Lautrette
authors.forename: Christophe
authors.initials: C
authors.lastname: Aube
authors.forename: Yann
authors.initials: Y
authors.lastname: Ferrand
authors.forename: Muriel
authors.initials: M
authors.lastname: Pipelier
authors.forename: Virginie
authors.initials: V
authors.lastname: Blot
authors.forename: Christine
authors.initials: C
authors.lastname: Thobie
authors.forename: Brice
authors.initials: B
authors.lastname: Kauffmann
authors.forename: Didier
authors.initials: D
authors.lastname: Dubreuil
authors.forename: Ivan
authors.initials: I
authors.lastname: Huc
keywords: X-ray diffraction; electrochemistry; foldamers; molecular capsules; molecular recognition