title: Isosteric substitutions of urea to thiourea and selenourea in aliphatic oligourea foldamers: site-specific perturbation of the helix geometry.
abstract: Nearly isosteric oxo to thioxo substitution was employed to interrogate the structure of foldamers with a urea backbone and explore the relationship between helical folding and hydrogen-bonding interactions. A series of oligomers with urea bonds substituted by thiourea bonds at discrete or all positions in the sequence have been prepared and their folding propensity was studied by using a combination of spectroscopic methods and X-ray diffraction. The outcome of oxo to thioxo replacements on the helical folding was found to depend on whether central or terminal ureas were modified. The canonical helix geometry was not affected upon insertion of thioureas close to the negative end of the helix dipole, whereas thioureas close to the positive pole were found to increase the terminal flexibility and cause helix fraying. Perturbation was amplified when a selenourea was incorporated instead, leading to a structure that is only partly folded.
authors.affiliation: Université de Bordeaux, CNRS, Institut Polytechnique de Bordeaux, UMR 5248 CBMN, Institut Européen de Chimie et Biologie, 2 rue Robert Escarpit, 33607 Pessac (France).
authors.forename: Yella Reddy
authors.initials: YR
authors.lastname: Nelli
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Antunes
authors.forename: Arnaud
authors.initials: A
authors.lastname: Salaün
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Thinon
authors.forename: Stéphane
authors.initials: S
authors.lastname: Massip
authors.forename: Brice
authors.initials: B
authors.lastname: Kauffmann
authors.forename: Céline
authors.initials: C
authors.lastname: Douat
authors.forename: Gilles
authors.initials: G
authors.lastname: Guichard
mesh_terms: D002942:Circular Dichroism; D008958:Models, Molecular; D015394:Molecular Structure; D016566:Organoselenium Compounds / Q000737:chemistry*; D013890:Thiourea / Q000737:chemistry*; D014508:Urea / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D016566:Organoselenium Compounds; C081959:selenourea; D014508:Urea; D013890:Thiourea
keywords: X-ray diffraction; foldamers; helical structures; thioureas; unfolding