title: Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers.
abstract: The folding and aggregation behavior of a pair of oligo(phenylene ethynylene) (OPE) foldamers are investigated by means of UV/Vis absorption and circular dichroism spectroscopy. With identical OPE backbones, two foldamers, 1 with alkyl side groups and 2 with triethylene glycol side chains, manifest similar helical conformations in solutions in n-hexane and methanol, respectively. However, disparate and competing folding and aggregation processes are observed in alternative solvents. In cyclohexane, oligomer 1 initially adopts the helical conformation, but the self-aggregation of unfolded chains, as a minor component, gradually drives the folding-unfolding transition eventually to the unfolded aggregate state completely. In contrast, in aqueous solution (CH3 OH/H2 O) both folded and unfolded oligomer 2 appear to undergo self-association; aggregates of the folded chains are thermodynamically more stable. In solutions with a high H2 O content, self-aggregation among unfolded oligomers is kinetically favored; these oligomers very slowly transform into aggregates of helical structures with greater thermodynamic stability. The folded-unfolded conformational switch thus takes place with the free (nonaggregated) molecules, and the very slow folding transition is due to the low concentration of molecularly dispersed oligomers.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, Center of Soft Matter Science and Engineering and Key Laboratory, of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing, P.R. China.
authors.forename: Zhouyang
authors.initials: Z
authors.lastname: Luo
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, Center of Soft Matter Science and Engineering and Key Laboratory, of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing, P.R. China.
authors.forename: Ningbo
authors.initials: N
authors.lastname: Zhu
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, Center of Soft Matter Science and Engineering and Key Laboratory, of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing, P.R. China. dhzhao@pku.edu.cn.
authors.forename: Dahui
authors.initials: D
authors.lastname: Zhao
keywords: aggregation; foldamers; helical structures; oligomers; self-assembly