Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers

• Published in: Chemistry : a European journal 2016-07, Vol.22 (31), p.11028-11034
• Main Authors: Luo, Zhouyang, Zhu, Ningbo, Zhao, Dahui
• Format: Article
• Language: English
• Subjects: Absorption; Agglomeration; Aggregates; aggregation; Aggregation behavior; Aqueous solutions; Backbone; Chains; Circular dichroism; Circularity; Conformation; Cyclohexane; Dichroism; Dispersion; foldamers; Folding; helical structures; Methanol; Methyl alcohol; Molecular conformation; n-Hexane; Oligomers; self-assembly; Self-association; Solvents; Spectroscopy; Structural stability; Triethylene glycol
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201601804

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 (CH3OH/H2O) 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 H2O 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. Twist or clump? Two oligo(phenylene ethynylene) foldamers exhibit competing helical folding and aggregation processes. Although unfolded aggregates are the most stable state for molecules with alkyl side chains, analogous foldamers with oligo(ethylene glycol) side chains present stable aggregates in the folded conformation. However, unfolded aggregates form kinetic traps that slow the folding process (see figure).