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Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers
Author(s) -
Luo Zhouyang,
Zhu Ningbo,
Zhao Dahui
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201601804
Subject(s) - chemistry , circular dichroism , folding (dsp implementation) , oligomer , crystallography , alkyl , phenylene , side chain , molecule , protein folding , polymer chemistry , organic chemistry , polymer , biochemistry , electrical engineering , engineering
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 (CH 3 OH/H 2 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 H 2 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.