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Reversible Double‐Helix–Random‐Coil Transition Process of Bis{hexa(ethynylhelicene)}s
Author(s) -
Sugiura Hiroki,
Amemiya Ryo,
Yamaguchi Masahiko
Publication year - 2008
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.200700295
Subject(s) - chemistry , helix (gastropod) , folding (dsp implementation) , linker , random coil , intramolecular force , crystallography , exothermic reaction , reaction rate constant , foldamer , conformational isomerism , thermodynamics , kinetics , stereochemistry , organic chemistry , molecule , circular dichroism , physics , quantum mechanics , snail , computer science , electrical engineering , ecology , biology , engineering , operating system
Two compounds with two hexa(ethynylhelicene) parts connected by a flexible haxadecamethylene and a rigid butadiyne linker were synthesized. The 1 H NMR spectroscopic and CD analyses and vapor‐pressure osmometry (VPO) of these two compounds revealed intramolecular double‐helix formation. Upon heating a 5‐μ M solution in toluene, the double‐helix structure unfolded to form a random coil, and on cooling it folded again into a double helix. The thermodynamic stabilities of both structures were dependent on temperature, and the structural change in both compounds is due to the large enthalpies and entropies under equilibrium. The rate constants of their unfolding were obtained by assuming a pseudo‐first‐order reaction; the compound with a rigid linker unfolded slower than that with a flexible linker. The former has a larger activation energy, and its double‐helix and random‐coil conformers were separated by chromatography. The rate of folding was also faster for the flexible‐linker compound with larger activation energy. The rate constants for the folding of both compounds slightly decreased with increasing temperature, which was ascribed to the presence of exothermic pre‐equilibrium and rate‐determining steps. The folding was markedly accelerated with increasing random‐coil concentration, which suggests the involvement of self‐catalysis. A mechanism of folding was proposed. The involvement of different mechanisms of folding and unfolding was suggested by the kinetic studies, and it was confirmed by the presence of hysteresis in the melting profiles. The difference in linker structure also affected the thermal‐switching profiles of the double‐helix–random‐coil structural changes.