Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 412

Thepresenceoftwothioethergroupsin 12 allowed it to bind mercury ions. In 2 : 1
hexane/EA with 5% methanol, Hg 2þ quenched the dansyl emission efficiently and even
20 nM of Hg 2þ could be easily detected by the fluorescence spectroscopy. The binding
was determined by the Job's plot to be 1 : 1. Unlike conventional sensors, however, the
mercury binding took place in the folded conformation of 12 , as the methionines were too
far apart in the unfolded conformer. Binding was extremely strong when the folded con-
former was favored by the solvents and very weak when the unfolded form was favored.
Experimentally, the binding constant between 12 and Hg 2þ could be tuned over five
orders of magnitude by simple solvent changes [57].
The conformation of the sensor was the dominant factor controlling the binding affin-
ity. Interestingly, despite its relatively flexible nature, this foldamer demonstrated good
selectivity toward Hg 2þ in comparison to other divalent cations such as Mg 2þ ,Zn 2þ ,
Cu 2þ ,Co 2þ ,Ni 2þ , and even Pb 2þ . Quite likely, in addition to the different binding affin-
ity, the quenching effect of the metal ion was also important to the selectivity. Mercury is
known, for example, to quench dansyl more strongly than most other metal ions [58].
Because of the large hydrophobic surface of an oligocholate, even multiple
hydroxyl and amide groups are insufficient to give any water solubility to the fol-
damer. The Zhao group found that oligocholates could be solubilized by common
surfactant micelles in water and the micelles greatly impacted the conformations of
the foldamers [56,59,60]. Foldamer 12 couldbeincludedwithinthemicellesofsur-
factants of many types, including the cationic cetyltrimethylammonium bromide
(CTAB), anionic sodium dodecylsulfate (SDS), and nonionic Triton X-100 [61]. The
environmentally sensitive emission of the dansyl group made it easy to study the
conformation of the oligocholate in micelles. From the emission properties of the
micelle-solubilized 12 , it was clear that nonionic Triton X-100 formed micelles with
the most hydrophobic interior. The charge of the surfactant impacted the binding
strongly, following the order: cationic micelle

anionic
micelle. The different bindings were attributed to the electrostatic interactions
between the positively charge Hg 2þ and the micelle [61].
The Zhao group later reported dicarboxylated oligocholate 13 [62]. The foldamer
was functionalized with two pyrenyl groups at the chain ends. Due to its long fluo-
rescence lifetime, pyrene could form excimers quite readily [63]. The folding of 13 ,
therefore, could be detected by the excimer formation at 470 nm in the fluorescence
spectrum. The two carboxylic acid groups of 13 helped the folding significantly.
Whereas the parent hexamer could not fold in the binary methanol/EA mixture at
all, 13 couldfoldwithupto15%methanol.When 13 was titrated with zinc acetate,
the excimer/monomer ratio underwent characteristic changes. In 5 and 10% methanol,
the compound was already folded and the addition of Zn 2þ caused very little change to
the excimer/monomer ratio. Although the addition of Zn 2þ enhanced the excimer
formation above 15% methanol, the enhancement was the largest with 15% methanol.

nonionic micelle

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