Phenylene-Diimine-Capped Conjugate
of Lower Rim 1,3-Calix[4]Arene
As Molecular Receptor for Mg<sup>2+</sup> via Arm Conformational Changes
Followed by Aggregation and Mimicking the Species by Molecular Mechanics
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Abstract
A phenylene-diimine-capped
conjugate of lower rim 1,3-calix[4]arene
(<b>L</b>) was synthesized, characterized, and shown to selectively
bind to Mg<sup>2+</sup> using its capped arms. This results in a selective
recognition of Mg<sup>2+</sup> through eliciting fluorescence enhancement
of ∼70 fold with a detection limit of 40 ± 5 ppb. However,
in the presence of blood serum, the lowest detection limit is 209
± 10 ppb (0.2 μM). The binding of Mg<sup>2+</sup> to <b>L</b> is authenticated by absorption and <sup>1</sup>H NMR data.
The Job’s plot derived on the basis of the absorption data
showed 1:1 stoichiometry between the receptor and Mg<sup>2+</sup>.
The 1:1 species was further confirmed through ESI MS, that is, being
supported by the isotope peak pattern authenticating the presence
of Mg<sup>2+</sup> in the complex. The <b>L</b> binds Mg<sup>2+</sup> octahedrally using the tetradentate <b>L</b><sup>2–</sup> and two additional acetic acid moieties by bringing conformational
changes as studied on the basis of MM computations. The conformational
changes that occur in the capped arms upon Mg<sup>2+</sup> binding
were supported experimentally by NOESY. AFM and SEM studies showed
that spherical particles of <b>L</b> are modified into flower
and chain type aggregates upon complexation with Mg<sup>2+</sup>,
confirming the supramolecular behavior of the species formed