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Phenylene-Diimine-Capped Conjugate of Lower Rim 1,3-Calix[4]Arene As Molecular Receptor for Mg2+ via Arm Conformational Changes Followed by Aggregation and Mimicking the Species by Molecular Mechanics

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journal contribution
posted on 20.06.2014, 00:00 by Anita Nehra, Vijaya Kumar Hinge, Chebrolu Pulla Rao
A phenylene-diimine-capped conjugate of lower rim 1,3-calix[4]­arene (L) was synthesized, characterized, and shown to selectively bind to Mg2+ using its capped arms. This results in a selective recognition of Mg2+ 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 Mg2+ to L is authenticated by absorption and 1H NMR data. The Job’s plot derived on the basis of the absorption data showed 1:1 stoichiometry between the receptor and Mg2+. The 1:1 species was further confirmed through ESI MS, that is, being supported by the isotope peak pattern authenticating the presence of Mg2+ in the complex. The L binds Mg2+ octahedrally using the tetradentate L2– 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 Mg2+ binding were supported experimentally by NOESY. AFM and SEM studies showed that spherical particles of L are modified into flower and chain type aggregates upon complexation with Mg2+, confirming the supramolecular behavior of the species formed.