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Buffering Heavy Metal Ions with Photoactive CrownCast Cages
Author(s) -
Mbatia Hannah W.,
Kennedy Daniel P.,
Camire Casey E.,
Incarvito Christopher D.,
Burdette Shawn C.
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000673
Subject(s) - chemistry , aniline , selectivity , metal , metal ions in aqueous solution , photochemistry , molecule , aqueous solution , delocalized electron , divalent , stereochemistry , organic chemistry , catalysis
Abstract Historically, caged compounds have been used to interrogate the biological activity of organic molecules by using light; however, R. Tsien and others have developed methodologies over the last three decades for using nitrobenzyl‐derived caged complexes to study the signaling behavior of Ca 2+ . A series of cation‐selective N ‐phenyl‐azamacrocyclic receptors integrated with a 4,5‐dimethoxy‐2‐nitrobenzyl (DMNB) photoactive group act as cages for divalent metal ions. The uncaging mechanism of these complexes involves a photoreaction that converts the nitrobenzhydrol, which is para to the aniline nitrogen atom, into the corresponding nitrosobenzophenone. Resonance delocalization of the aniline into the distal carbonyl group of the photoproduct attenuates the ability of the nitrogen atom to interact with the guest. CrownCast‐1 ( 3 ) utilizes a 13‐phenyl‐1,4,7,10‐tetraoxa‐13‐azacyclopentadecane (A15C5, 2 ) receptor. Binding studies with CrownCast‐1 revealed a modest selectivity for Ca 2+ ; however, differences in the measured binding affinity uponphotolysis suggest that CrownCast‐1 is better suited to cage Mg 2+ . CrownCast‐2 ( 5 ) is derived from the Hg 2+ ‐selective10‐phenyl‐1,4‐dioxa‐7,13‐dithia‐10‐azacyclopentadecane(AT 2 15C5, 4 ) receptor. Aqueous binding studies demonstrate that CrownCast‐2 binds tightly to Hg 2+ , but the strong sulfur–mercury interactions in the complex mitigate the release of the metal ion upon photolysis. Based on previous reports of metal selectivity, CrownCast‐3 ( 7 ), which possesses a 16‐phenyl‐1,4,7,10,13‐pentaoxa‐16‐azacyclooctadecane (A18C6, 6 ) receptor, was designed to act as a Pb 2+ cage. Initial spectroscopic investigations suggest the uncaged ligand binds Pb 2+ with higher affinity than the parent cage. To address this unexpected observation, a turn‐on fluorescence sensor for Pb 2+ that couples the A18C6 ligand with a 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐ s ‐indacene (BODIPY) fluorophore was synthesized. In contrast to the titration data, photolysis of [Pb(CrownCast‐3)] 2+ in the presence of PbSensor‐1 ( 16 ) demonstrates that the uncaged ligand, CrownUnc‐3 ( 15 ), binds Pb 2+ with a lower affinity than the caged o ‐nitrobenzhydrol macrocycle.