2-Anthryltriazolyl-Containing Multidentate Ligands: Zinc-Coordination Mediated Photophysical Processes and Potential in Live-Cell Imaging Applications

Abstract

1,2,3-Triazol-4-yl (triazolyl)-containing tetradentate ligand 1 undergoes fluorescence enhancement upon binding to zinc ion (Zn(2+)) in both organic (acetonitrile) and aqueous solutions. A 1:1 complex of 1 with a trigonal bipyramidal Zn(2+) was characterized by X-ray crystallography. The cyclic voltammogram (CV) of 1 suggests that an intramolecular photoinduced electron transfer (PET) process is thermodynamically feasible which would quench the fluorescence of the 2-anthryltriazolyl fluorophore. On the basis of the X-ray and CV data, it was initially postulated that the 1:1 binding between Zn(2+) and ligand 1 shuts down the PET quenching pathway of the free ligand, which leads to the fluorescence enhancement of 1. However, the nuance of the interaction between 1 and Zn(2+) was revealed by isothermal titration calorimetry (ITC) and (1)H NMR titration experiments. A two-step binding process was observed which proceeds through an intermediate species of 2:1 (ligand/Zn(2+)) stoichiometry. Upon close examination of the fluorescence spectra of 1 during the Zn(2+) titration experiment, the fluorescence profile is in fact consistent with a two-step binding process in which a moderate fluorescence enhancement was observed during the early stage of the titration, followed by a bathochromic shift in conjunction with a more pronounced enhancement as Zn(2+) concentration increases. The studies on compounds 2-5 support the amended hypothesis that upon increasing Zn(2+) concentration, compound 1 first undergoes fluorescence enhancement because of the formation of a 2:1 (ligand to Zn(2+)) complex which slows down the PET quenching process. As Zn(2+) concentration increases, the 2:1 complex is converted into a 1:1 complex which facilitates an intramolecular exciplex formation between the excited 2-anthryltriazolyl fluorophore and the Zn(2+)-bound pyridyl moiety. Finally, the potential of compound 1 as an intracellular fluorescent indicator for Zn(2+) was evaluated. HeLa cells loaded with compound 1 grown in Zn(2+)-rich media show stronger fluorescence than those grown under Zn(2+)-deprived conditions, confirming the promise that the triazolyl-containing polyaza fluoroionophores can be developed into intracellular fluorescent indicators targeting biological Zn(2+).

Zn(ii)-coordination modulated ligand photophysical processes – the development of fluorescent indicators for imaging biological Zn(ii) ions

“…185 The formation of 1,2,3-triazole from Cu(I)-catalyzed azide-alkyne cycloaddition 186,187 not only provides a covalent linker between a fluorophore and a Zn(II) binding site, but enhances the binding affinity by providing an additional ligand to Zn(II) in the form of a 1,2,3-triazole. 185,188 Both scaffolds C 185,189 and D 190 have been characterized in the solid state. The dissociation constants of their Zn(II) complexes are in single nanomolar range.…”

“…185 A more careful examination of 1 H NMR and isothermal titration colorimetry (ITC) data of solution binding, and the stepwise fluorescence changes in the Zn(II) titration experiment revealed the nuances in this chemistry (Scheme 8). 189 Both 1 H NMR and ITC give evidence of a [Zn( 15 ) 2 ] 2+ complex. The structured fluorescence of the anthryl undergoes a modest enhancement during the initial stage of Zn(II) titration.…”

“…The second stage of fluorescence change – a red shift and the appearance of an unstructured emission band – is characteristic of an intramolecular exciplex formation between the excited anthryl and a pyridyl group in the 1:1 [Zn( 15 )] 2+ complex. 189 …”

“…Both ITC and 1 H NMR experiments were critical in determining the stepwise coordination of ligand 15 with Zn(II) (Scheme 8), which contributed to a more detailed, and presumably a more accurate interpretation of the Zn(II)-coordination-mediated fluorescence change of 15 than the fluorescence or absorption spectrometric titration experiments alone. 189 …”

ZnII and PbII coordination chemistry of 2,6-bis(1,2,3-triazol-4-yl)pyridine (clickate) and the metal ion-dependent emission of ‘clickate’–appended anthracene

“…Our previous study in the area of 1,2,3-triazole coordination chemistry includes the incorporation of the triazolyl group in non-conjugated polyaza multidentate ligands (26)(27)(28)(29)(30), and the exploration of their utilities as components in fluorescent indicators for Zn II (26,31). The conjugated, planar tridentate clickate is also an attractive candidate for a metal ion accepting component in fluorescent indicators, due to its ease of synthetic derivatisation with a fluorophore and its potential to undergo fluorescence change upon metal ion binding (32,33).…”