Integrated and Passive 1,2,3-Triazolyl Groups in Fluorescent Indicators for Zinc(II) Ions: Thermodynamic and Kinetic Evaluations

In addition to being a covalent linker
in molecular conjugation
chemistry, the function of a 1,2,3-triazolyl moiety resulting from
the copper­(I)-catalyzed azide–alkyne cycloaddition reaction
as a ligand for metal ions is receiving considerable attention. In
this work, we characterize the thermodynamic and kinetic effects of
incorporating a 1,2,3-triazolyl group in a multidentate ligand scaffold
on metal coordination in the context of fluorescent zinc­(II) indicator
development. Ligands L14, BrL14, and FL14 (1,4-isomers) contain the 1,4-disubstituted-1,2,3-triazolyl
group that is capable of binding with zinc­(II) in conjunction with
a di­(2-picolylamino) (DPA) moiety within a multidentate ligand scaffold.
Therefore, the 1,2,3-triazolyl in the 1,4-isomers is “integrated”
in chelation. The 1,5-isomers L15, BrL15, and FL15 contain 1,2,3-triazolyls that are excluded
from participating in zinc­(II) coordination. These 1,2,3-triazolyls
are “passive linkers”. Zinc­(II) complexes of 2:1 (ligand/metal)
stoichiometry are identified in solution using 1H NMR spectroscopy
and isothermal titration calorimetry (ITC) and, in one case, characterized
in the solid state. The 1:1 ligand/zinc­(II) affinity ratio of L14 over L15, which is attributed to the affinity
enhancement of a 1,2,3-triazolyl group to zinc­(II) over that of the
solvent acetonitrile, is quantified at 18 (−1.7 kcal/mol at
298 K) using an ITC experiment. Fluorescent ligands FL14 and FL15 are evaluated for their potential in zinc­(II)
sensing applications under pH neutral aqueous conditions. The 1,4-isomer FL14 binds zinc­(II) both stronger and faster than the 1,5-isomer FL15. Visualization of free zinc­(II) ion distribution in live
HeLa cells is achieved using both FL14 and FL15. The superiority of FL14 in staining endogenous zinc­(II)
ions in live rat hippocampal slices is evident. In summation, this
work is a fundamental study of 1,2,3-triazole coordination chemistry,
with a demonstration of its utility in developing fluorescent indicators.