lanthanide ion electron density. Upon chelation of an electronega-
tive helper ligand, the electron density of the lanthanide is shifted
away from open coordination sites. The resulting redistribution of
charge produces a more electropositive region on the Ln^3 þsurface
with greater affinity for an anionic analyte.
We have demonstrated that helper ligands can greatly enhance
the binding affinity of an oxyanion with a lanthanide cation,
thereby establishing this property as a powerful element in
receptor design. What is more, the ligand-induced gadolinium
break makes it very attractive to use terbium as the lanthanide
of choice in sensors, as the readily polarized electron density of
Tb^3 þwill produce the greatest increases in binding affinities if
coupled to the right helper ligands.
From their extraction and separation to even the most basic
density functional theory (DFT) calculations, life is always more
complex when lanthanides are involved and receptor design is no
exception. Steric considerations alone do not dictate ana priori
choice of helper ligand for receptors based on sensitized lantha-
nide luminescence. To a first approximation, parameters such
as ligand denticity and coordination geometry can provide a good
basis for designing a functional receptor complex, but in practice,
it is difficult to predict how these complexes will interact with
their target analytes. We have found that small changes
in helper ligand denticity and binding motif can lead to
surprising behavior in analyte affinity, which has led us to view
these phenomena in a different light.
ABBREVIATIONSacac acetylacetonate
ADHD attention deficit hyperactivity disorder
AETE absorption-energy transfer-emission
ATP adenosine triphosphate
BAC binding affinity by competition
CA catecholamine
Cat catechol, or 1,2-dihydroxybenzene, oro-benzenediol
DA dopamine
DFT density functional theory
DMABA p-dimethylaminobenzoic acid
DO2A 1,4,7,10-tetraazacyclododecane-1,7-bisacetate
DO3A 1,4,7,10-tetraazacyclododecane-1,4,7-trisacetate
DOTA 1,4,7,10-tetrakiscarboxymethyl-1,4,7,10-tetraazacyclododecane
DPA dipicolinic acid, or pyridine-2,6-dicarboxylic acid
2,4-DPA pyridine-2,4-dicarboxylic acid
3,5-DPA pyridine-3,5-dicarboxylic acid
EDTA ethylenediaminetetraacetic acid
EnT energy transfer
LUMINESCENT LANTHANIDE SENSORS 39