features, suggesting the Tb^3 þcoordination environment is similar
and therefore that all three ligands chelate in the same manner.
As picolinate has only one carboxylate moiety, this indicates a
bidentate binding mode involving the Pyr nitrogen and a single
carboxylate oxygen. The decrease in intensity for 3,5-dipicolinate
also supports this motif, as the two donors (Pyr N and carboxylate
O) are spaced further apart, weakening chelation for this isomer.
Restriction of the number of binding sites to two using the
heptadentate DO3A ligand results in little change for picolinate,
meaning this analyte remains in the same configuration—bound
to Tb^3 þwith one nitrogen and one oxygen. For the dipicolinate
isomers, however, things have changed. All three exhibit analo-
gous emission spectra with less splitting for the [Tb(DO3A)]
complex, signifying a change in the composition and/or geometry
of the lanthanide coordination sphere and consequently a different
binding mode. Chelation to the lanthanide via anZ^2 -carboxylate
FIG. 14 Method of continuous variations to determine the binding
stoichiometry of SA^2 – to [Tb(EDTA)]–. Tb^3 þand SA^2 – concentrations
varied inversely from 0 to 120mM in 10mM increments with 1.00 mM
EDTA in 50.0 mM CAPS buffer, pH 13.5 (lex¼314 nm). Emission
intensity integrated from 530 to 560 nm. Equilibration time of 24 h.
LUMINESCENT LANTHANIDE SENSORS 33