carboxylates and the Pyr nitrogen as donors, shifting one or both
of the carboxyl moieties around the ring will allow multiple
bidentate binding modes (Fig. 15).
As expected, emission intensities of the Tb^3 þ chelates in
solution follow the order Pyr Pic<DPA. The finding that
monodentate Pyr was the poorest performing ligand and
tridentate dipicolinate was best demonstrated that as denticity
increases the chromophore can bind with greater affinity to the
lanthanide, which in turn leads to more efficient EnT via an
AETE mechanism. A blue shift in the Tb^3 þ–picolinate excitation
spectrum (approximately 5 nm compared to the analogous
dipicolinate complex) also suggests that more energy is required
for sensitization as the electron density of the ligand is shifted
back onto the Pyr ring. A similar blue shift was also observed
with dipicolinate fluorinated in the 4-position, which similarly
shifts electron density away from the chelating side of the ligand.
Results from picolinate and the DPA isomers are of interest:
using ancillary ligands to limit the available binding sites on the
Tb^3 þion, we were able to deduce the likely binding motif of each
isomer based on the intensity and degree of splitting in the emis-
sion spectrum (Fig. 16). Binding of picolinate, 2,4-DPA, and 3,5-
DPA to [Tb(DO2A)]þ all produced spectra with comparable
FIG. 13 Emission spectra (lex¼326 nm) of various Tb(ligand)(SA)
complexes in 50 mM CAPS buffer, pH 12.5 (DO2A complex in CHES
buffer, same concentration, and pH). 10mM SA, 1.0 mM Tb(ligand)
complex. [Tb(EDTA)(SA)] (gray) has the greatest emission intensity.
32 MORGAN L. CABLEet al.