(NE). The triplet energy levels of CAs (23,800–24,000 cm–^1 )( 122 )
lie in the appropriate range for efficient EnT to the^5 D 4 energy
level of Tb^3 þ(20,500 cm–^1 )( 89 ). CAs coordinate to Cu^2 þ, Co^2 þ,
Ni^2 þ, Mn^2 þ, and Zn^2 þ via the two phenolic hydroxyl groups
(127,128). CAs have also been reported to complex Al^3 þ with
submicromolar affinity, though citrate and adenosine triphos-
phate (ATP) can interfere ( 129 ). When bound to Y^3 þ, Ca^2 þ, or
La^3 þ, a CA still coordinates through the two phenolic groups at
neutral pH, but one of the two groups remains protonated
( 130 – 132 ). Therefore, to bind to lanthanides in a bidentate fash-
ion with high affinity to allow for efficient EnT, both of these phe-
nol groups must be deprotonated ( 133 – 135 ), which requires
strongly basic conditions (Table IV) where lanthanides rapidly
precipitate as hydroxide salts.
Ancillary ligands such as ethylenediaminetetraacetic acid
(EDTA), which bind to lanthanides with high affinities, are able
to arrest lanthanide precipitation by preventing hydroxide from
coordinating, and previous work using [Tb(EDTA)]–following cap-
illary electrophoresis has demonstrated that detection limits on
the order of 0.1mM for CAs (DA, NE, Epi, and others) can be
achieved( 136 ). However, no CA complexes with Ln^3 þ–macrocycles
have been reported in the literature, despite the fact that these
macrocyclic platforms bind lanthanides with even greater
affinities than their linear analogues (Table V) (96,137,138). We
sought to compare acyclic (linear) to cyclic ancillary ligands and
determine the optimal lanthanide–ligand combination to attain
better limits of detection for these biologically relevant analytes.
Four ancillary ligands were investigated: three cyclic (DO2A,
DO3A, and DOTA) and one acyclic (EDTA), with denticities rang-
ing from hexa- to octadentate (Fig. 9). Assuming chelation via the
two hydroxyl moieties, CAs should bind in a bidentate fashion,
leaving seven coordination sites remaining on the lanthanide
for the helper ligand. We therefore assume that [Tb(DOTA)]–
TABLE IV
PROTONATIONCONSTANTS FORCATECHOL ANDVARIOUSCATECHOLAMINES.Analyte pKa1 pKa2 pKa3 References
Cat 9.48 12.08 – ( 133 )
Epi 8.64 9.84 13.1 ( 134 )
NE 8.58 9.53 12.9 ( 134 )
DA 8.89 10.41 13.1 ( 135 )
Cat, catechol; Epi, epinephrine; NE, norepinephrine; DA, dopamine.
LUMINESCENT LANTHANIDE SENSORS 23