species in the former case would reduce the overall binding affin-
ity due to a decrease in electrostatic attraction to the dipositive
complex. Differences in pKavalues for Eu and Gd macrocyclic
complexes with DOTA derivatives have been noted in the litera-
ture, though it is not clear if this trend includes Tb, or if these
macrocyclic ligands can be compared directly. We have found
no evidence for such a hydroxylated species in our pH depen-
dence studies, as there is no change in stability of [Eu(DO2A)
(DPA)]–compared to [Tb(DO2A)(DPA)]–at lower pH. With this
model ruled out, we return to the previous discussion of per-
turbations of Ln^3 þelectron density as a possible way to explain
both the gadolinium break and the improved binding affinity.
The ability of a chelating ligand to perturb the electron density
of an Ln^3 þcation is dependent on (1) the number and arrange-
ment of electron-withdrawing groups in the ligand and (2) the
susceptibility of the lanthanide to polarization. The former prop-
erty can be tuned by judicial choice of ligand; the latter is defined
by how easily the electron density of the lanthanide can be exter-
nally influenced, one measure of which is ionization energy.
A lanthanide with a low Ln^3 þ!Ln^4 þionization energy requires
less energy to remove an electron and is arguably more suscepti-
ble to perturbation by chelating species than a lanthanide with
high ionization energy. As shown inFig. 7, the observed trend
in dipicolinate binding affinity is mirrored quite closely by the
lanthanide ionization energy. The Tb^3 þion has the lowest ioniza-
tion energy of all the lanthanides investigated, due primarily to
TABLE IIIBINDINGAFFINITIES FORVARIOUSTB ANDEUCOMPLEXES FOROXYANIONS,PH 7.4–7.5,
295 – 298 K, SHOWING THEDIFFERENCEBETWEEN THETB ANDEUSPECIES,D(LN).
Ligand Analyte logK* D(Ln)
Tb EuDO2A DPA 9.25 8.39 0.86
L 1 HCO 3 – 3.8 2.6 1.2
CH 3 CO 2 – 2.3 <1.0 >1.3
HPO 42 – 4.7 4.15 0.55
L 2 HCO 3 – 4.7 3.75 0.95
CH 3 CO 2 – 3.5 2.4 1.1
HPO 42 – >4.7 >4.7 Unknown
L 1 ¼(SSS)-1,4,7-tris[1-(1-phenyl)ethylcarbamoylmethyl]-1,4,7,10-tetraazacyclododecane,
L 2 ¼(SSS)-1,4,7-tris[1-(1-phenyl)ethylcarbamoylmethyl]-10-methyl-1,4,7,10-
tetraazacyclododecane.
*DPA values from Ref.( 92 ), all others from Ref.( 106 ) with errors0.2.
LUMINESCENT LANTHANIDE SENSORS 19