inorganic chemistry

resistant to solvent-quenching effects. It had not been anti-
cipated that the ancillary ligand also could have a more direct
effect—increasing the attraction between lanthanide and
analyte.
While investigating the effects of ancillary ligands on the
binding of dipicolinate, we developed a binding affinity by
competition (BAC) assay to quantify the binding constant of
analyte to lanthanide complex (Eq.3) ( 102 ).

LnðDO2AÞþþDPA^2 Ð

K^0 a

LnðDO2AÞðDPAÞ

K^0 a¼

½LnðDO2AÞðDPAÞŠeq

½LnðDO2AÞþŠeq½DPA^2 Šeq

ð 3 Þ

This value (K^0 a) cannot be measured directly, so we performed a
competition experiment to obtain a competition equilibrium
constant (Kc), as shown in Eq.(4).

LnðDO2AÞðDPAÞþLn^3 þÐ

Kc

LnðDO2AÞþþLnðDPAÞþ

Kc¼

½LnðDO2AÞþŠeq½LnðDPAÞþŠeq

½LnðDO2AÞðDPAÞŠeq½Ln^3 þŠeq

ð 4 Þ

This competition equilibrium constant is inversely proportional
to the association constantK^0 a(Eq. 5):

Kc¼

Ka

K^0 a

ð 5 Þ

whereKais the association constant of the lanthanide aquo ion
with dipicolinate. Using this technique, we were able to compare
the dipicolinate binding affinity for the lanthanide alone with that
of the lanthanide complex under virtually identical conditions.
This assay revealed an enhancement in dipicolinate binding
affinity by about an order of magnitude for [Ln(DO2A)]þas com-
pared to the Ln^3 þaquo ion (Fig. 6) ( 92 ). Interestingly, this increase
in binding affinity is contrary to predictions based purely on
electrostatics. The dianionic DPA^2 – analyte should be more
strongly attracted to tripositive Ln^3 þthan the less-electropositive
[Ln(DO2A)]þ complex. The DO2A helper ligand affords an
enhancement in DPA affinity that more than counteracts the
reduction in positive charge, as dipicolinate binding increases
10-fold.

16 MORGAN L. CABLEet al.