C O M M U N I C A T I O N S
seem promising for applications in highly sensitive biological
assays. Our efforts are now turning toward the synthesis of
polydentate ligands with other topologies by using higher denticity
linkers (e.g., H22 ) N,N,N′,N′-tetrakis(2-aminoethyl)ethane-1,2-
diamine). Removing amide NH oscillators via methylation has also
proven13 to be a method of improving the radiative decay, although
in this case this modification may negatively impact the complex
structure and stability. These efforts are ongoing, together with the
preparation of functionalized derivatives suitable for bioconjugation.
Acknowledgment. This work was partially supported by the
NIH (Grant HL69832) and the Director, Office of Science, Office
of Advanced Scientific Computing Research, Office of Basic
Energy Sciences (U.S. Department of Energy) under contract DE-
AC02-05CH11231. This technology is licensed to Lumiphore, Inc.
in which some of the authors have a financial interest. Financial
support was provided to C.J. by the German Research Foundation
(DFG). The authors thank Amanda Samuel for experimental
assistance.
Figure 2. Electronic absorption spectrum (left) and steady-state emission
spectrum (right) (λex ) 330 nm, 5 nm band-pass) for the [Eu(5LIO-1,2-
HOPO)2]- complex in 0.1 M Tris buffer at pH 7.4.
Table 1. Protonation Constants of 5LIO-1,2-HOPO and Formation
Constants of ML, ML2, and ML2H Complexes with Eu(III)
pK1
pK2
pEu
4.19(3)
5.79(1)
18.64(10)
â110
â120
â121
12.46(2)
22.85(10)
25.21(3)
Table 2. Summary of Experimental and Calculated29 Photophysical
Supporting Information Available: Detailed synthesis of 5LIO-
1,2-HOPO, experimental data for potentiometric, spectrophotometric,
and spectrofluorometric titrations, additional photophysical data, and
X-ray crystallographic files (in CIF format). This material is available
Parameters for the [Eu(5LIO-1,2-HOPO)2]- Complex
ꢀmax
233 nm
333 nm
(H2O)
73860 M-1 cm-1
19250 M-1 cm-1
727 ( 1.2 µs
1012 ( 2.5 µs
0.215 ( 0.03
1640 µs
krad (calcd)
0.609 ms-1
0.807 ms-1
τobs
knonrad (calcd)
(D2O)
Φtot
τrad
(H2O)
(calcd)
Φ
Eu (calcd)
ηsens (calcd)
0.43
0.49
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q ) A′(kH - kD - B)
where A′ and B are empirical constants of 1.11 and 0.31 ms-1 for
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As a result of their highly luminescent properties with Eu(III)
and excellent thermodynamic stability, 1,2-HOPO-based chelators
JA062597+
9
J. AM. CHEM. SOC. VOL. 128, NO. 33, 2006 10649